Variable and Constants

Variable: Variable is a named location in memory used to hold a value that can be modified by a program. Variables are used to hold numbers, strings and complex data for manipulation.

• In C, we use variable to allocate certain memory where certain data can store.
• Every variable used in the program should be declared to the compiler.
• It tells the compiler about variable name.
• It specifies what type of data the variable will hold.

Constants and Literals: Constants are fixed values that cannot be altered by the program and it could be numbers, characters, or strings. Constants are sometimes called literals. Literals are data items that never change their value during the entire run of program. A variable can be considered as a name given to the location into the memory where the constant is stored. Eg: int x = 5, float mark = 50.5 etc. There are four types of constant, they are;

1. Integer constant: An integer constant 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 constant 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.

212                  /* Legal */
215u                /* Legal */
0xFeeL             /* Legal */
078                 /* Illegal: 8 is not an octal digit */
032UU              /* Illegal: cannot repeat a suffix*/

85                   /*decimal */
0213                 /*octal */
0x4b                 /*hexadecimal */
30                   /*int */
30u                  /* unsigned int */
30l                  /*long */ 
30ul                 /* unsigned long */

2. Floating-point Constants: 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.

3.14159      /*Legal */
314159E-5L   /* Legal */
510E        /*Illegal: incomplete exponent */
210f        /*Illegal: no decimal or exponent */
.e55       /*Illegal: missing integer or fraction */ 

3. Character Constants: Character literals are enclosed in single quotes, e.g. 'x' which 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:

#include <stdio.h>
int main()
{
printf("Hello\tWorld\n\n");
  return 0;
} 

Hello World 

4. String constants: A string constant is a set of character enclosed in double quotes " ". A string contains characters that are similar to character literals: plain characters, escape sequences, and universal characters. You can break a long line into multiple lines using string literals and separating them using whitespaces. Here are some examples of string literals. All the three forms are identical strings.

"hello, dear"
"hello, \dear"
"hello, " "d" "ear" 

• Using #define preprocessor
• Using const keyword

#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;
}

#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;
}

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