Constants in C

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Opposite to variables, constants are those values that do not change throughout the execution of the program. They are also called literals. Like variables, a constant can be of any data type like integer, float, or string.

Note: Constants should be declared using upper case names so that they can be distinguished with the variables.

We can define a constant by any of these two ways –

Const Keyword: This keyword can be used to declare a constant. Memory is allocated to the constant when declared through this method.

Syntax

const datatype constant_name;

Code

#include <stdio.h>
int main()
{
    const int LENGTH = 15;
    printf("%d", LENGTH);
    return 0;
}

Output:

15

We cannot redefine or change the value of a constant.

#include <stdio.h>
int main()
{
    const int LENGTH = 15;
    printf("%d", LENGTH);
    LENGTH = 17; //Error would be generated
    return 0;
}

It doesn’t matter whether the const keyword is used before or after the datatype.

const int LENGTH = 15;

is same as

int const LENGTH = 15;

#define pre-processor: There is a preprocessor directive #define using which we can declare a constant.
No memory is allocated to the constant in this approach.

#include <stdio.h>
#define LENGTH 11
int main()
{
    printf("%d", LENGTH);
    return 0;
}

Output:

11

Constant types in C:

There are two primary categories of constants-

 

  • Numeric Constants
  • Character Constants

 

These two categories are further classified into –

Numeric Constants

 

  • Integer Constants
  • Real Constants

 

Character Constants

 

  • Single Character Constants.
  • String Constants
  • Backslash Character Constants

 

 

Numeric Constants

 

1. Integer Constants: It is a sequence of digits. Integers can be of three types-

 

  • Decimal Integer
  • Octal Integer
  • Hexadecimal Integer

 

Examples-

Decimal Integer-12, -24

Octal Integer-

Hexadecimal Integer- 0x2a

2. Real Constants: They contains fractional values like 19.24.

Character Constants

1. Single Character Constants: They contain a single character enclosed within single quotes (‘ and ‘). It is to be noted that the character ‘5’ is not the same as 5. Character constants having numeric character have a specific value known as ASCII values (American Standard Code for Information Interchange).

For example- ‘A’, ‘t’ etc

2. String Constants: These are a sequence of characters enclosed in double-quotes, and they may include letters, digits, special characters, and blank spaces. It should be noted that “G” and ‘G’ are different. “G” represents a string as it is enclosed within a pair of double quotes whereas ‘G’ represents a single character as it is enclosed within single quotes.

3. Backslash Character Constant: C has some character constants having a backslash in front of them. The lists of backslash characters have a specific meaning which is known to the compiler. They are also called “Escape Sequence”.

For example- \n is used to give a line break.

List of Escape Sequences in C

Escape Sequence Representation
\0 Null
\\ Backslash
\” Double quote
\’ Single quote
\a Beep sound
\b Backspace
\f Form feed
\n New line
\r Carriage return
\t Horizontal tab
\v Vertical tab

 

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