Constants in C

In C programming, a constant is a value that cannot be changed during the execution of a program.

Once a constant is defined, its value remains fixed throughout the program. This helps in writing safe, readable, and reliable code.

Why Constants Are Important

Constants are used when a value should never change. They help:

• Prevent accidental modification of important values
• Improve program readability
• Make maintenance easier
• Reduce logical errors

For example, values like PI (3.14), tax rates, or maximum limits should remain constant.

Types of Constants in C

C supports two main ways to define constants:

• Using the const keyword
• Using #define preprocessor directive

1. Using const Keyword

The const keyword is used to declare variables whose values cannot be changed after initialization.

const int MAX_AGE = 100;

If you try to change MAX_AGE later, the compiler will generate an error.

MAX_AGE = 120;   // Error: cannot modify a constant

2. Using #define Directive

The #define directive is used to create symbolic constants.

These constants do not occupy memory; they are replaced by the compiler before compilation.

#define PI 3.14
#define MAX_STUDENTS 60

Wherever PI appears in the program, the compiler replaces it with 3.14.

Numerical Example Using Constants

Let us calculate the area of a circle using a constant.

#include <stdio.h>
#define PI 3.14

int main() {
    float radius = 5;
    float area;

    area = PI * radius * radius;

    printf("Area of circle = %.2f", area);
    return 0;
}

Here, PI is a constant and should never be modified.

Difference Between const and #define

Although both are used for constants, they are different.

• const uses memory and follows data types
• #define does not use memory and has no data type
• const provides better type checking
• #define is replaced during preprocessing

Real-World Thinking

Think of constants like:

• Maximum speed limit on a road
• Number of days in a week
• Fixed tax percentage

These values should not change during program execution.

Mini Practice

• Create a constant for the number of days in a week
• Create a constant for tax rate
• Use them in a simple calculation

Quick Quiz

Q1. What is a constant?

A constant is a value that cannot be changed during program execution.

Q2. Which keyword is used to create constants in C?

The const keyword is used to create constants.

Q3. Does #define use memory?

No, #define does not use memory.

Q4. Which provides better type safety: const or #define?

const provides better type safety.

Q5. Can the value of a constant be modified later?

No, constants cannot be modified after definition.