Preprocessor

/*
 * Preprocessor directives
 * Copyright (c) 2003 Chris Provenzano
 */

/* Comments are stripped by the preprocessor */



#include <stdio.h>

#define PI 3.14159

int main() {
    double radius = 2.0;

#if defined(PI)
printf("We have PI defined as %lf\n", PI);
#endif

#undef PI

#ifdef PI
printf("We never print this line.\n");
#else
printf("PI is no longer defined.\n");
#endif

#if 0
printf("This line is never printed\n");
#endif

#ifndef PI                  // Same as #if !defined(PI)
#define PI 3.1415927        // More precision
#endif

#define AREA(x)	\
    (x) * (x) * PI

    printf("Area of circle radius %lf is %lf\n", radius, AREA(radius));

    printf("The incorrect area of circle radius %lf is %lf\n", 
      radius, AREA(++radius));
    return(0);

}