How to Use #define in C: Simple Guide with Examples

In C, #define is a preprocessor directive used to create symbolic constants or macros. It replaces all occurrences of a name with a value or code snippet before compilation. For example, #define PI 3.14 replaces every PI with 3.14 in your code.

Syntax

The #define directive has two main forms:

• Constant definition: #define NAME value replaces NAME with value.
• Macro definition: #define NAME(parameters) code replaces NAME(args) with code using those arguments.

This happens before the program compiles, so no memory is used for these replacements.

c

#define NAME value
#define SQUARE(x) ((x) * (x))

Example

This example shows how to use #define to create a constant and a macro function. It calculates the area of a circle using the constant PI and the macro SQUARE.

c

#include <stdio.h>

#define PI 3.14
#define SQUARE(x) ((x) * (x))

int main() {
    float radius = 5.0;
    float area = PI * SQUARE(radius);
    printf("Area of circle with radius %.1f is %.2f\n", radius, area);
    return 0;
}

Output

Area of circle with radius 5.0 is 78.50

Common Pitfalls

Common mistakes when using #define include:

• Not using parentheses in macros, which can cause wrong calculations.
• Defining macros that evaluate arguments multiple times, causing side effects.
• Using #define for constants instead of const variables, which can be safer.

Always wrap macro parameters and the entire macro body in parentheses to avoid errors.

c

#define BAD_SQUARE(x) x * x  // Wrong: no parentheses
#define GOOD_SQUARE(x) ((x) * (x))  // Correct: safe macro

// Example usage:
// BAD_SQUARE(1 + 2) expands to 1 + 2 * 1 + 2 = 1 + 2 + 2 = 5 (wrong)
// GOOD_SQUARE(1 + 2) expands to ((1 + 2) * (1 + 2)) = 9 (correct)

Quick Reference

Usage	Description	Example
Constant	Defines a constant value	#define MAX 100
Simple Macro	Defines a macro without parameters	#define PI 3.14
Function-like Macro	Defines a macro with parameters	#define SQUARE(x) ((x) * (x))
Multi-line Macro	Defines a macro spanning multiple lines	#define PRINT(x) \ printf("%d\n", x)

Key Takeaways

Use #define to create constants or macros that replace code before compilation.

Always wrap macro parameters and the entire macro body in parentheses to avoid errors.

Macros do not use memory but can cause unexpected behavior if not carefully written.

Prefer const variables for constants when possible for better type safety.

Multi-line macros use a backslash (\) at the end of each line except the last.