Constants in C

A constant is a fixed value that cannot be changed during the execution of a program. It is also called a read-only value. Constants must be initialized at the time of declaration; otherwise, they may hold garbage values.

Ways to Define Constants

1. Using the `const` keyword

const data_type var_name = value;
const float PI = 3.14;

2. Using `#define` (Preprocessor Macro)

#define CONSTANT_NAME value
#define PI 3.14

Examples

Using const:

#include <stdio.h>
int main() {
    const float PI = 3.14;
    printf("Value of PI: %.2f\n", PI);
    return 0;
}

Using #define:

#include <stdio.h>
#define PI 3.14
int main() {
    printf("Value of PI: %.2f\n", PI);
    return 0;
}

Both produce the same output: Value of PI: 3.14

Attempting to Change a Constant

const int a = 10;
a = 20;   // ❌ Error: assignment of read-only variable 'a'

`const` vs `#define`

Feature	`const`	`#define`
Handled by	Compiler	Preprocessor
Memory allocated	Yes	No
Type safety	Enforced	Not enforced
Scope	Follows block rules	File-wide only
Error checking	Compile-time	None
Debugging	Easier — has a name	Harder — replaced before compiling

Best practice: Prefer const over #define for typed constants in modern C. Use #define for values that are truly compile-time substitutions (like include guards).

Data Types Variables

​Constants in C

​Ways to Define Constants

​1. Using the const keyword

​2. Using #define (Preprocessor Macro)

​Examples

​Attempting to Change a Constant

​const vs #define