C Programming

C is a general-purpose, high-level language that was originally developed by Dennis M. Ritchie to develop the UNIX operating system at Bell Labs. C was originally first implemented on the DEC PDP-11 computer in 1972.In 1978, Brian Kernighan and Dennis Ritchie produced the first publicly available description of C, now known as the K&R standard.

The UNIX operating system, the C compiler, and essentially all UNIX application programs have been written in C. C has now become a widely used professional language for various reasons:

• Easy to learn.Structured Language.
• It produces efficient programs.
• It can handle low level activity.
• It can compiled on a variety of computer platforms.

Some Facts about C:-

• C was invented to write an operating system called UNIX.
• C is a successor of B language which was introduced around the early 1970s.
• The UNIX OS was totally written in C.
• Today C is the most widely used and popular System Programming Language.
• Most of the state-of-the-art software have been implemented using C.
• Today's most popular Linux OS and RDBMS MySQL have been written in C.
• The language was formalized in 1988 by the American National Standard Institute (ANSI).

Why Use C?

C was initially used for system development work, particularly the programs that make-up the operating system. C was adopted as a system development language because it produces code that runs nearly as fast as the code written in assembly language. Some examples of the use of C might be:

• Operating Systems
• Language Compilers
• Assembler
• Text Editors
• Print Spoolers
• Network Drivers
• Modern Programs
• Databases
• Language Interpreters
• Utilities

Getting started with c..

In order to run a C program, you need a compiler. Compiler change source code(code written by programmer) to object code(code that computer understands) and creates executable file. There are many free and professional compilers available. For the sake of this course, GNU GCC compiler is used. All the examples in this course are tested and verified in GNU GCC compiler.

A C program can vary from 3 lines to millions of lines and it should be written into one or more text files with extension ".c"; for example, hello.c. You can use "vi", "vim" or any other text editor to write your C program into a file.

This tutorial assumes that you know how to edit a text file and how to write source code inside a program file.

Environment setup for c:-

If you want to set up your environment for C programming language, you need the following two software tools available on your computer,(a) Text Editor and (b) The C Compiler.

(a)Text Editor:-

This will be used to type your program. Examples of a few editors include Windows Notepad, OS Edit command, Brief, Epsilon, EMACS, and vim or vi.The name and version of text editors can vary on different operating systems. For example, Notepad will be used on Windows, and vim or vi can be used on Windows as well as on Linux or UNIX.The files you create with your editor are called the source files and they contain the program source codes. The source files for C programs are typically named with the extension ".c".Before starting your programming, make sure you have one text editor in place and you have enough experience to write a computer program, save it in a file, compile it and finally execute it.

(b)The C compiler:-

The source code written in source file is the human readable source for your program. It needs to be "compiled" into machine language so that your CPU can actually execute the program as per the instructions given.

The compiler compiles the source codes into final executable programs. The most frequently used and free available compiler is the GNU C/C++ compiler, otherwise you can have compilers either from HP or Solaris if you have the respective operating systems.

The following section explains how to install GNU C/C++ compiler on various OS. m We keep mentioning C/C++ together because GNU gcc compiler works for both C and C++ programming languages.

The Environment setups :-

Installation on UNIX OR LINUX:-

If you are using Linux or UNIX, then check whether GCC is installed on your system by entering the following command from the command line:

                                                               $ gcc -v

If you have GNU compiler installed on your machine, then it should print a message as follows:

                                           Using built-in specs.

                                           Target: i386-redhat-linux

                                           Configured with: ../configure --prefix=/usr .......

                                           Thread model: posix

                                            gcc version 4.1.2 20080704 (Red Hat 4.1.2-46)

If GCC is not installed, then you will have to install it yourself using the detailed instructions available at http://gcc.gnu.org/install/.

This tutorial has been written based on Linux and all the given examples have been compiled on the Cent OS flavour of the Linux system.

Installation on Mac OS:-

If you use Mac OS X, the easiest way to obtain GCC is to download the Xcode development environment from Apple's web site and follow the simple installation instructions. Once you have Xcode setup, you will be able to use GNU compiler for C/C++.

Xcode is currently available at developer.apple.com/technologies/tools/.

Installation on Windows:-

To install GCC on Windows, you need to install MinGW. To install MinGW, go to the MinGW homepage, www.mingw.org, and follow the link to the MinGW download page. Download the latest version of the MinGW installation program, which should be named MinGW-<version>.exe.

While installing MinGW, at a minimum, you must install gcc-core, gcc-g++, binutils, and the MinGW runtime, but you may wish to install more.

Add the bin subdirectory of your MinGW installation to your PATH environment variable, so that you can specify these tools on the command line by their simple names.

After the installation is complete, you will be able to run gcc, g++, ar, ranlib, dlltool, and several other GNU tools from the Windows command line.

The C Program Structure:-

Before we study the basic building blocks of the C programming language, let us look at a bare minimum C program structure so that we can take it as a reference in the upcoming topics.

Hello world Example:-

A C program basically consists of the following parts:

• Preprocessor 
• Commands
• Functions
• Variables
• Statements & ExpressionsComments

C hello world program: c programming language code to print hello world. This program prints hello world, printf library function is used to display text on screen, '\n' places cursor on the beginning of next line, stdio.h header file contains declaration of printf function. The code will work on all operating systems may be its Linux, Mac or any other and compilers. To learn a programming language you must start writing programs in it and may be your first c code while learning programming.

                                          

                                                 //C hello world example

                                                 #include <stdio.h>

                                                 int main()

                                                 {

                                                  printf("Hello world\n");

                                                 return 0;

                                                 }

Purpose of Hello world program may be to say hello to people or the users if your software or application.

Output of program:-

Let us take a look at the various parts of the above program:-

1. The first line of the program #include <stdio.h> is a preprocessor command, which tells a C compiler to include stdio.h file before going to actual compilation.

2. The next line int main() is the main function where the program execution begins.

3. The next line /*...*/ will be ignored by the compiler and it has been put to add additional comments in the program. So such lines are called comments in the program.

4. The next line printf(...) is another function available in C which causes the message "Hello, World!" to be displayed on the screen. 

5. The next line return 0; terminates the main() function and returns the value 0.

Compile and Execute C Program:-

Let us see how to save the source code in a file, and how to compile and run it. Following are the simple steps:

1. Open a text editor and add the above-mentioned code. 

2. Save the file as hello.c

3. Open a command prompt and go to the directory where you have saved the file. 

4. Type gcc hello.c and press enter to compile your code.

5. If there are no errors in your code, the command prompt will take you to the next line and would generate a.out executable file. 

6. Now, type a.out to execute your program.

7. You will see the output "Hello World" printed on the screen.

                                                 $ gcc hello.c

                                                 $ ./a.out

                                                 Hello, World!

Make sure the gcc compiler is in your path and that you are running it in the directory containing the source file hello.c.

Basic Syntax:-

You have seen the basic structure of a C program, so it will be easy to understand other basic building blocks of the C programming language.

Tokens in C:-
A C program consists of various tokens and a token is either a keyword, an identifier, a constant, a string literal, or a symbol. For example, the following C statement consists of five tokens:

                                     printf("Hello, World! \n");

The individual tokens are:

                                     printf
                                     (
                                      "Hello, World! \n"
                                      )

                                      ;
Semicolons:-
In a C program, the semicolon is a statement terminator. That is, each individual statement must be ended with a semicolon. It indicates the end of one logical entity.

Given below are two different statements:

printf("Hello, World! \n");

return 0;

Comments:-
Comments are like helping text in your C program and they are ignored by the compiler. They start with /* and terminate with the characters */ as shown below:

                        /* my first program in C */
You cannot have comments within comments and they do not occur within a string or character literals.

Keywords and Identifiers:-


A C identifier is a name used to identify a variable, function, or any other user-defined item. An identifier starts with a letter A to Z, a to z, or an underscore ‘_’ followed by zero or more letters, underscores, and digits (0 to 9).
C does not allow punctuation characters such as @, $, and % within identifiers. C is a case-sensitive programming language. Thus, Manpower and manpower are two different identifiers in C. Here are some examples of acceptable identifiers:

mohd              zara      abc   move_name    a_123
myname50     _temp     j     a23b9               retVal

The following list shows the reserved words in C. These reserved words may not be used as constants or variables or any other identifier names.

Rules for writing an identifier:-

1. A valid identifier can have letters (both uppercase and lowercase letters), digits and underscore only.
2. The first letter of an identifier should be either a letter or an underscore. However, it is discouraged to start an identifier name with an underscore. It is because identifier that starts with an underscore can conflict with system names.
   In such cases, compiler will complain about it. Some system names that start with underscore are _fileno, _iob, _wfopen etc.
3. There is no rule on the length of an identifier. However, the first 31 characters of identifiers are discriminated by the compiler. So, the first 31 letters of two identifiers in a program should be different.

White-space in C:-

A line containing only whitespace, possibly with a comment, is known as a blank line, and a C compiler totally ignores it.

Whitespace is the term used in C to describe blanks, tabs, newline characters and comments. Whitespace separates one part of a statement from another and enables the compiler to identify where one element in a statement, such as int, ends and the next element begins. Therefore, in the following statement:

                           int age;

there must be at least one whitespace character (usually a space) between int and age for the compiler to be able to distinguish them. On the other hand, in the following statement:

          fruit = apples + oranges; // get the total fruit

no whitespace characters are necessary between fruit and =, or between = and apples, although you are free to include some if you wish to increase readability.

Data types:-

Data types in C refer to an extensive system used for declaring variables or functions of different types. The type of a variable determines how much space it occupies in storage and how the bit pattern stored is interpreted.

The types in C can be classified as follows:

Integer data types:-

Integers are whole numbers that can have both positive and negative values, but no decimal values. Example: 0, -5, 10

In C programming, keyword int is used for declaring integer variable. For example:

int id;

Here, id is a variable of type integer.

You can declare multiple variable at once in C programming. For example:

int id, age;

The size of int is either 2 bytes(In older PC's) or 4 bytes. If you consider an integer having size of 4 byte( equal to 32 bits), it can take 232 distinct states as: -231,-231+1, ...,-2, -1, 0, 1, 2, ..., 231-2, 231-1

Similarly, int of 2 bytes, it can take 216 distinct states from -215 to 215-1. If you try to store larger number than 231-1, i.e,+2147483647 and smaller number than -231, i.e, -2147483648, program will not run correctly.

Floating types:-

Floating type variables can hold real numbers such as: 2.34, -9.382, 5.0 etc. You can declare a floating point variable in C by using either float or double keyword. For example:

float accountBalance;
double bookPrice;

Here, both accountBalance and bookPrice are floating type variables.

In C, floating values can be represented in exponential form as well. For example:

float normalizationFactor = 22.442e2;

Difference between float and double

The size of float (single precision float data type) is 4 bytes. And the size ofdouble (double precision float data type) is 8 bytes. Floating point variables has a precision of 6 digits whereas the the precision of double is 14 digits.

Character types:-

Keyword char is used for declaring character type variables. For example:

char test = 'h'

Here, test is a character variable. The value of test is 'h'.

The size of character variable is 1 byte.

C Qualifiers:-

Qualifiers alters the meaning of base data types to yield a new data type.

Size qualifiers

Size qualifiers alters the size of a basic type. There are two size qualifiers, longand short. For example:

long double i;

The size of float is 8 bytes. However, when long keyword is used, that variable becomes 10 bytes.

If you know that the value of a variable will not be large, short can be used.

Sign qualifiers

Integers and floating point variables can hold both negative and positive values. However, if a variable needs to hold positive value only, unsigned data types are used. For example:

// unsigned variables cannot hold negative value 
unsigned int positiveInteger;

There is another qualifier signed which can hold both negative and positive only. However, it is not necessary to define variable signed since a variable is signed by default.

An integer variable of 4 bytes can hold data from -231 to 231-1. However, if the variable is defined as unsigned, it can hold data from 0 to 232-1.

It is important to note that, sign qualifiers can be applied to int and char types only.

Constant qualifiers

An identifier can be declared as a constant. To do so const keyword is used.

const int cost = 20;

The value of cost cannot be changed in the program.

Volatile qualifiers

A variable should be declared volatile whenever its value can be changed by some external sources outside the program. Keyword volatile is used for creating volatile variables.

Variables and Constants:-

A variable is nothing but a name given to a storage area that our programs can manipulate. Each variable in C has a specific type, which determines the size and layout of the variable's memory; the range of values that can be stored within that memory; and the set of operations that can be applied to the variable.
The name of a variable can be composed of letters, digits, and the underscore character. It must begin with either a letter or an underscore. Upper and lowercase letters are distinct because C is case-sensitive. Based on the basic types explained in the previous chapter, there will be the following basic variable types:

C programming language also allows to define various other types of variables, which we will cover in subsequent chapters like Enumeration, Pointer, Array, Structure, Union, etc. For this chapter, let us study only basic variable types.

Variable Definition:-

A variable definition tells the compiler where and how much storage to create for the variable. A variable definition specifies a data type and contains a list of one or more variables of that type as follows:

                                     type variable_list;

Here, type must be a valid C data type including char, w_char, int, float, double, bool, or any user-defined object; and variable_list may consist of one or more identifier names separated by commas. Some valid declarations are shown here:

int           i, j, k;

char        c, ch;

float        f, salary;

double    d;

The line int i, j, k; declares and defines the variables i, j and k; which instruct the compiler to create variables named i, j, and k of type int.

Variables can be initialized (assigned an initial value) in their declaration. The initializer consists of an equal sign followed by a constant expression as follows:

type variable_name = value;

For definition without an initializer: variables with static storage duration are implicitly initialized with NULL (all bytes have the value 0); the initial value of all other variables are undefined.

Variable Declaration:-

A variable declaration provides assurance to the compiler that there exists a variable with the given type and name so that the compiler can proceed for further compilation without requiring the complete detail about the variable. A variable declaration has its meaning at the time of compilation only, the compiler needs actual variable declaration at the time of linking the program.

A variable declaration is useful when you are using multiple files and you define your variable in one of the files which will be available at the time of linking the program. You will use the keyword extern to declare a variable at any place. Though you can declare a variable multiple times in your C program, it can be defined only once in a file, a function, or a block of code.

Example

Try the following example, where variables have been declared at the top, but they have been defined and initialized inside the main function:

 #include <stdio.h>

// Variable declaration:

extern int a, b;

extern int c;

extern float f;

int main ()

{

/* variable definition: */

int a, b;

int c;

float f;

/* actual initialization */

a = 10;

b = 20;

c = a + b;

printf("value of c : %d \n", c);

f = 70.0/3.0;

printf("value of f : %f \n", f);

return 0;

}

When the above code is compiled and executed, it produces the following result:

value of c : 30

value of f : 23.333334

Constants and Literals:-

Constants refer to fixed values that the program may not alter during its execution. These fixed values are also called literals.

Constants can be of any of the basic data types like an integer constant, a floating constant, a character constant, or a string literal. There are enumeration constants as well.

Constants are treated just like regular variables except that their values cannot be modified after their definition.

Integer Literals:-

An integer literal can be a decimal, octal, or hexadecimal constant. A prefix specifies the base or radix: 0x or 0X for hexadecimal, 0 for octal, and nothing for decimal.

An integer literal can also have a suffix that is a combination of U and L, for unsigned and long, respectively. The suffix can be uppercase or lowercase and can be in any order.

Here are some examples of integer literals:

212 /* Legal */

215u /* Legal */

0xFeeL /* Legal */

078 /* Illegal: 8 is not an octal digit */

032UU /* Illegal: cannot repeat a suffix */

Following are other examples of various types of integer literals:

85 /* decimal */

0213 /* octal */

0x4b /* hexadecimal */

30 /* int */

30u /* unsigned int */

30l /* long */

30ul /* unsigned long */

Floating Point Literals:-

A floating-point literal has an integer part, a decimal point, a fractional part, and an exponent part. You can represent floating point literals either in decimal form or exponential form.

While representing decimal form, you must include the decimal point, the exponent, or both; and while representing exponential form, you must include the integer part, the fractional part, or both. The signed exponent is introduced by e or E.

Here are some examples of floating-point literals:

3.14159 /* Legal */

314159E-5L /* Legal */

510E /* Illegal: incomplete exponent */

210f /* Illegal: no decimal or exponent */

.e55 /* Illegal: missing integer or fraction */

Character Constants:- 

Character literals are enclosed in single quotes, e.g., 'x' can be stored in a simple variable of char type.

A character literal can be a plain character (e.g., 'x'), an escape sequence (e.g., '\t'), or a universal character (e.g., '\u02C0').

There are certain characters in C that represent special meaning when preceded by a backslash, for example, newline (\n) or tab (\t). Here, you have a list of such escape sequence codes:

Following is the example to show a few escape sequence characters:

#include <stdio.h>

int main()

{

printf("Hello\tWorld\n\n");

return 0;

}

When the above code is compiled and executed, it produces the following result:

Hello World

Defining Constants:-

There are two simple ways in C to define constants:

1. Using #define preprocessor
2. Using const keyword

The #define Preprocessor

Given below is the form to use #define preprocessor to define a constant:

                                         #define identifier value

The following example explains it in detail:

#include <stdio.h>

#define LENGTH 10

#define WIDTH 5

#define NEWLINE '\n'

int main()

{

int area;

area = LENGTH * WIDTH;

printf("value of area : %d", area);

printf("%c", NEWLINE);

return 0;

}

When the above code is compiled and executed, it produces the following result:

                     value of area : 50

The const Keyword:-

You can use const prefix to declare constants with a specific type as follows:

const type variable = value;

The following example explains it in detail:

#include <stdio.h>

int main()

{

const int LENGTH = 10;

const int WIDTH = 5;

const char NEWLINE = '\n';

int area;

area = LENGTH * WIDTH;

printf("value of area : %d", area);

printf("%c", NEWLINE);

return 0;

}

When the above code is compiled and executed, it produces the following result:

                      value of area : 50

Note that it is a good programming practice to define constants in CAPITALS.

Storage classes:-

A storage class defines the scope (visibility) and life-time of variables and/or functions within a C Program. They precede the type that they modify. We have four different storage classes in a C program:

• auto
• register
• static
• extern

The Auto Storage class:-

The auto storage class is the default storage class for all local variables.

{

int mount;

auto int month;

}

The example above defines two variables within the same storage class. ‘auto’ can only be used within functions, i.e., local variables.

The Register Storage Class:-

The register storage class is used to define local variables that should be stored in a register instead of RAM. This means that the variable has a maximum size equal to the register size (usually one word) and can't have the unary '&' operator applied to it (as it does not have a memory location).

{

register int miles;

}

The register should only be used for variables that require quick access such as counters. It should also be noted that defining 'register' does not mean that the variable will be stored in a register. It means that it MIGHT be stored in a register depending on hardware and implementation restrictions.

The Static Storage Class:-

The static storage class instructs the compiler to keep a local variable in existence during the life-time of the program instead of creating and destroying it each time it comes into and goes out of scope. Therefore, making local variables static allows them to maintain their values between function calls.

The static modifier may also be applied to global variables. When this is done, it causes that variable's scope to be restricted to the file in which it is declared.

In C programming, when static is used on a class data member, it causes only one copy of that member to be shared by all the objects of its class.

#include <stdio.h>

/* function declaration */

void func(void);

static int count = 5; /* global variable */

main()

{

while(count--)

{

func();

}

return 0;

}

/* function definition */

void func( void )

{

static int i = 5; /* local static variable */

i++;

printf("i is %d and count is %d\n", i, count);

}

When the above code is compiled and executed, it produces the following result:

                       i is 6 and count is 4

                       i is 7 and count is 3

                       i is 8 and count is 2

                       i is 9 and count is 1

                       i is 10 and count is 0

The Extern Storage Class:-

The extern storage class is used to give a reference of a global variable that is visible to ALL the program files. When you use 'extern', the variable cannot be initialized, however, it points the variable name at a storage location that has been previously defined.

When you have multiple files and you define a global variable or function, which will also be used in other files, then extern will be used in another file to provide the reference of defined variable or function. Just for understanding, extern is used to declare a global variable or function in another file.

The extern modifier is most commonly used when there are two or more files sharing the same global variables or functions as explained below.

First File: main.c

#include <stdio.h>

int count;

extern void write_extern();

main()

{

count = 5;

write_extern();

}

Second File: support.c

#include <stdio.h>

extern int count;

void write_extern(void)

{

printf("count is %d\n", count);

}

Here, extern is being used to declare count in the second file, whereas it has its definition in the first file, main.c. Now, compile these two files as follows:

                  $gcc main.c support.c

It will produce the executable program a.out. When this program is executed, it produces the following result:

                  5

Operators in C :-

An operator is a symbol that tells the compiler to perform specific mathematical or logical functions. C language is rich in built-in operators and provides the following types of operators:

• Arithmetic Operators
• Relational Operators
• Logical Operators
• Bitwise Operators
• Assignment Operators
• Misc Operators

We will, in this chapter, look into the way each operator works.

Arithmetic Operators:-

The following table shows all the arithmetic operators supported by the C language. Assume variable A holds 10 and variable B holds 20, then:

Example

Try the following example to understand all the arithmetic operators available in C:

#include <stdio.h>

main()

{

int a = 21;

int b = 10;

int c ;

c = a + b;

printf("Line 1 - Value of c is %d\n", c );

c = a - b;

printf("Line 2 - Value of c is %d\n", c );

c = a * b;

printf("Line 3 - Value of c is %d\n", c );

c = a / b;

printf("Line 4 - Value of c is %d\n", c );

c = a % b;

printf("Line 5 - Value of c is %d\n", c );

c = a++;

printf("Line 6 - Value of c is %d\n", c );

c = a--;

printf("Line 7 - Value of c is %d\n", c );

}

When you compile and execute the above program, it produces the following result:

Line 1 - Value of c is 31

Line 2 - Value of c is 11

Line 3 - Value of c is 210

Line 4 - Value of c is 2

Line 5 - Value of c is 1

Line 6 - Value of c is 21

Line 7 - Value of c is 22

Relational Operators:-

The following table shows all the relational operators supported by C. Assume variable A holds 10 and variable B holds 20, then:

Example

Try the following example to understand all the relational operators available in C:

#include <stdio.h>

main()

{

int a = 21;

int b = 10;

int c ;

if( a == b )

{

printf("Line 1 - a is equal to b\n" );

}

else

{

printf("Line 1 - a is not equal to b\n" );

}

if ( a < b )

{

printf("Line 2 - a is less than b\n" );

}

else

{

printf("Line 2 - a is not less than b\n" );

}

if ( a > b )

{

printf("Line 3 - a is greater than b\n" );

}

else

{

printf("Line 3 - a is not greater than b\n" );

}

/* Lets change value of a and b */

a = 5;

b = 20;

if ( a <= b )

{

printf("Line 4 - a is either less than or equal to b\n" );

}

if ( b >= a )

{

printf("Line 5 - b is either greater than or equal to b\n" );

}

}

When you compile and execute the above program, it produces the following result:

Line 1 - a is not equal to b

Line 2 - a is not less than b

Line 3 - a is greater than b

Line 4 - a is either less than or equal to b

Line 5 - b is either greater than or equal to b

Logical Operators:-

Following table shows all the logical operators supported by C language. Assume variable A holds 1 and variable B holds 0, then:

Example

Try the following example to understand all the logical operators available in C:

#include <stdio.h>

main()

{

int a = 5;

int b = 20;

int c ;

if ( a && b )

{

printf("Line 1 - Condition is true\n" );

}

if ( a || b )

{

printf("Line 2 - Condition is true\n" );

}

/* lets change the value of a and b */

a = 0;

b = 10;

if ( a && b )

{

printf("Line 3 - Condition is true\n" );

}

else

{

printf("Line 3 - Condition is not true\n" );

}

if ( !(a && b) )

{

printf("Line 4 - Condition is true\n" );

}

}

When you compile and execute the above program, it produces the following result:

Line 1 - Condition is true

Line 2 - Condition is true

Line 3 - Condition is not true

Line 4 - Condition is true

Bitwise Operators:-

Bitwise operators work on bits and perform bit-by-bit operation. The truth table for &, |, and ^ is as follows:

Assume A = 60 and B = 13; in binary format, they will be as follows:

A = 0011 1100

B = 0000 1101

-----------------

A&B = 0000 1100

A|B = 0011 1101

A^B = 0011 0001

~A = 1100 0011

The following table lists the bitwise operators supported by C. Assume variable ‘A’ holds 60 and variable ‘B’ holds 13, then:

Example

Try the following example to understand all the bitwise operators available in C:

#include <stdio.h>

main()

{

unsigned int a = 60; /* 60 = 0011 1100 */

unsigned int b = 13; /* 13 = 0000 1101 */

int c = 0;

c = a & b; /* 12 = 0000 1100 */

printf("Line 1 - Value of c is %d\n", c );

c = a | b; /* 61 = 0011 1101 */

printf("Line 2 - Value of c is %d\n", c );

c = a ^ b; /* 49 = 0011 0001 */

printf("Line 3 - Value of c is %d\n", c );

c = ~a; /*-61 = 1100 0011 */

printf("Line 4 - Value of c is %d\n", c );

c = a << 2; /* 240 = 1111 0000 */

printf("Line 5 - Value of c is %d\n", c );

c = a >> 2; /* 15 = 0000 1111 */

printf("Line 6 - Value of c is %d\n", c );

}

When you compile and execute the above program, it produces the following result:

Line 1 - Value of c is 12

Line 2 - Value of c is 61

Line 3 - Value of c is 49

Line 4 - Value of c is -61

Line 5 - Value of c is 240

Line 6 - Value of c is 15

Assignment Operator:-

The following tables lists the assignment operators supported by the C language:

Example

Try the following example to understand all the assignment operators available in C:

#include <stdio.h>

main()

{

int a = 21;

int c ;

c = a;

printf("Line 1 - = Operator Example, Value of c = %d\n", c );

c += a;

printf("Line 2 - += Operator Example, Value of c = %d\n", c );

c -= a;

printf("Line 3 - -= Operator Example, Value of c = %d\n", c );

c *= a;

printf("Line 4 - *= Operator Example, Value of c = %d\n", c );

c /= a;

printf("Line 5 - /= Operator Example, Value of c = %d\n", c );

c = 200;

c %= a;

printf("Line 6 - %= Operator Example, Value of c = %d\n", c );

c <<= 2;

printf("Line 7 - <<= Operator Example, Value of c = %d\n", c );

c >>= 2;

printf("Line 8 - >>= Operator Example, Value of c = %d\n", c );

c &= 2;

printf("Line 9 - &= Operator Example, Value of c = %d\n", c );

c ^= 2;

printf("Line 10 - ^= Operator Example, Value of c = %d\n", c );

c |= 2;

printf("Line 11 - |= Operator Example, Value of c = %d\n", c );

}

When you compile and execute the above program, it produces the following result:

Line 1 - = Operator Example, Value of c = 21

Line 2 - += Operator Example, Value of c = 42

Line 3 - -= Operator Example, Value of c = 21

Line 4 - *= Operator Example, Value of c = 441

Line 5 - /= Operator Example, Value of c = 21

Line 6 - %= Operator Example, Value of c = 11

Line 7 - <<= Operator Example, Value of c = 44

Line 8 - >>= Operator Example, Value of c = 11

Line 9 - &= Operator Example, Value of c = 2

Line 10 - ^= Operator Example, Value of c = 0

Line 11 - |= Operator Example, Value of c = 2

Misc Operators ↦ sizeof & ternary:-

Besides the operators discussed above, there are a few other important operators including sizeof and ? : supported by the C Language.

Example

Try following example to understand all the miscellaneous operators available in C:

#include <stdio.h>

main()

{

int a = 4;

short b;

double c;

int* ptr;

/* example of sizeof operator */

printf("Line 1 - Size of variable a = %d\n", sizeof(a) );

printf("Line 2 - Size of variable b = %d\n", sizeof(b) );

printf("Line 3 - Size of variable c= %d\n", sizeof(c) );

/* example of & and * operators */

ptr = &a; /* 'ptr' now contains the address of 'a'*/

printf("value of a is %d\n", a);

printf("*ptr is %d.\n", *ptr);

/* example of ternary operator */

a = 10;

b = (a == 1) ? 20: 30;

printf( "Value of b is %d\n", b );

b = (a == 10) ? 20: 30;

printf( "Value of b is %d\n", b );

}

When you compile and execute the above program, it produces the following result:

value of a is 4

*ptr is 4.

Value of b is 30

Value of b is 20

Operator Precedence in C:-

Operator precedence determines the grouping of terms in an expression and decides how an expression is evaluated. Certain operators have higher precedence than others; for example, the multiplication operator has a higher precedence than the addition operator.

For example, x = 7 + 3 * 2; here, x is assigned 13, not 20 because operator * has a higher precedence than +, so it first gets multiplied with 3*2 and then adds into 7.

Here, operators with the highest precedence appear at the top of the table, those with the lowest appear at the bottom. Within an expression, higher precedence operators will be evaluated first.

Example

Try the following example to understand operator precedence in C:

#include <stdio.h>

main()

{

int a = 20;

int b = 10;

int c = 15;

int d = 5;

int e;

e = (a + b) * c / d; // ( 30 * 15 ) / 5

printf("Value of (a + b) * c / d is : %d\n", e );

e = ((a + b) * c) / d; // (30 * 15 ) / 5

printf("Value of ((a + b) * c) / d is : %d\n" , e );

e = (a + b) * (c / d); // (30) * (15/5)

printf("Value of (a + b) * (c / d) is : %d\n", e );

e = a + (b * c) / d; // 20 + (150/5)

printf("Value of a + (b * c) / d is : %d\n" , e );

return 0;

}

When you compile and execute the above program, it produces the following result:

Value of (a + b) * c / d is : 90

Value of ((a + b) * c) / d is : 90

Value of (a + b) * (c / d) is : 90

Value of a + (b * c) / d is : 50

Decision Making Statements:-

Decision-making structures require that the programmer specifies one or more conditions to be evaluated or tested by the program, along with a statement or statements to be executed if the condition is determined to be true, and optionally, other statements to be executed if the condition is determined to be false.

Shown below is the general form of a typical decision-making structure found in most of the programming languages:

C programming language assumes any non-zero and non-null values as true, and if it is either zero or null, then it is assumed as false value.

C programming language provides the following types of decision-making statements.

if Statement:-

An if statement consists of a Boolean expression followed by one or more statements.

Syntax

The syntax of an ‘if’ statement in C programming language is:

if(boolean_expression)

{

/* statement(s) will execute if the boolean expression is true */

}

If the Boolean expression evaluates to true, then the block of code inside the ‘if’ statement will be executed. If the Boolean expression evaluates to false, then the first set of code after the end of the ‘if’ statement (after the closing curly brace) will be executed.

C programming language assumes any non-zero and non-null values as true and if it is either zero or null, then it is assumed as false value.

Flow Diagram

Example

#include <stdio.h>

int main ()

{

/* local variable definition */

int a = 10;

/* check the boolean condition using if statement */

if( a < 20 )

{

/* if condition is true then print the following */

printf("a is less than 20\n" );

}

printf("value of a is : %d\n", a);

return 0;

}

When the above code is compiled and executed, it produces the following result:

a is less than 20;

value of a is : 10

if..else Statement:-

An if statement can be followed by an optional else statement, which executes when the Boolean expression is false.

Syntax

The syntax of an if...else statement in C programming language is:

if(boolean_expression)

{

/* statement(s) will execute if the boolean expression is true */

}

else

{

/* statement(s) will execute if the boolean expression is false */

}

If the Boolean expression evaluates to true, then the if block will be executed, otherwise, the else block will be executed.

C programming language assumes any non-zero and non-null values as true, and if it is either zero or null, then it is assumed as false value.

Flow Diagram

Example

#include <stdio.h>

int main ()

{

/* local variable definition */

int a = 100;

/* check the boolean condition */

if( a < 20 )

{

/* if condition is true then print the following */

printf("a is less than 20\n" );

}

else

{

/* if condition is false then print the following */

printf("a is not less than 20\n" );

}

printf("value of a is : %d\n", a);

return 0;

}

When the above code is compiled and executed, it produces the following result:

a is not less than 20;

value of a is : 100

if...else if...else Statement:-

An if statement can be followed by an optional else if...else statement, which is very useful to test various conditions using single if...else if statement.

When using if…else if…else statements, there are few points to keep in mind:

• An if can have zero or one else's and it must come after any else if's.
• An if can have zero to many else if's and they must come before the else.
• Once an else if succeeds, none of the remaining else if's or else's will be tested.

Syntax

The syntax of an if...else if...else statement in C programming language is:

if(boolean_expression 1)

{

/* Executes when the boolean expression 1 is true */

}

else if( boolean_expression 2)

{

/* Executes when the boolean expression 2 is true */

}

else if( boolean_expression 3)

{

/* Executes when the boolean expression 3 is true */

}

else

{

/* executes when the none of the above condition is true */

}

Example

#include <stdio.h>

int main ()

{

/* local variable definition */

int a = 100;

/* check the boolean condition */

if( a == 10 )

{

/* if condition is true then print the following */

printf("Value of a is 10\n" );

}

else if( a == 20 )

{

/* if else if condition is true */

printf("Value of a is 20\n" );

}

else if( a == 30 )

{

/* if else if condition is true */

printf("Value of a is 30\n" );

}

else

{

/* if none of the conditions is true */

printf("None of the values is matching\n" );

}

printf("Exact value of a is: %d\n", a );

return 0;

}

When the above code is compiled and executed, it produces the following result:

None of the values is matching

Exact value of a is: 100

Nested if Statement:-

It is always legal in C programming to nest if-else statements, which means you can use one if or else if statement inside another if or else if statement(s).

Syntax

The syntax for a nested if statement is as follows:

if( boolean_expression 1)

{

/* Executes when the boolean expression 1 is true */

if(boolean_expression 2)

{

/* Executes when the boolean expression 2 is true */

}

}

You can nest else if...else in the similar way as you have nested if statements.

Example

#include <stdio.h>

int main ()

{

/* local variable definition */

int a = 100;

int b = 200;

/* check the boolean condition */

if( a == 100 )

{

/* if condition is true then check the following */

if( b == 200 )

{

/* if condition is true then print the following */

printf("Value of a is 100 and b is 200\n" );

}

}

printf("Exact value of a is : %d\n", a );

printf("Exact value of b is : %d\n", b );

return 0;

}

When the above code is compiled and executed, it produces the following result:

Value of a is 100 and b is 200

Exact value of a is : 100

Exact value of b is : 200

Switch Statement:-

A switch statement allows a variable to be tested for equality against a list of values. Each value is called a case, and the variable being switched on is checked for each switch case.

Syntax

The syntax for a switch statement in C programming language is as follows:

switch(expression){

case constant-expression :

statement(s);

break; /* optional */

case constant-expression :

statement(s);

break; /* optional */

/* you can have any number of case statements */

default : /* Optional */

statement(s);

}

The following rules apply to a switch statement:

• The expression used in a switch statement must have an integral or enumerated type, or be of a class type in which the class has a single conversion function to an integral or enumerated type.
• You can have any number of case statements within a switch. Each case is followed by the value to be compared to and a colon.
• The constant-expression for a case must be the same data type as the variable in the switch, and it must be a constant or a literal.
• When the variable being switched on is equal to a case, the statements following that case will execute until a break statement is reached.
• When a break statement is reached, the switch terminates, and the flow of control jumps to the next line following the switch statement.
• Not every case needs to contain a break. If no break appears, the flow of control will fall through to subsequent cases until a break is reached.
• A switch statement can have an optional default case, which must appear at the end of the switch. The default case can be used for performing a task when none of the cases is true. No break is needed in the default case.

Flow Diagram:-

Example

#include <stdio.h>

int main ()

{

/* local variable definition */

char grade = 'B';

switch(grade)

{

case 'A' :

printf("Excellent!\n" );

break;

case 'B' :

case 'C' :

printf("Well done\n" );

break;

case 'D' :

printf("You passed\n" );

break;

case 'F' :

printf("Better try again\n" );

break;

default :

printf("Invalid grade\n" );

}

printf("Your grade is %c\n", grade );

return 0;

}

When the above code is compiled and executed, it produces the following result:

Well done

Your grade is B

The ? : Operator:-

We have covered conditional operator ? : in the previous chapter which can be used to replace if...else statements. It has the following general form:

Exp1 ? Exp2 : Exp3;

Where Exp1, Exp2, and Exp3 are expressions. Notice the use and placement of the colon.

The value of a ? expression is determined like this:

1. Exp1 is evaluated. If it is true, then Exp2 is evaluated and becomes the value of the entire ? expression.

2. If Exp1 is false, then Exp3 is evaluated and its value becomes the value of the expression.

Loops:-

You may encounter situations when a block of code needs to be executed several number of times. In general, statements are executed sequentially: The first statement in a function is executed first, followed by the second, and so on.

Programming languages provide various control structures that allow for more complicated execution paths.

A loop statement allows us to execute a statement or group of statements multiple times. Given below is the general form of a loop statement in most of the programming languages:

C programming language provides the following types of loops to handle looping requirements.

while Loop:-

A while loop in C programming repeatedly executes a target statement as long as a given condition is true.

Syntax

The syntax of a while loop in C programming language is:

while(condition)

{

statement(s);

}

Here, statement(s) may be a single statement or a block of statements. The condition may be any expression, and true is any nonzero value. The loop iterates while the condition is true.

When the condition becomes false, the program control passes to the line immediately following the loop.

Flow Diagram

Here, the key point to note is that a while loop might not execute at all. When the condition is tested and the result is false, the loop body will be skipped and the first statement after the while loop will be executed.

Example

#include <stdio.h>

int main ()

{

/* local variable definition */

int a = 10;

/* while loop execution */

while( a < 20 )

{

printf("value of a: %d\n", a);

a++;

}

61

return 0;

}

When the above code is compiled and executed, it produces the following result:

value of a: 10

value of a: 11

value of a: 12

value of a: 13

value of a: 14

value of a: 15

value of a: 16

value of a: 17

value of a: 18

value of a: 19

for Loop:-

A for loop is a repetition control structure that allows you to efficiently write a loop that needs to execute a specific number of times.

Syntax

The syntax of a for loop in C programming language is:

for ( init; condition; increment )

{

statement(s);

}

Here is the flow of control in a ‘for’ loop:

1. The init step is executed first, and only once. This step allows you to declare and initialize any loop control variables. You are not required to put a statement here, as long as a semicolon appears.

2. Next, the condition is evaluated. If it is true, the body of the loop is executed. If it is false, the body of the loop does not execute and the flow of control jumps to the next statement just after the ‘for’ loop.

3. After the body of the ‘for’ loop executes, the flow of control jumps back up to the increment statement. This statement allows you to update any loop control variables. This statement can be left blank, as long as a semicolon appears after the condition.

4. The condition is now evaluated again. If it is true, the loop executes and the process repeats itself (body of loop, then increment step, and then again condition). After the condition becomes false, the ‘for’ loop terminates.

Flow Diagram

Example

#include <stdio.h>

int main ()

{

/* for loop execution */

for( int a = 10; a < 20; a = a + 1 )

{

printf("value of a: %d\n", a);

}

return 0;

}

When the above code is compiled and executed, it produces the following result:

value of a: 10

value of a: 11

value of a: 12

value of a: 13

value of a: 14

value of a: 15

value of a: 16

value of a: 17

value of a: 18

value of a: 19

do...while Loop:-

Unlike for and while loops, which test the loop condition at the top of the loop, the do...while loop in C programming checks its condition at the bottom of the loop.

A do...while loop is similar to a while loop, except the fact that it is guaranteed to execute at least one time.

Syntax

The syntax of a do...while loop in C programming language is:

do

{

statement(s);

}while( condition );

Notice that the conditional expression appears at the end of the loop, so the statement(s) in the loop executes once before the condition is tested.

If the condition is true, the flow of control jumps back up to do, and the statement(s) in the loop executes again. This process repeats until the given condition becomes false.

Flow Diagram

Example

#include <stdio.h>

int main ()

{

/* local variable definition */

int a = 10;

/* do loop execution */

do

{

printf("value of a: %d\n", a);

a = a + 1;

}while( a < 20 );

return 0;

}

When the above code is compiled and executed, it produces the following result:

value of a: 10

value of a: 11

value of a: 12

value of a: 13

value of a: 14

value of a: 15

value of a: 16

value of a: 17

value of a: 18

value of a: 19

Nested Loops:-

C programming allows to use one loop inside another loop. The following section shows a few examples to illustrate the concept.

Syntax

The syntax for a nested for loop statement in C is as follows:

for ( init; condition; increment )

{

for ( init; condition; increment )

{

statement(s);

}

statement(s);

}

The syntax for a nested while loop statement in C programming language is as follows:

while(condition)

{

while(condition)

{

statement(s);

}

statement(s);

}

The syntax for a nested do...while loop statement in C programming language is as follows:

do

{

statement(s);

do

{

statement(s);

}while( condition );

}while( condition );

A final note on loop nesting is that you can put any type of loop inside any other type of loop. For example, a ‘for’ loop can be inside a ‘while’ loop or vice versa.

Example

The following program uses a nested for loop to find the prime numbers from 2 to 100:

#include <stdio.h>

int main ()

{

/* local variable definition */

int i, j;

for(i=2; i<100; i++) {

for(j=2; j <= (i/j); j++)

if(!(i%j)) break; // if factor found, not prime

if(j > (i/j)) printf("%d is prime\n", i);

}

return 0;

}

When the above code is compiled and executed, it produces the following result:

2 is prime

3 is prime

5 is prime

7 is prime

11 is prime

13 is prime

17 is prime

19 is prime

23 is prime

29 is prime

31 is prime

37 is prime

41 is prime

43 is prime

47 is prime

53 is prime

59 is prime

61 is prime

67 is prime

71 is prime

73 is prime

79 is prime

83 is prime

89 is prime

97 is prime

Loop Control Statements:-

Loop control statements change execution from its normal sequence. When execution leaves a scope, all automatic objects that were created in that scope are destroyed.

C supports the following control statements.

break Statement:-

The break statement in C programming has the following two usages:

• When a break statement is encountered inside a loop, the loop is immediately terminated and the program control resumes at the next statement following the loop.
• It can be used to terminate a case in the switch statement (covered in the next chapter).

If you are using nested loops, the break statement will stop the execution of the innermost loop and start executing the next line of code after the block.

Syntax

The syntax for a break statement in C is as follows:

break;

Flow Diagram

Example

#include <stdio.h>

int main ()

{

/* local variable definition */

int a = 10;

/* while loop execution */

while( a < 20 )

{

printf("value of a: %d\n", a);

a++;

if( a > 15)

{

/* terminate the loop using break statement */

break;

}

}

return 0;

}

When the above code is compiled and executed, it produces the following result:

value of a: 10

value of a: 11

value of a: 12

value of a: 13

value of a: 14

value of a: 15

continue Statement:-

The continue statement in C programming works somewhat like the break statement. Instead of forcing termination, it forces the next iteration of the loop to take place, skipping any code in between.

For the for loop, continue statement causes the conditional test and increment portions of the loop to execute. For the while and do...while loops, continue statement causes the program control to pass to the conditional tests.

Syntax

The syntax for a continue statement in C is as follows:

continue;

Flow Diagram

Example

#include <stdio.h>

int main ()

{

/* local variable definition */

int a = 10;

/* do loop execution */

do

{

if( a == 15)

{

/* skip the iteration */

a = a + 1;

continue;

}

printf("value of a: %d\n", a);

a++;

}while( a < 20 );

return 0;

}

When the above code is compiled and executed, it produces the following result:

value of a: 10

value of a: 11

value of a: 12

value of a: 13

value of a: 14

value of a: 16

value of a: 17

value of a: 18

value of a: 19

goto Statement:-

A goto statement in C programming provides an unconditional jump from the ‘goto’ to a labeled statement in the same function.

NOTE: Use of goto statement is highly discouraged in any programming language because it makes difficult to trace the control flow of a program, making the program hard to understand and hard to modify. Any program that uses a goto can be rewritten to avoid them.

Syntax

The syntax for a goto statement in C is as follows:

goto label;

..

.

label: statement;

Here label can be any plain text except C keyword and it can be set anywhere in the C program above or below to goto statement.

Flow Diagram

Example

#include <stdio.h>

int main ()

{

/* local variable definition */

int a = 10;

/* do loop execution */

LOOP:do

{

if( a == 15)

{

/* skip the iteration */

a = a + 1;

goto LOOP;

}

printf("value of a: %d\n", a);

a++;

}while( a < 20 );

return 0;

}

When the above code is compiled and executed, it produces the following result:

value of a: 10

value of a: 11

value of a: 12

value of a: 13

value of a: 14

value of a: 16

value of a: 17

value of a: 18

value of a: 19

The infinite Loop:-

A loop becomes an infinite loop if a condition never becomes false. The for loop is traditionally used for this purpose. Since none of the three expressions that form the ‘for’ loop are required, you can make an endless loop by leaving the conditional expression empty.

#include <stdio.h>

int main ()

{

for( ; ; )

{

printf("This loop will run forever.\n");

}

return 0;

}

When the conditional expression is absent, it is assumed to be true. You may have an initialization and increment expression, but C programmers more commonly use the for(;;) construct to signify an infinite loop.

NOTE: You can terminate an infinite loop by pressing Ctrl + C keys.

Functions:-

A function is a group of statements that together perform a task. Every C program has at least one function, which is main(), and all the most trivial programs can define additional functions.

You can divide up your code into separate functions. How you divide up your code among different functions is up to you, but logically the division is such that each function performs a specific task.

A function declaration tells the compiler about a function's name, return type, and parameters. A function definition provides the actual body of the function.

The C standard library provides numerous built-in functions that your program can call. For example, strcat() to concatenate two strings, memcpy() to copy one memory location to another location, and many more functions.

A function can also be referred as a method or a sub-routine or a procedure, etc.

Defining a Function:-

The general form of a function definition in C programming language is as follows:

return_type function_name( parameter list )

{

body of the function

}

A function definition in C programming consists of a function header and a function body. Here are all the parts of a function:

• Return Type: A function may return a value. The return_type is the data type of the value the function returns. Some functions perform the desired operations without returning a value. In this case, the return_type is the keyword void.
• Function Name: This is the actual name of the function. The function name and the parameter list together constitute the function signature.
• Parameters: A parameter is like a placeholder. When a function is invoked, you pass a value to the parameter. This value is referred to as actual parameter or argument. The parameter list refers to the type, order, and number of the parameters of a function. Parameters are optional; that is, a function may contain no parameters.
• Function Body: The function body contains a collection of statements that define what the function does.

Example

Given below is the source code for a function called max(). This function takes two parameters num1 and num2 and returns the maximum value between the two:

/* function returning the max between two numbers */

int max(int num1, int num2)

{

/* local variable declaration */

int result;

if (num1 > num2)

result = num1;

else

result = num2;

return result;

}

Function Declaration:-

A function declaration tells the compiler about a function name and how to call the function. The actual body of the function can be defined separately.

A function declaration has the following parts:

return_type function_name( parameter list );

For the above defined function max(),the function declaration is as follows:

int max(int num1, int num2);

Parameter names are not important in function declaration, only their type is required, so the following is also a valid declaration:

int max(int, int);

Function declaration is required when you define a function in one source file and you call that function in another file. In such case, you should declare the function at the top of the file calling the function.

Calling a Function:-

While creating a C function, you give a definition of what the function has to do. To use a function, you will have to call that function to perform the defined task.

When a program calls a function, the program control is transferred to the called function. A called function performs a defined task and when its return statement is executed or when its function-ending closing brace is reached, it returns the program control back to the main program.

To call a function, you simply need to pass the required parameters along with the function name, and if the function returns a value, then you can store the returned value. 

For example:

#include <stdio.h>

/* function declaration */

int max(int num1, int num2);

int main ()

{

/* local variable definition */

int a = 100;

int b = 200;

int ret;

/* calling a function to get max value */

ret = max(a, b);

printf( "Max value is : %d\n", ret );

return 0;

}

/* function returning the max between two numbers */

int max(int num1, int num2)

{

/* local variable declaration */

int result;

if (num1 > num2)

result = num1;

else

result = num2;

return result;

}

We have kept max()along with main() and compiled the source code. While running the final executable, it would produce the following result:

Max value is : 200

Function Arguments:-

If a function is to use arguments, it must declare variables that accept the values of the arguments. These variables are called the formal parameters of the function.

Formal parameters behave like other local variables inside the function and are created upon entry into the function and destroyed upon exit.

While calling a function, there are two ways in which arguments can be passed to a function:

Call by value:-

The call by value method of passing arguments to a function copies the actual value of an argument into the formal parameter of the function. In this case, changes made to the parameter inside the function have no effect on the argument.

By default, C programming uses call by value to pass arguments. In general, it means the code within a function cannot alter the arguments used to call the function. Consider the function swap() definition as follows.

/* function definition to swap the values */

void swap(int x, int y)

{

int temp;

temp = x; /* save the value of x */

x = y; /* put y into x */

y = temp; /* put temp into y */

return;

}

Now, let us call the function swap() by passing actual values as in the following example:

#include <stdio.h>

/* function declaration */

void swap(int x, int y);

int main ()

{

/* local variable definition */

int a = 100;

int b = 200;

printf("Before swap, value of a : %d\n", a );

printf("Before swap, value of b : %d\n", b );

/* calling a function to swap the values */

swap(a, b);

printf("After swap, value of a : %d\n", a );

printf("After swap, value of b : %d\n", b );

return 0;

}

Let us put the above code in a single C file, compile and execute it, it will produce the following result:

Before swap, value of a :100

Before swap, value of b :200

After swap, value of a :100

After swap, value of b :200

It shows that there are no changes in the values, though they had been changed inside the function.

Call by Reference:-

The call by reference method of passing arguments to a function copies the address of an argument into the formal parameter. Inside the function, the address is used to access the actual argument used in the call. It means the changes made to the parameter affect the passed argument.

To pass a value by reference, argument pointers are passed to the functions just like any other value. So accordingly, you need to declare the function parameters as pointer types as in the following function swap(), which exchanges the values of the two integer variables pointed to, by their arguments.

/* function definition to swap the values */

void swap(int *x, int *y)

{

int temp;

temp = *x; /* save the value at address x */

*x = *y; /* put y into x */

*y = temp; /* put temp into y */

return;

}

Let us now call the function swap() by passing values by reference as in the following example:

#include <stdio.h>

/* function declaration */

void swap(int *x, int *y);

int main ()

{

/* local variable definition */

int a = 100;

int b = 200;

printf("Before swap, value of a : %d\n", a );

printf("Before swap, value of b : %d\n", b );

/* calling a function to swap the values.

* &a indicates pointer to a i.e. address of variable a and

* &b indicates pointer to b i.e. address of variable b.

*/

swap(&a, &b);

printf("After swap, value of a : %d\n", a );

printf("After swap, value of b : %d\n", b );

return 0;

}

Let us put the above code in a single C file, compile and execute it, to produce the following result:

Before swap, value of a :100

Before swap, value of b :200

After swap, value of a :200

After swap, value of b :100

It shows that the change has reflected outside the function as well, unlike call by value where the changes do not reflect outside the function.

By default, C uses call by value to pass arguments. In general, it means the code within a function cannot alter the arguments used to call the function.

Scope Rules:-

A scope in any programming is a region of the program where a defined variable can have its existence and beyond that variable it cannot be accessed. There are three places where variables can be declared in C programming language:

• Inside a function or a block which is called local variables,
• Outside of all functions which is called global variables.
• In the definition of function parameters which are called formal parameters.

Let us understand what are local and global variables, and formal parameters.

Local Variables:-

Variables that are declared inside a function or block are called local variables. They can be used only by statements that are inside that function or block of code. Local variables are not known to functions outside their own. The following example shows how local variables are used. Here all the variables a, b, and c are local to main() function.

#include <stdio.h>

int main ()

{

/* local variable declaration */

int a, b;

int c;

/* actual initialization */

a = 10;

b = 20;

c = a + b;

printf ("value of a = %d, b = %d and c = %d\n", a, b, c);

return 0;

}

Global Variables:-

Global variables are defined outside a function, usually on top of the program. Global variables hold their values throughout the lifetime of your program and they can be accessed inside any of the functions defined for the program.

A global variable can be accessed by any function. That is, a global variable is available for use throughout your entire program after its declaration. The following program shows how global variables are used in a program.

#include <stdio.h>

/* global variable declaration */

int g;

int main ()

{

/* local variable declaration */

int a, b;

/* actual initialization */

a = 10;

b = 20;

g = a + b;

printf ("value of a = %d, b = %d and g = %d\n", a, b, g);

return 0;

}

A program can have same name for local and global variables but the value of local variable inside a function will take preference. Here is an example:

#include <stdio.h>

/* global variable declaration */

int g = 20;

int main ()

{

/* local variable declaration */

int g = 10;

printf ("value of g = %d\n", g);

return 0;

}

When the above code is compiled and executed, it produces the following result:

value of g = 10

Formal Parameters:-

Formal parameters are treated as local variables with-in a function and they take precedence over global variables. Following is an example:

#include <stdio.h>

/* global variable declaration */

int a = 20;

int main ()

{

/* local variable declaration in main function */

int a = 10;

int b = 20;

int c = 0;

printf ("value of a in main() = %d\n", a);

c = sum( a, b);

printf ("value of c in main() = %d\n", c);

return 0;

}

/* function to add two integers */

int sum(int a, int b)

{

printf ("value of a in sum() = %d\n", a);

printf ("value of b in sum() = %d\n", b);

return a + b;

}

When the above code is compiled and executed, it produces the following result:

value of a in main() = 10

value of a in sum() = 10

value of b in sum() = 20

value of c in main() = 30

Initializing Local and Global Variables:-

When a local variable is defined, it is not initialized by the system, you must initialize it yourself. Global variables are initialized automatically by the system when you define them, as follows:

It is a good programming practice to initialize variables properly, otherwise your program may produce unexpected results, because uninitialized variables will take some garbage value already available at their memory location.

Arrays:-

Arrays a kind of data structure that can store a fixed-size sequential collection of elements of the same type. An array is used to store a collection of data, but it is often more useful to think of an array as a collection of variables of the same type.

Instead of declaring individual variables, such as number0, number1, ..., and number99, you declare one array variable such as numbers and use numbers[0], numbers[1], and ..., numbers[99] to represent individual variables. A specific element in an array is accessed by an index.

All arrays consist of contiguous memory locations. The lowest address corresponds to the first element and the highest address to the last element.

Declaring Arrays:-

To declare an array in C, a programmer specifies the type of the elements and the number of elements required by an array as follows:

type arrayName [ arraySize ];

This is called a single-dimensional array. The arraySize must be an integer constant greater than zero and type can be any valid C data type. For example, to declare a 10-element array called balance of type double, use this statement:

double balance[10];

Here, balance is a variable array which is sufficient to hold up to 10 double numbers.

Initializing Arrays:-

You can initialize an array in C either one by one or using a single statement as follows:

double balance[5] = {1000.0, 2.0, 3.4, 7.0, 50.0};

The number of values between braces { } cannot be larger than the number of elements that we declare for the array between square brackets [ ].

If you omit the size of the array, an array just big enough to hold the initialization is created. Therefore, if you write:

double balance[] = {1000.0, 2.0, 3.4, 7.0, 50.0};

You will create exactly the same array as you did in the previous example. Following is an 

example to assign a single element of the array:

balance[4] = 50.0;

The above statement assigns the 5th element in the array with a value of 50.0. All arrays have 0 as the index of their first element which is also called the base index and the last index of an array will be total size of the array minus 1. Shown below is the pictorial representation of the array we discussed above:

Accessing Array Elements:-

An element is accessed by indexing the array name. This is done by placing the index of the element within square brackets after the name of the array. 

For example:

double salary = balance[9];

The above statement will take the 10th element from the array and assign the value to salary variable. The following example shows how to use all the three above-mentioned concepts viz. declaration, assignment, and accessing arrays:

#include <stdio.h>

int main ()

{

int n[ 10 ]; /* n is an array of 10 integers */

int i,j;

/* initialize elements of array n to 0 */

for ( i = 0; i < 10; i++ )

{

n[ i ] = i + 100; /* set element at location i to i + 100 */

}

/* output each array element's value */

for (j = 0; j < 10; j++ )

{

printf("Element[%d] = %d\n", j, n[j] );

}

return 0;

}

When the above code is compiled and executed, it produces the following result:

Element[0] = 100

Element[1] = 101

Element[2] = 102

Element[3] = 103

Element[4] = 104

Element[5] = 105

Element[6] = 106

Element[7] = 107

Element[8] = 108

Element[9] = 109

Arrays in Detail:-

Arrays are important to C and should need a lot more attention. The following important concepts related to array should be clear to a C programmer:

Multidimensional Array:-

C programming language allows multidimensional arrays. Here is the general form of a multidimensional array declaration:

type name[size1][size2]...[sizeN];

For example, the following declaration creates a three-dimensional integer array:

int threedim[5][10][4];

Two-dimensional Array:-

The simplest form of multidimensional array is the two-dimensional array. A two-dimensional array is, in essence, a list of one-dimensional arrays. To declare a two-dimensional integer array of size [x][y], you would write something as follows:

type arrayName [ x ][ y ];

Where type can be any valid C data type and arrayName will be a valid C identifier. A two-dimensional array can be considered as a table which will have x number of rows and y number of columns. A two-dimensional array a, which contains three rows and four columns can be shown as follows:

Thus, every element in the array a is identified by an element name of the form a[ i ][ j ], where ‘a’ is the name of the array, and ‘i' and ‘j’ are the subscripts that uniquely identify each element in ‘a’.

Initializing Two-Dimensional Arrays:-

Multidimensional arrays may be initialized by specifying bracketed values for each row. Following is an array with 3 rows and each row has 4 columns.

int a[3][4] = {+++

{0, 1, 2, 3} , /* initializers for row indexed by 0 */

{4, 5, 6, 7} , /* initializers for row indexed by 1 */

{8, 9, 10, 11} /* initializers for row indexed by 2 */

};

The nested braces, which indicate the intended row, are optional. The following initialization is equivalent to the previous example:

int a[3][4] = {0,1,2,3,4,5,6,7,8,9,10,11};

Accessing Two-Dimensional Array Elements:-

An element in a two-dimensional array is accessed by using the subscripts, i.e., row index and column index of the array. For example:

int val = a[2][3];

The above statement will take the 4th element from the 3rd row of the array. You can verify it in the above figure. Let us check the following program where we have used a nested loop to handle a two-dimensional array:

#include <stdio.h>

int main ()

{

/* an array with 5 rows and 2 columns*/

int a[5][2] = { {0,0}, {1,2}, {2,4}, {3,6},{4,8}};

int i, j;

/* output each array element's value */

for ( i = 0; i < 5; i++ )

{

for ( j = 0; j < 2; j++ )

{

printf("a[%d][%d] = %d\n", i,j, a[i][j] );

}

}

return 0;

}

When the above code is compiled and executed, it produces the following result:

a[0][0]: 0

a[0][1]: 0

a[1][0]: 1

a[1][1]: 2

a[2][0]: 2

a[2][1]: 4

a[3][0]: 3

a[3][1]: 6

a[4][0]: 4

a[4][1]: 8

As explained above, you can have arrays with any number of dimensions, although it is likely that most of the arrays you create will be of one or two dimensions.

Passing Array to function:-

If you want to pass a single-dimension array as an argument in a function, you would have to declare a formal parameter in one of following three ways and all three declaration methods produce similar results because each tells the compiler that an integer pointer is going to be received. Similarly, you can pass multi-dimensional arrays as formal parameters.

Way-1

Formal parameters as a pointer:

void myFunction(int *param)

{

.

.

.

}

Way-2

Formal parameters as a sized array:

void myFunction(int param[10])

{

.

.

.

}

Way-3

Formal parameters as an unsized array:

void myFunction(int param[])

{

.

.

.

}

Example

Now, consider the following function, which takes an array as an argument along with another argument and based on the passed arguments, it returns the average of the numbers passed through the array as follows:

double getAverage(int arr[], int size)

{

int i;

double avg;

double sum;

for (i = 0; i < size; ++i)

{

sum += arr[i];

}

avg = sum / size;

return avg;

}

Now, let us call the above function as follows:

#include <stdio.h>

/* function declaration */

double getAverage(int arr[], int size);

int main ()

{

/* an int array with 5 elements */

int balance[5] = {1000, 2, 3, 17, 50};

double avg;

/* pass pointer to the array as an argument */

avg = getAverage( balance, 5 ) ;

/* output the returned value */

printf( "Average value is: %f ", avg );

return 0;

}

When the above code is compiled together and executed, it produces the following result:

Average value is: 214.400000

As you can see, the length of the array doesn't matter as far as the function is concerned because C performs no bounds checking for formal parameters.

Return Array from a Function:-

C programming does not allow to return an entire array as an argument to a function. However, you can return a pointer to an array by specifying the array's name without an index.

If you want to return a single-dimension array from a function, you would have to declare a function returning a pointer as in the following example:

int * myFunction()

{

.

.

.

}

Second point to remember is that C does not advocate to return the address of a local variable to outside of the function, so you would have to define the local variable as static variable.

Now, consider the following function which will generate 10 random numbers and return them using an array and call this function as follows:

#include <stdio.h>

/* function to generate and return random numbers */

int * getRandom( )

{

static int r[10];

int i;

/* set the seed */

srand( (unsigned)time( NULL ) );

for ( i = 0; i < 10; ++i)

{

r[i] = rand();

printf( "r[%d] = %d\n", i, r[i]);

}

return r;

}

/* main function to call above defined function */

int main ()

{

/* a pointer to an int */

int *p;

int i;

p = getRandom();

for ( i = 0; i < 10; i++ )

{

printf( "*(p + %d) : %d\n", i, *(p + i));

}

return 0;

}

When the above code is compiled together and executed, it produces the following result:

r[0] = 313959809

r[1] = 1759055877

r[2] = 1113101911

r[3] = 2133832223

r[4] = 2073354073

r[5] = 167288147

r[6] = 1827471542

r[7] = 834791014

r[8] = 1901409888

r[9] = 1990469526

*(p + 0) : 313959809

*(p + 1) : 1759055877

*(p + 2) : 1113101911

*(p + 3) : 2133832223

*(p + 4) : 2073354073

*(p + 5) : 167288147

*(p + 6) : 1827471542

*(p + 7) : 834791014

*(p + 8) : 1901409888

*(p + 9) : 1990469526

Pointer to an array:-

It is most likely that you would not understand this section until you are through with the chapter ‘Pointers’.

Assuming you have some understanding of pointers in C, let us start: An array name is a constant pointer to the first element of the array. Therefore, in the declaration:

double balance[50];

balance is a pointer to &balance[0], which is the address of the first element of the array balance. Thus, the following program fragment assigns p as the address of the first element of balance:

double *p;

double balance[10];

p = balance;

It is legal to use array names as constant pointers, and vice versa. Therefore, *(balance + 4) is a legitimate way of accessing the data at balance[4].

Once you store the address of the first element in ‘p’, you can access the array elements using *p, *(p+1), *(p+2), and so on. Given below is the example to show all the concepts discussed above:

#include <stdio.h>

int main ()

{

/* an array with 5 elements */

double balance[5] = {1000.0, 2.0, 3.4, 17.0, 50.0};

double *p;

int i;

p = balance;

/* output each array element's value */

printf( "Array values using pointer\n");

for ( i = 0; i < 5; i++ )

{

printf("*(p + %d) : %f\n", i, *(p + i) );

}

printf( "Array values using balance as address\n");

for ( i = 0; i < 5; i++ )

{

printf("*(balance + %d) : %f\n", i, *(balance + i) );

}

return 0;

}

When the above code is compiled and executed, it produces the following result:

Array values using pointer

*(p + 0) : 1000.000000

*(p + 1) : 2.000000

*(p + 2) : 3.400000

*(p + 3) : 17.000000

*(p + 4) : 50.000000

Array values using balance as address

*(balance + 0) : 1000.000000

*(balance + 1) : 2.000000

*(balance + 2) : 3.400000

*(balance + 3) : 17.000000

*(balance + 4) : 50.000000

In the above example, p is a pointer to double, which means it can store the address of a variable of double type. Once we have the address in p, *p will give us the value available at the address stored in p, as we have shown in the above example.

Pointers:-

Pointers in C are easy and fun to learn. Some C programming tasks are performed more easily with pointers, and other tasks, such as dynamic memory allocation, cannot be performed without using pointers. So it becomes necessary to learn pointers to become a perfect C programmer. Let's start learning them in simple and easy steps.

As you know, every variable is a memory location and every memory location has its address defined which can be accessed using ampersand (&) operator, which denotes an address in memory. Consider the following example, which prints the address of the variables defined:

#include <stdio.h>

int main ()

{

int var1;

char var2[10];

printf("Address of var1 variable: %x\n", &var1 );

printf("Address of var2 variable: %x\n", &var2 );

return 0;

}

When the above code is compiled and executed, it produces the following result:

Address of var1 variable: bff5a400

Address of var2 variable: bff5a3f6

What are Pointers:-

A pointer is a variable whose value is the address of another variable, i.e., direct address of the memory location. Like any variable or constant, you must declare a pointer before using it to store any variable address. The general form of a pointer variable declaration is:

type *var-name;

Here, type is the pointer's base type; it must be a valid C data type and var-name is the name of the pointer variable. The asterisk * used to declare a pointer is the same asterisk used for multiplication. However, in this statement, the asterisk is being used to designate a variable as a pointer. Take a look at some of the valid pointer declarations:

int            *ip;       /* pointer to an integer */

double    *dp;     /* pointer to a double */

float         *fp;    /* pointer to a float */

char         *ch   /* pointer to a character */

The actual data type of the value of all pointers, whether integer, float, character, or otherwise, is the same, a long hexadecimal number that represents a memory address. The only difference between pointers of different data types is the data type of the variable or constant that the pointer points to.

How to Use Pointers:-

There are a few important operations, which we will do with the help of pointers very frequently. (a) We define a pointer variable, (b) assign the address of a variable to a pointer, and (c) finally access the value at the address available in the pointer variable. This is done by using unary operator * that returns the value of the variable located at the address specified by its operand. The following example makes use of these operations:

#include <stdio.h>

int main ()

{

int var = 20;                                      /* actual variable declaration */

int *ip;                                              /* pointer variable declaration */

ip = &var;                                       /* store address of var in pointer variable*/

printf("Address of var variable: %x\n", &var );

/* address stored in pointer variable */

printf("Address stored in ip variable: %x\n", ip );

/* access the value using the pointer */

printf("Value of *ip variable: %d\n", *ip );

return 0;

}

When the above code is compiled and executed, it produces the following result:

Address of var variable: bffd8b3c

Address stored in ip variable: bffd8b3c

Value of *ip variable: 20

Null Pointers:-

It is always a good practice to assign a NULL value to a pointer variable in case you do not have an exact address to be assigned. This is done at the time of variable declaration. A pointer that is assigned NULL is called a null pointer.

The NULL pointer is a constant with a value of zero defined in several standard libraries. Consider the following program:

#include <stdio.h>

int main ()

{

int *ptr = NULL;

printf("The value of ptr is : %x\n", ptr );

return 0;

}

When the above code is compiled and executed, it produces the following result:

The value of ptr is 0

In most of the operating systems, programs are not permitted to access memory at address 0 because that memory is reserved by the operating system. However, the memory address 0 has special significance; it signals that the pointer is not intended to point to an accessible memory location. But by convention, if a pointer contains the null (zero) value, it is assumed to point to nothing.

To check for a null pointer, you can use an ‘if’ statement as follows:

if(ptr) /* succeeds if p is not null */

if(!ptr) /* succeeds if p is null */

Pointers in Detail:-

Pointers have many but easy concepts and they are very important to C programming. The following important pointer concepts should be clear to any C programmer:

Pointer Arithmetic:-

A pointer in C is an address, which is a numeric value. Therefore, you can perform arithmetic operations on a pointer just as you can on a numeric value. There are four arithmetic operators that can be used on pointers: ++, --, +, and -

To understand pointer arithmetic, let us consider that ptr is an integer pointer which points to the address 1000. Assuming 32-bit integers, let us perform the following arithmetic operation on the pointer:

ptr++

After the above operation, the ptr will point to the location 1004 because each time ptr is incremented, it will point to the next integer location which is 4 bytes next to the current location. This operation will move the pointer to the next memory location without impacting the actual value at the memory location. If ptr points to a character whose address is 1000, then the above operation will point to the location 1001 because the next character will be available at 1001.

Incrementing a Pointer:-

We prefer using a pointer in our program instead of an array because the variable pointer can be incremented, unlike the array name which cannot be incremented because it is a constant pointer. The following program increments the variable pointer to access each succeeding element of the array:

#include <stdio.h>

const int MAX = 3;

int main ()

{

int var[] = {10, 100, 200};

int i, *ptr;

/* let us have array address in pointer */

ptr = var;

for ( i = 0; i < MAX; i++)

{

printf("Address of var[%d] = %x\n", i, ptr );

printf("Value of var[%d] = %d\n", i, *ptr );

/* move to the next location */

ptr++;

}

return 0;

}

When the above code is compiled and executed, it produces the following result:

Address of var[0] = bf882b30

Value of var[0] = 10

Address of var[1] = bf882b34

Value of var[1] = 100

Address of var[2] = bf882b38

Value of var[2] = 200

Decrementing a Pointer:-

The same considerations apply to decrementing a pointer, which decreases its value by the number of bytes of its data type as shown below:

#include <stdio.h>

const int MAX = 3;

int main ()

{

int var[] = {10, 100, 200};

int i, *ptr;

/* let us have array address in pointer */

ptr = &var[MAX-1];

for ( i = MAX; i > 0; i--)

{

printf("Address of var[%d] = %x\n", i, ptr );

printf("Value of var[%d] = %d\n", i, *ptr );

/* move to the previous location */

ptr--;

}

return 0;

}

When the above code is compiled and executed, it produces the following result:

Address of var[3] = bfedbcd8

Value of var[3] = 200

Address of var[2] = bfedbcd4

Value of var[2] = 100

Address of var[1] = bfedbcd0

Value of var[1] = 10

Array of Pointers:-

Before we understand the concept of arrays of pointers, let us consider the following example, which uses an array of 3 integers:

#include <stdio.h>

const int MAX = 3;

int main ()

{

int var[] = {10, 100, 200};

int i;

for (i = 0; i < MAX; i++)

{

printf("Value of var[%d] = %d\n", i, var[i] );

}

return 0;

}

When the above code is compiled and executed, it produces the following result:

Value of var[0] = 10

Value of var[1] = 100

Value of var[2] = 200

There may be a situation when we want to maintain an array, which can store pointers to an int or char or any other data type available. Following is the declaration of an array of pointers to an integer:

int *ptr[MAX];

It declares ptr as an array of MAX integer pointers. Thus, each element in ptr holds a pointer to an int value. The following example uses three integers, which are stored in an array of pointers, as follows:

#include <stdio.h>

const int MAX = 3;

int main ()

{

int var[] = {10, 100, 200};

int i, *ptr[MAX];

for ( i = 0; i < MAX; i++)

{

ptr[i] = &var[i]; /* assign the address of integer. */

}

for ( i = 0; i < MAX; i++)

{

printf("Value of var[%d] = %d\n", i, *ptr[i] );

}

return 0;

}

When the above code is compiled and executed, it produces the following result:

Value of var[0] = 10

Value of var[1] = 100

Value of var[2] = 200

You can also use an array of pointers to character to store a list of strings as follows:

#include <stdio.h>

const int MAX = 4;

int main ()

{

char *names[] = {

"Zara Ali",

"Hina Ali",

"Nuha Ali",

"Sara Ali",

};

int i = 0;

for ( i = 0; i < MAX; i++)

{

printf("Value of names[%d] = %s\n", i, names[i] );

}

return 0;

}

When the above code is compiled and executed, it produces the following result:

Value of names[0] = Zara Ali

Value of names[1] = Hina Ali

Value of names[2] = Nuha Ali

Value of names[3] = Sara Ali

Pointer to Pointer:-

A pointer to a pointer is a form of multiple indirection, or a chain of pointers. Normally, a pointer contains the address of a variable. When we define a pointer to a pointer, the first pointer contains the address of the second pointer, which points to the location that contains the actual value as shown below.

A variable that is a pointer to a pointer must be declared as such. This is done by placing an additional asterisk in front of its name. For example, the following declaration declares a pointer to a pointer of type int:

int **var;

When a target value is indirectly pointed to by a pointer to a pointer, accessing that value requires that the asterisk operator be applied twice, as is shown below in the example:

#include <stdio.h>

int main ()

{

int var;

int *ptr;

int **pptr;

var = 3000;

/* take the address of var */

ptr = &var;

/* take the address of ptr using address of operator & */

pptr = &ptr;

/* take the value using pptr */

printf("Value of var = %d\n", var );

printf("Value available at *ptr = %d\n", *ptr );

printf("Value available at **pptr = %d\n", **pptr);

return 0;

}

When the above code is compiled and executed, it produces the following result:

Value of var = 3000

Value available at *ptr = 3000

Value available at **pptr = 3000

Passing Pointers to Functions:-

C programming allows passing a pointer to a function. To do so, simply declare the function parameter as a pointer type.

Following is a simple example where we pass an unsigned long pointer to a function and change the value inside the function which reflects back in the calling function:

#include <stdio.h>

#include <time.h>

void getSeconds(unsigned long *par);

int main ()

{

unsigned long sec;

getSeconds( &sec );

/* print the actual value */

printf("Number of seconds: %ld\n", sec );

return 0;

}

void getSeconds(unsigned long *par)

{

/* get the current number of seconds */

*par = time( NULL );

return;

}

When the above code is compiled and executed, it produces the following result:

Number of seconds :1294450468

The function, which can accept a pointer, can also accept an array as shown in the following example:

#include <stdio.h>

/* function declaration */

double getAverage(int *arr, int size);

int main ()

{

/* an int array with 5 elements */

int balance[5] = {1000, 2, 3, 17, 50};

double avg;

/* pass pointer to the array as an argument */

avg = getAverage( balance, 5 ) ;

/* output the returned value */

printf("Average value is: %f\n", avg );

return 0;

}

double getAverage(int *arr, int size)

{

int i, sum = 0;

double avg;

for (i = 0; i < size; ++i)

{

sum += arr[i];

}

avg = (double)sum / size;

return avg;

}

When the above code is compiled together and executed, it produces the following result:

Average value is: 214.40000

Return Pointer From Functions:-

We have seen in the last chapter how C programming allows to return an array from a function. Similarly, C also allows to return a pointer from a function. To do so, you would have to declare a function returning a pointer as in the following example:

int * myFunction()

{

.

.

.

}

Second point to remember is that, it is not a good idea to return the address of a local variable outside the function, so you would have to define the local variable as static variable.

Now, consider the following function which will generate 10 random numbers and return them using an array name which represents a pointer, i.e., address of first array element.

#include <stdio.h>

#include <time.h>

/* function to generate and retrun random numbers. */

int * getRandom( )

{

static int r[10];

int i;

/* set the seed */

srand( (unsigned)time( NULL ) );

for ( i = 0; i < 10; ++i)

{

r[i] = rand();

printf("%d\n", r[i] );

}

return r;

}

/* main function to call above defined function */

int main ()

{

/* a pointer to an int */

int *p;

int i;

p = getRandom();

for ( i = 0; i < 10; i++ )

{

printf("*(p + [%d]) : %d\n", i, *(p + i) );

}

return 0;

}

When the above code is compiled together and executed, it produces the following result:

1523198053

1187214107

1108300978

430494959

1421301276

930971084

123250484

106932140

1604461820

149169022

*(p + [0]) : 1523198053

*(p + [1]) : 1187214107

*(p + [2]) : 1108300978

*(p + [3]) : 430494959

*(p + [4]) : 1421301276

*(p + [5]) : 930971084

*(p + [6]) : 123250484

*(p + [7]) : 106932140

*(p + [8]) : 1604461820

*(p + [9]) : 149169022

Strings in C:-

Strings are actually one-dimensional array of characters terminated by a null character '\0'. Thus a null-terminated string contains the characters that comprise the string followed by a null.

The following declaration and initialization create a string consisting of the word "Hello". To hold the null character at the end of the array, the size of the character array containing the string is one more than the number of characters in the word "Hello."

char greeting[6] = {'H', 'e', 'l', 'l', 'o', '\0'};

If you follow the rule of array initialization, then you can write the above statement as follows:

char greeting[] = "Hello";

Following is the memory presentation of the above defined string in C/C++:

Actually, you do not place the null character at the end of a string constant. The C compiler automatically places the '\0' at the end of the string when it initializes the array. Let us try to print the above mentioned string:

#include <stdio.h>

int main ()

{

char greeting[6] = {'H', 'e', 'l', 'l', 'o', '\0'};

printf("Greeting message: %s\n", greeting );

return 0;

}

When the above code is compiled and executed, it produces the following result:

Greeting message: Hello

C supports a wide range of functions that manipulate null-terminated strings:

The following example uses some of the above-mentioned functions:

#include <stdio.h>

#include <string.h>

int main ()

{

char str1[12] = "Hello";

char str2[12] = "World";

char str3[12];

int len ;

/* copy str1 into str3 */

strcpy(str3, str1);

printf("strcpy( str3, str1) : %s\n", str3 );

/* concatenates str1 and str2 */

strcat( str1, str2);

printf("strcat( str1, str2): %s\n", str1 );

/* total lenghth of str1 after concatenation */

len = strlen(str1);

printf("strlen(str1) : %d\n", len );

return 0;

}

When the above code is compiled and executed, it produces the following result:

strcpy( str3, str1) : Hello

strcat( str1, str2): HelloWorld

strlen(str1) : 10

Structures in C:-

Arrays allow to define type of variables that can hold several data items of the same kind. Similarly, structure is another user-defined data type available in C that allows to combine data items of different kinds.

Structures are used to represent a record. Suppose you want to keep track of your books in a library. You might want to track the following attributes about each book:

• Title
• Author
• Subject
• Book ID

Defining a Structure:-

To define a structure, you must use the struct statement. The struct statement defines a new data type, with more than one member. The format of the struct statement is as follows:

struct [structure tag]

{

member definition;

member definition;

...

member definition;

} [one or more structure variables];

The structure tag is optional and each member definition is a normal variable definition, such as int i; or float f; or any other valid variable definition. At the end of the structure's definition, before the final semicolon, you can specify one or more structure variables but it is optional. Here is the way you would declare the Book structure:

struct Books

{

char title[50];

char author[50];

char subject[100];

int book_id;

} book;

Accessing Structure Members:-

To access any member of a structure, we use the member access operator (.). The member access operator is coded as a period between the structure variable name and the structure member that we wish to access. You would use the keyword struct to define variables of structure type. The following example shows how to use a structure in a program:

#include <stdio.h>

#include <string.h>

struct Books

{

char title[50];

char author[50];

char subject[100];

int book_id;

};

int main( )

{

struct Books Book1;                          /* Declare Book1 of type Book */

struct Books Book2;                        /* Declare Book2 of type Book */

/* book 1 specification */

strcpy( Book1.title, "C Programming");

strcpy( Book1.author, "Nuha Ali");

strcpy( Book1.subject, "C Programming Tutorial");

Book1.book_id = 6495407;

/* book 2 specification */

strcpy( Book2.title, "Telecom Billing");

strcpy( Book2.author, "Zara Ali");

strcpy( Book2.subject, "Telecom Billing Tutorial");

Book2.book_id = 6495700;

/* print Book1 info */

printf( "Book 1 title : %s\n", Book1.title);

printf( "Book 1 author : %s\n", Book1.author);

printf( "Book 1 subject : %s\n", Book1.subject);

printf( "Book 1 book_id : %d\n", Book1.book_id);

/* print Book2 info */

printf( "Book 2 title : %s\n", Book2.title);

printf( "Book 2 author : %s\n", Book2.author);

printf( "Book 2 subject : %s\n", Book2.subject);

printf( "Book 2 book_id : %d\n", Book2.book_id);

return 0;

}

When the above code is compiled and executed, it produces the following result:

Book 1 title : C Programming

Book 1 author : Nuha Ali

Book 1 subject : C Programming Tutorial

Book 1 book_id : 6495407

Book 2 title : Telecom Billing

Book 2 author : Zara Ali

Book 2 subject : Telecom Billing Tutorial

Book 2 book_id : 6495700

Structures as Function Arguments:-

You can pass a structure as a function argument in the same way as you pass any other variable or pointer.

#include <stdio.h>

#include <string.h>

struct Books

{

char title[50];

char author[50];

char subject[100];

int book_id;

};

/* function declaration */

void printBook( struct Books book );

int main( )

{

struct Books Book1;                      /* Declare Book1 of type Book */

struct Books Book2;                    /* Declare Book2 of type Book */

/* book 1 specification */

strcpy( Book1.title, "C Programming");

strcpy( Book1.author, "Nuha Ali");

strcpy( Book1.subject, "C Programming Tutorial");

Book1.book_id = 6495407;

/* book 2 specification */

strcpy( Book2.title, "Telecom Billing");

strcpy( Book2.author, "Zara Ali");

strcpy( Book2.subject, "Telecom Billing Tutorial");

Book2.book_id = 6495700;

/* print Book1 info */

printBook( Book1 );

/* Print Book2 info */

printBook( Book2 );

return 0;

}

void printBook( struct Books book )

{

printf( "Book title : %s\n", book.title);

printf( "Book author : %s\n", book.author);

printf( "Book subject : %s\n", book.subject);

printf( "Book book_id : %d\n", book.book_id);

}

When the above code is compiled and executed, it produces the following result:

Book title : C Programming

Book author : Nuha Ali

Book subject : C Programming Tutorial

Book book_id : 6495407

Book title : Telecom Billing

Book author : Zara Ali

Book subject : Telecom Billing Tutorial

Book book_id : 6495700

Pointers to Structures:-

You can define pointers to structures in the same way as you define pointer to any other variable:

struct Books *struct_pointer;

Now, you can store the address of a structure variable in the above-defined pointer variable. To find the address of a structure variable, place the ‘&’ operator before the structure's name as follows:

struct_pointer = &Book1;

To access the members of a structure using a pointer to that structure, you must use the -> operator as follows:

struct_pointer->title;

Let us rewrite the above example using structure pointer.

#include <stdio.h>

#include <string.h>

struct Books

{

char title[50];

char author[50];

char subject[100];

int book_id;

};

/* function declaration */

void printBook( struct Books *book );

int main( )

{

struct Books Book1;                             /* Declare Book1 of type Book */

struct Books Book2;                           /* Declare Book2 of type Book */

/* book 1 specification */

strcpy( Book1.title, "C Programming");

strcpy( Book1.author, "Nuha Ali");

strcpy( Book1.subject, "C Programming Tutorial");

Book1.book_id = 6495407;

/* book 2 specification */

strcpy( Book2.title, "Telecom Billing");

strcpy( Book2.author, "Zara Ali");

strcpy( Book2.subject, "Telecom Billing Tutorial");

Book2.book_id = 6495700;

/* print Book1 info by passing address of Book1 */

printBook( &Book1 );

/* print Book2 info by passing address of Book2 */

printBook( &Book2 );

return 0;

}

void printBook( struct Books *book )

{

printf( "Book title : %s\n", book->title);

printf( "Book author : %s\n", book->author);

printf( "Book subject : %s\n", book->subject);

printf( "Book book_id : %d\n", book->book_id);

}

When the above code is compiled and executed, it produces the following result:

Book title : C Programming

Book author : Nuha Ali

Book subject : C Programming Tutorial

Book book_id : 6495407

Book title : Telecom Billing

Book author : Zara Ali

Book subject : Telecom Billing Tutorial

Book book_id : 6495700

Unions in C:-

A union is a special data type available in C that allows to store different data types in the same memory location. You can define a union with many members, but only one member can contain a value at any given time. Unions provide an efficient way of using the same memory location for multiple purpose.

Defining a Union:-

To define a union, you must use the union statement in the same way as you did while defining a structure. The union statement defines a new data type with more than one member for your program. The format of the union statement is as follows:

union [union tag]

{

member definition;

member definition;

...

member definition;

} [one or more union variables];

The union tag is optional and each member definition is a normal variable definition, such as int i; or float f; or any other valid variable definition. At the end of the union's definition, before the final semicolon, you can specify one or more union variables, but it is optional. Here is the way you would define a union type named Data having three members i, f, and str:

union Data

{

int i;

float f;

char str[20];

} data;

Now, a variable of Data type can store an integer, a floating-point number, or a string of characters. It means a single variable, i.e., same memory location, can be used to store multiple types of data. You can use any built-in or user-defined data types inside a union based on your requirement.

The memory occupied by a union will be large enough to hold the largest member of the union. For example, in the above example, Data type will occupy 20 bytes of memory space because this is the maximum space which can be occupied by a character string. The following example displays the total memory size occupied by the above union:

#include <stdio.h>

#include <string.h>

union Data

{

int i;

float f;

char str[20];

};

int main( )

{

union Data data;

printf( "Memory size occupied by data : %d\n", sizeof(data));

return 0;

}

When the above code is compiled and executed, it produces the following result:

Memory size occupied by data : 20

Accessing Union Members:-

To access any member of a union, we use the member access operator (.). The member access operator is coded as a period between the union variable name and the union member that we wish to access. You would use the keyword union to define variables of union type. The following example shows how to use unions in a program:

#include <stdio.h>

#include <string.h>

union Data

{

int i;

float f;

char str[20];

};

int main( )

{

union Data data;

data.i = 10;

data.f = 220.5;

strcpy( data.str, "C Programming");

printf( "data.i : %d\n", data.i);

printf( "data.f : %f\n", data.f);

printf( "data.str : %s\n", data.str);

return 0;

}

When the above code is compiled and executed, it produces the following result:

data.i : 1917853763

data.f : 4122360580327794860452759994368.000000

data.str : C Programming

Here, we can see that the values of i and f members of union got corrupted because the final value assigned to the variable has occupied the memory location and this is the reason that the value of str member is getting printed very well.

Now let's look into the same example once again where we will use one variable at a time which is the main purpose of having unions:

#include <stdio.h>

#include <string.h>

union Data

{

int i;

float f;

char str[20];

};

int main( )

{

union Data data;

data.i = 10;

printf( "data.i : %d\n", data.i);

data.f = 220.5;

printf( "data.f : %f\n", data.f);

strcpy( data.str, "C Programming");

printf( "data.str : %s\n", data.str);

return 0;

}

When the above code is compiled and executed, it produces the following result:

data.i : 10

data.f : 220.500000

data.str : C Programming

Here, all the members are getting printed very well because one member is being used at a time.

Typedef in C:-

The C programming language provides a keyword called typedef, which you can use to give a type, a new name. Following is an example to define a term BYTE for one-byte numbers:

typedef unsigned char BYTE;

After this type definition, the identifier BYTE can be used as an abbreviation for the type unsigned char, for example:

BYTE b1, b2;

By convention, uppercase letters are used for these definitions to remind the user that the type name is really a symbolic abbreviation, but you can use lowercase, as follows:

typedef unsigned char byte;

You can use typedef to give a name to your user-defined data types as well. For example, you can use typedef with structure to define a new data type and then use that data type to define structure variables directly as follows:

#include <stdio.h>

#include <string.h>

typedef struct Books

{

char title[50];

char author[50];

char subject[100];

int book_id;

} Book;

int main( )

{

Book book;

strcpy( book.title, "C Programming");

strcpy( book.author, "Nuha Ali");

strcpy( book.subject, "C Programming Tutorial");

book.book_id = 6495407;

printf( "Book title : %s\n", book.title);

printf( "Book author : %s\n", book.author);

printf( "Book subject : %s\n", book.subject);

printf( "Book book_id : %d\n", book.book_id);

return 0;

}

When the above code is compiled and executed, it produces the following result:

Book title : C Programming

Book author : Nuha Ali

Book subject : C Programming Tutorial

Book book_id : 6495407

typedef Vs #define:-

#define is a C-directive which is also used to define the aliases for various data types similar to typedef but with the following differences:

• typedef is limited to giving symbolic names to types only, whereas #define can be used to define alias for values as well, e.g., you can define 1 as ONE, etc.
• typedef interpretation is performed by the compiler whereas #define statements are processed by the preprocessor.

The following example shows how to use #define in a program:

#include <stdio.h>

#define TRUE 1

#define FALSE 0

int main( )

{

printf( "Value of TRUE : %d\n", TRUE);

printf( "Value of FALSE : %d\n", FALSE);

return 0;

}

When the above code is compiled and executed, it produces the following result:

Value of TRUE : 1

Value of FALSE : 0

C-Input and Output:-

When we say Input, it means to feed some data into a program. An input can be given in the form of a file or from the command line. C programming provides a set of built-in functions to read the given input and feed it to the program as per requirement.

When we say Output, it means to display some data on screen, printer, or in any file. C programming provides a set of built-in functions to output the data on the computer screen as well as to save it in text or binary files.

The Standard Files:-

C programming treats all the devices as files. So devices such as the display are addressed in the same way as files and the following three files are automatically opened when a program executes to provide access to the keyboard and screen.

The file pointers are the means to access the file for reading and writing purpose. This section explains how to read values from the screen and how to print the result on the screen.

The getchar() and putchar Functions:-

The int getchar(void) function reads the next available character from the screen and returns it as an integer. This function reads only single character at a time. You can use this method in the loop in case you want to read more than one character from the screen.

The int putchar(int c) function puts the passed character on the screen and returns the same character. This function puts only single character at a time. You can use this method in the loop in case you want to display more than one character on the screen. Check the following example:

#include <stdio.h>

int main( )

{

int c;

printf( "Enter a value :");

c = getchar( );

printf( "\nYou entered: ");

putchar( c );

return 0;

}

When the above code is compiled and executed, it waits for you to input some text. When you enter a text and press enter, then the program proceeds and reads only a single character and displays it as follows:

$./a.out

Enter a value : this is test

You entered: t

The gets() and puts() Functions:-

The char *gets(char *s) function reads a line from stdin into the buffer pointed to by s until either a terminating newline or EOF (End of File).

The int puts(const char *s) function writes the string ‘s’ and ‘a’ trailing newline to stdout.

#include <stdio.h>

int main( )

{

char str[100];

printf( "Enter a value :");

gets( str );

printf( "\nYou entered: ");

puts( str );

return 0;

}

When the above code is compiled and executed, it waits for you to input some text. When you enter a text and press enter, then the program proceeds and reads the complete line till end, and displays it as follows:

$./a.out

Enter a value : this is test

You entered: This is test

The scanf() and printf() Function:-

The int scanf(const char *format, ...) function reads the input from the standard input stream stdin and scans that input according to the format provided.

The int printf(const char *format, ...) function writes the output to the standard output stream stdout and produces the output according to the format provided.

The format can be a simple constant string, but you can specify %s, %d, %c, %f, etc., to print or read strings, integer, character, or float, respectively. There are many other formatting options available which can be used based on requirements. Let us now proceed with a simple example to understand the concepts better:

#include <stdio.h>

int main( )

{

char str[100];

int i;

printf( "Enter a value :");

scanf("%s %d", str, &i);

printf( "\nYou entered: %s %d ", str, i);

return 0;

}

When the above code is compiled and executed, it waits for you to input some text. When you enter a text and press enter, then program proceeds and reads the input and displays it as follows:

$./a.out

Enter a value : seven 7

You entered: seven 7

Here, it should be noted that scanf() expects input in the same format as you provided %s and %d, which means you have to provide valid inputs like "string integer". If you provide "string string" or "integer integer", then it will be assumed as wrong input. Secondly, while reading a string, scanf() stops reading as soon as it encounters a space, so "this is test" are three strings for scanf().

File I/O in C:-

The last chapter explained the standard input and output devices handled by C programming language. This chapter covers how C programmers can create, open, close text or binary files for their data storage.

A file represents a sequence of bytes, regardless of it being a text file or a binary file. C programming language provides access on high-level functions as well as low-level (OS level) calls to handle file on your storage devices. This chapter will take you through the important calls for file management.

Opening Files:-

You can use the fopen( ) function to create a new file or to open an existing file. This call will initialize an object of the type FILE, which contains all the information necessary to control the stream. The prototype of this function call is as follows:

FILE *fopen( const char * filename, const char * mode );

Here, filename is a string literal, which you will use to name your file, and access mode can have one of the following values:

If you are going to handle binary files, then you will use the following access modes instead of the above-mentioned ones:

"rb", "wb", "ab", "rb+", "r+b", "wb+", "w+b", "ab+", "a+b"

Closing a File:-

To close a file, use the fclose( ) function. The prototype of this function is:

int fclose( FILE *fp );

The fclose() function returns zero on success, or EOF if there is an error in closing the file. This function actually flushes any data still pending in the buffer to the file, closes the file, and releases any memory used for the file. The EOF is a constant defined in the header file stdio.h.

There are various functions provided by C standard library to read and write a file, character by character, or in the form of a fixed length string.

Writing a File:-

Following is the simplest function to write individual characters to a stream:

int fputc( int c, FILE *fp );

The function fputc() writes the character value of the argument c to the output stream referenced by fp. It returns the written character written on success otherwise EOF if there is an error. You can use the following functions to write a null-terminated string to a stream:

int fputs( const char *s, FILE *fp );

The function fputs() writes the string s to the output stream referenced by fp. It returns a non-negative value on success, otherwise EOF is returned in case of any error. You can use int fprintf(FILE *fp,const char *format, ...) function as well to write a string into a file. Try the following example.

Make sure you have /tmp directory available. If it is not, then before proceeding, you must create this directory on your machine.

#include <stdio.h>

main()

{

FILE *fp;

fp = fopen("/tmp/test.txt", "w+");

fprintf(fp, "This is testing for fprintf...\n");

fputs("This is testing for fputs...\n", fp);

fclose(fp);

}

When the above code is compiled and executed, it creates a new file test.txt in /tmp directory and writes two lines using two different functions. Let us read this file in the next section.

Reading a File:-

Given below is the simplest function to read a single character from a file:

int fgetc( FILE * fp );

The fgetc() function reads a character from the input file referenced by fp. The return value is the character read, or in case of any error, it returns EOF. The following function allows to read a string from a stream:

char *fgets( char *buf, int n, FILE *fp );

The functions fgets() reads up to n - 1 characters from the input stream referenced by fp. It copies the read string into the buffer buf, appending a null character to terminate the string.

If this function encounters a newline character '\n' or the end of the file EOF before they have read the maximum number of characters, then it returns only the characters read up to that point including the new line character. You can also use int fscanf(FILE *fp, const char *format, ...) function to read strings from a file, but it stops reading after encountering the first space character.

#include <stdio.h>

main()

{

FILE *fp;

char buff[255];

fp = fopen("/tmp/test.txt", "r");

fscanf(fp, "%s", buff);

printf("1 : %s\n", buff );

fgets(buff, 255, (FILE*)fp);

printf("2: %s\n", buff );

fgets(buff, 255, (FILE*)fp);

printf("3: %s\n", buff );

fclose(fp);

}

When the above code is compiled and executed, it reads the file created in the previous section and produces the following result:

1 : This

2: is testing for fprintf...

3: This is testing for fputs...

Let's see a little more in detail about what happened here. First, fscanf() reads just This because after that, it encountered a space, second call is for fgets() which reads the remaining line till it encountered end of line. Finally, the last call fgets() reads the second line completely.

Binary I/O Functions:-

There are two functions that can be used for binary input and output:

size_t fread(void *ptr, size_t size_of_elements,

size_t number_of_elements, FILE *a_file);

size_t fwrite(const void *ptr, size_t size_of_elements,

size_t number_of_elements, FILE *a_file);

Both of these functions should be used to read or write blocks of memories - usually arrays or structures.

Preprocessors:-

The C Preprocessor is not a part of the compiler, but is a separate step in the compilation process. In simple terms, a C Preprocessor is just a text substitution tool and it instructs the compiler to do required preprocessing before the actual compilation. We'll refer to the C Preprocessor as CPP.

All preprocessor commands begin with a hash symbol (#). It must be the first nonblank character, and for readability, a preprocessor directive should begin in the first column. The following section lists down all the important preprocessor directives:

Header Files:-

A header file is a file with extension .h which contains C function declarations and macro definitions to be shared between several source files. There are two types of header files: the files that the programmer writes and the files that comes with your compiler.

You request to use a header file in your program by including it with the C preprocessing directive #include, like you have seen inclusion of stdio.h header file, which comes along with your compiler.

Including a header file is equal to copying the content of the header file but we do not do it because it will be error-prone and it is not a good idea to copy the content of a header file in the source files, especially if we have multiple source files in a program.

A simple practice in C or C++ programs is that we keep all the constants, macros, system wide global variables, and function prototypes in the header files and include that header file wherever it is required.

Include Syntax:-

Both the user and the system header files are included using the preprocessing directive #include. It has the following two forms:

#include <file>

This form is used for system header files. It searches for a file named ‘file’ in a standard list of system directories. You can prepend directories to this list with the -I option while compiling your source code.

#include "file"

This form is used for header files of your own program. It searches for a file named ‘file’ in the directory containing the current file. You can prepend directories to this list with the -I option while compiling your source code.

Include Operation:-

The #include directive works by directing the C preprocessor to scan the specified file as input before continuing with the rest of the current source file. The output from the preprocessor contains the output already generated, followed by the output resulting from the included file, followed by the output that comes from the text after the #include directive. For example, if you have a header file header.h as follows:

char *test (void);

and a main program called program.c that uses the header file, like this:

int x;

#include "header.h"

int main (void)

{

puts (test ());

}

the compiler will see the same token stream as it would if program.c read.

int x;

char *test (void);

int main (void)

{

puts (test ());

}

Once-Only Headers:-

If a header file happens to be included twice, the compiler will process its contents twice and it will result in an error. The standard way to prevent this is to enclose the entire real contents of the file in a conditional, like this:

#ifndef HEADER_FILE

#define HEADER_FILE

the entire header file file

#endif

This construct is commonly known as a wrapper #ifndef. When the header is included again, the conditional will be false, because HEADER_FILE is defined. The preprocessor will skip over the entire contents of the file, and the compiler will not see it twice.

Type Casting in C:-

Type casting is a way to convert a variable from one data type to another data type. For example, if you want to store a ‘long’ value into a simple integer, then you can type cast ‘long’ to ‘int’. You can convert the values from one type to another explicitly using the cast operator as follows:

(type_name) expression

Consider the following example where the cast operator causes the division of one integer variable by another to be performed as a floating-point operation:

#include <stdio.h>

main()

{

int sum = 17, count = 5;

double mean;

mean = (double) sum / count;

printf("Value of mean : %f\n", mean );

}

When the above code is compiled and executed, it produces the following result:

Value of mean : 3.400000

It should be noted here that the cast operator has precedence over division, so the value of sum is first converted to type double and finally it gets divided by count yielding a double value.

Type conversions can be implicit which is performed by the compiler automatically, or it can be specified explicitly through the use of the cast operator. It is considered good programming practice to use the cast operator whenever type conversions are necessary.

Integer Promotion:-

Integer promotion is the process by which values of integer type "smaller" than int or unsigned int are converted either to int or unsigned int. Consider an example of adding a character with an integer:

#include <stdio.h>

main()

{

int i = 17;

char c = 'c'; /* ascii value is 99 */

int sum;

sum = i + c;

printf("Value of sum : %d\n", sum );

}

When the above code is compiled and executed, it produces the following result:

Value of sum : 116

Here, the value of sum is 116 because the compiler is doing integer promotion and converting the value of 'c' to ASCII before performing the actual addition operation.

Usual Arithmetic Conversion:-

The usual arithmetic conversions are implicitly performed to cast their values to a common type. The compiler first performs integer promotion; if the operands still have different types, then they are converted to the type that appears highest in the following hierarchy:

The usual arithmetic conversions are not performed for the assignment operators, nor for the logical operators && and ||. Let us take the following example to understand the concept:

#include <stdio.h>

main()

{

int i = 17;

char c = 'c'; /* ascii value is 99 */

float sum;

sum = i + c;

printf("Value of sum : %f\n", sum );

}

When the above code is compiled and executed, it produces the following result:

Value of sum : 116.000000

Here, it is simple to understand that first c gets converted to integer, but as the final value is double, usual arithmetic conversion applies and the compiler converts i and c into ‘float’ and adds them yielding a ‘float’ result.

Error Handling in C:-

As such, C programming does not provide direct support for error handling but being a sytem programming language, it provides you access at lower level in the form of return values. Most of the C or even Unix function calls return -1 or NULL in case of any error and set an error code errno. It is set as a global variable and indicates an error occurred during any function call. You can find various error codes defined in <error.h> header file.

So a C programmer can check the returned values and can take appropriate action depending on the return value. It is a good practice to set errno to 0 at the time of initializing a program. A value of 0 indicates that there is no error in the program.

errno(),perror() and sterror():-

The C programming language provides perror() and strerror() functions which can be used to display the text message associated with errno.

• The perror() function displays the string you pass to it, followed by a colon, a space, and then the textual representation of the current errno value.
• The strerror() function, which returns a pointer to the textual representation of the current errno value.

Let's try to simulate an error condition and try to open a file which does not exist. Here I'm using both the functions to show the usage, but you can use one or more ways of printing your errors. Second important point to note is that you should use stderr file stream to output all the errors.

#include <stdio.h>

#include <errno.h>

#include <string.h>

extern int errno ;

int main ()

{

FILE * pf;

int errnum;

pf = fopen ("unexist.txt", "rb");

if (pf == NULL)

{

errnum = errno;

fprintf(stderr, "Value of errno: %d\n", errno);

perror("Error printed by perror");

fprintf(stderr, "Error opening file: %s\n", strerror( errnum ));

}

else

{

fclose (pf);

}

return 0;

}

When the above code is compiled and executed, it produces the following result:

Value of errno: 2

Error printed by perror: No such file or directory

Error opening file: No such file or directory

Recursion in C:-

Recursion is the process of repeating items in a self-similar way. In programming languages, if a program allows you to call a function inside the same function, then it is called a recursive call of the function.

void recursion()

{

recursion(); /* function calls itself */

}

int main()

{

recursion();

}

The C programming language supports recursion, i.e., a function to call itself. But while using recursion, programmers need to be careful to define an exit condition from the function, otherwise it will go into an infinite loop.

Recursive functions are very useful to solve many mathematical problems, such as calculating the factorial of a number, generating Fibonacci series, etc.

Number Factorial:-

The following example calculates the factorial of a given number using a recursive function:

#include <stdio.h>

int factorial(unsigned int i)

{

if(i <= 1)

{

return 1;

}

return i * factorial(i - 1);

}

int main()

{

int i = 15;

printf("Factorial of %d is %d\n", i, factorial(i));

return 0;

}

When the above code is compiled and executed, it produces the following result:

Factorial of 15 is 2004310016

Fibonacci Series:-

The following example generates the Fibonacci series for a given number using a recursive function:

#include <stdio.h>

int fibonaci(int i)

{

if(i == 0)

{

return 0;

}

if(i == 1)

{

return 1;

}

return fibonaci(i-1) + fibonaci(i-2);

}

int main()

{

int i;

for (i = 0; i < 10; i++)

{

printf("%d\t%n", fibonaci(i));

}

return 0;

}

When the above code is compiled and executed, it produces the following result:

0  1  1  2  3  5  8  13  21  34

Variable Arguments:-

Sometimes, you may come across a situation, when you want to have a function, which can take variable number of arguments, i.e., parameters, instead of predefined number of parameters. The C programming language provides a solution for this situation and you are allowed to define a function which can accept variable number of parameters based on your requirement. The following example shows the definition of such a function.

int func(int, ... )

{

.

.

.

}

int main()

{

func(1, 2, 3);

func(1, 2, 3, 4);

}

It should be noted that the function func() has its last argument as ellipses, i.e., three dotes (...) and the one just before the ellipses is always an int which will represent the total number variable arguments passed. To use such functionality, you need to make use of stdarg.h header file which provides the functions and macros to implement the functionality of variable arguments and follow the given steps:

1. Define a function with its last parameter as ellipses and the one just before the ellipses is always an int which will represent the number of arguments.

2. Create a va_list type variable in the function definition. This type is defined in stdarg.h header file.

3. Use int parameter and va_start macro to initialize the va_list variable to an argument list. The macro va_start is defined in stdarg.h header file.

4. Use va_arg macro and va_list variable to access each item in argument list.

5. Use a macro va_end to clean up the memory assigned to va_list variable.

Now let us follow the above steps and write down a simple function which can take the variable number of parameters and return their average:

#include <stdio.h>

#include <stdarg.h>

double average(int num,...)

{

va_list valist;

double sum = 0.0;

int i;

/* initialize valist for num number of arguments */

va_start(valist, num);

/* access all the arguments assigned to valist */

for (i = 0; i < num; i++)

{

sum += va_arg(valist, int);

}

/* clean memory reserved for valist */

va_end(valist);

return sum/num;

}

int main()

{

printf("Average of 2, 3, 4, 5 = %f\n", average(4, 2,3,4,5));

printf("Average of 5, 10, 15 = %f\n", average(3, 5,10,15));

}

When the above code is compiled and executed, it produces the following result. It should be noted that the function average() has been called twice and each time the first argument represents the total number of variable arguments being passed. Only ellipses will be used to pass variable number of arguments.

Average of 2, 3, 4, 5 = 3.500000

Average of 5, 10, 15 = 10.000000

Memory Management in C:-

This topic explains dynamic memory management in C. The C programming language provides several functions for memory allocation and management. These functions can be found in the <stdlib.h> header file.

Dynamic Memory Allocation:-

While programming, if you are aware of the size of an array, then it is easy and you can define it as an array. For example, to store a name of any person, it can go up to a maximum of 100 characters, so you can define something as follows:

char name[100];

But now let us consider a situation where you have no idea about the length of the text you need to store, for example, you want to store a detailed description about a topic. Here we need to define a pointer to character without defining how much memory is required and later, based on requirement, we can allocate memory as shown in the below example:

#include <stdio.h>

#include <stdlib.h>

#include <string.h>

int main()

{

char name[100];

char *description;

strcpy(name, "Zara Ali");

/* allocate memory dynamically */

description = malloc( 200 * sizeof(char) );

if( description == NULL )

{

fprintf(stderr, "Error - unable to allocate required memory\n");

}

else

{

strcpy( description, "Zara ali a DPS student in class 10th");

}

printf("Name = %s\n", name );

printf("Description: %s\n", description );

}

When the above code is compiled and executed, it produces the following result.

Name = Zara Ali

Description: Zara ali a DPS student in class 10th

Same program can be written using calloc(); only thing is you need to replace malloc with 

calloc as follows:

calloc(200, sizeof(char));

So you have complete control and you can pass any size value while allocating memory, unlike arrays where once the size is defined, you cannot change it.

Command Line Arguments:-

It is possible to pass some values from the command line to your C programs when they are executed. These values are called command line arguments and many times they are important for your program especially when you want to control your program from outside instead of hard coding those values inside the code.

The command line arguments are handled using main() function arguments where argc refers to the number of arguments passed, and argv[] is a pointer array which points to each argument passed to the program. Following is a simple example which checks if there is any argument supplied from the command line and take action accordingly:

#include <stdio.h>

int main( int argc, char *argv[] )

{

if( argc == 2 )

{

printf("The argument supplied is %s\n", argv[1]);

}

else if( argc > 2 )

{

printf("Too many arguments supplied.\n");

}

else

{

printf("One argument expected.\n");

}

}

When the above code is compiled and executed with a single argument, it produces the following result.

$./a.out testing

The argument supplied is testing

When the above code is compiled and executed with two arguments, it produces the following result.

$./a.out testing1 testing2

Too many arguments supplied.

When the above code is compiled and executed without passing any argument, it produces the following result.

$./a.out

One argument expected

It should be noted that argv[0] holds the name of the program itself and argv[1] is a pointer to the first command line argument supplied, and *argv[n] is the last argument. If no arguments are supplied, argc will be one, and if you pass one argument, then argc is set at 2.

You pass all the command line arguments separated by a space, but if argument itself has a space then you can pass such arguments by putting them inside double quotes "" or single quotes ''. Let us re-write above example once again where we will print program name and we also pass a command line argument by putting inside double quotes:

#include <stdio.h>

int main( int argc, char *argv[] )

{

printf("Program name %s\n", argv[0]);

if( argc == 2 )

{

printf("The argument supplied is %s\n", argv[1]);

}

else if( argc > 2 )

{

printf("Too many arguments supplied.\n");

}

else

{

printf("One argument expected.\n");

}

}

When the above code is compiled and executed with a single argument separated by space but inside double quotes, it produces the following result.

$./a.out "testing1 testing2"

Progranm name ./a.out

The argument supplied is testing1 testing2