Constants in C

A constant is a value that is intended to remain unchanged while the program is running.

const int MAX_MARKS = 100;

Here, MAX_MARKS is declared as a constant integer.

Constants are useful because they:

• Prevent accidental changes to fixed values.
• Make programs easier to understand.
• Make repeated values easier to manage.
• Improve code readability.
• Make changes easier when a fixed value needs updating.

The const keyword can be used when declaring an object whose value should not be modified through that object.

const int age = 20;

The variable age cannot be assigned a new value through this declaration.

An integer constant can be declared using const int.

const int DAYS = 7;

The value of DAYS is initialized to 7.

A floating-point object can also be declared with const.

const float PI = 3.14f;

The value is intended to remain unchanged through the object PI.

The double type can also be used with const.

const double TAX_RATE = 0.18;

This can be useful when a fixed value requires floating-point precision.

A character can also be declared as a constant.

const char GRADE = 'A';

The character value should not be changed through the object GRADE.

For a const object with automatic storage duration, initialization should be provided when it is declared because its value cannot later be assigned in the normal way.

const int MAX = 100;

A program should not attempt to assign a new value to a const-qualified object.

const int age = 20;

// age = 25;   // Not allowed

The assignment is invalid because age is const-qualified.

There is no requirement that const variable names be uppercase. However, uppercase names are commonly used by programmers to make fixed values easy to recognize.

const int MAX_SIZE = 100;
const float PI = 3.14f;

The preprocessor directive #define can be used to create a macro representing a value.

#define MAX_MARKS 100

The preprocessor replaces occurrences of the macro name according to the macro definition before compilation.

#include <stdio.h>

#define MAX_MARKS 100

int main()
{
    printf("%d", MAX_MARKS);

    return 0;
}

The output is:

100

A preprocessor macro definition normally does not end with a semicolon.

#define PI 3.14

Do not write:

#define PI 3.14;

when you intend PI to expand to the numeric value 3.14.

Numbers written directly in a C program are integer constants when they represent integer values.

10
100
-25
0

These values can be used in expressions and assignments.

Values containing a decimal point are commonly written as floating-point constants.

10.5
3.14
99.99

The compiler determines their type according to C's literal rules.

A character constant is normally written inside single quotation marks.

'A'
'B'
'7'

These are character constants, not string literals.

Text written inside double quotation marks is a string literal.

"Hello"
"Welcome to C"
"Programming"

A string literal can contain multiple characters.

Escape sequences represent special characters inside character and string literals.

\n
\t
\\
\"

For example, \n represents a newline character.

#include <stdio.h>

int main()
{
    const int PRICE = 100;
    int quantity = 3;

    int total = PRICE * quantity;

    printf("Total = %d", total);

    return 0;
}

Here, the constant PRICE is used in a calculation.

#include <stdio.h>

int main()
{
    const int MAX_MARKS = 100;

    printf("Maximum Marks = %d", MAX_MARKS);

    return 0;
}

#include <stdio.h>

int main()
{
    const int DAYS_IN_WEEK = 7;

    printf("Days = %d", DAYS_IN_WEEK);

    return 0;
}

#include <stdio.h>

int main()
{
    const double TAX_RATE = 0.18;

    printf("Tax Rate = %.2f", TAX_RATE);

    return 0;
}

A program can contain multiple constants.

const int DAYS = 7;
const int MONTHS = 12;
const double PI = 3.14159;

Each constant can represent a different fixed value.

#include <stdio.h>

int main()
{
    const int PASS_MARKS = 40;
    int marks = 75;

    if (marks >= PASS_MARKS)
    {
        printf("Pass");
    }

    return 0;
}

Using a named constant makes the condition easier to understand.

Compare these two expressions:

if (marks >= 40)

and:

const int PASS_MARKS = 40;

if (marks >= PASS_MARKS)

The named constant clearly explains what the value 40 represents.

• Trying to assign a new value to a const object.
• Confusing character constants with string literals.
• Adding an unwanted semicolon to a simple #define macro.
• Using unclear names for important fixed values.
• Changing repeated literal values separately throughout a program.

#include <stdio.h>

int main()
{
    const int DAYS = 7;
    const float PI = 3.14f;
    const char GRADE = 'A';

    printf("Days = %d\n", DAYS);
    printf("PI = %.2f\n", PI);
    printf("Grade = %c\n", GRADE);

    return 0;
}

#include <stdio.h>

#define MAX_SIZE 100

int main()
{
    const int MIN_SIZE = 10;

    printf("Minimum = %d\n", MIN_SIZE);
    printf("Maximum = %d\n", MAX_SIZE);

    return 0;
}

Both const objects and preprocessor macros can represent fixed values, but they work differently.

#include <stdio.h>

#define MAX_MARKS 100

int main()
{
    const int PASS_MARKS = 40;
    int marks = 85;

    printf("Maximum Marks = %d\n", MAX_MARKS);
    printf("Passing Marks = %d\n", PASS_MARKS);
    printf("Student Marks = %d\n", marks);

    if (marks >= PASS_MARKS)
    {
        printf("Result: Pass");
    }

    return 0;
}

This example shows how named fixed values can make a C program easier to read and maintain.