C Preprocessor & Macros

#define, #include and conditional compilation.

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Lines starting with # are handled by the preprocessor before compilation:

#include: insert a header file.
#define PI 3.14159: a constant macro.
#define SQUARE(x) ((x) * (x)): a function-like macro (always wrap arguments in parentheses).
#ifdef / #ifndef / #endif: compile code only in certain conditions, e.g. debug builds.

Key Takeaways: C Preprocessor & Macros

Mastering C Preprocessor & Macros helps you write clean and reliable C code. Follow standard conventions, test edge cases, and keep logic simple. Try running the code example below and complete the exercises to build confidence.

C Preprocessor & Macros Code Example

#include <stdio.h>

#define PI 3.14159
#define SQUARE(x) ((x) * (x))
#define DEBUG

int main() {
    double r = 2.0;
    printf("Area: %.2f\n", PI * SQUARE(r));

#ifdef DEBUG
    printf("[debug] radius = %.1f\n", r);
#endif
    return 0;
}

Output