Question

I’m learning how to program in C, but I can’t figure out how to loop using a constant. Its my understanding that the best way to use a constant value in C is through the #define statement (correct me if I’m wrong). But it doesn’t seem to be working.

Here’s my code.

#include <stdio.h>

#define NUM = 3

void main(int argc, char *argv[]){

  int i=0;

  while(i<NUM){
    printf("foo ");
    i++;
  }

  return;
 }

When I try to compile the code I get the following error.

helloWorld.c: In function ‘main’:
helloWorld.c:9: error: expected expression before ‘=’ token

(For those of you who don’t want to count, line 9 is the while loop declaration).
How can I do this using preprocessor functions, and is that the best way to use constant values in C? I can get it to work using ‘const’ but I don’t think thats best.

Answer 1

You use #define to declare a macro, which you can kind of think of as a type of constant in some cases.

Actually, what happens is the compiler replaces any occurrences of your macro with your macro text. In your case, = 3. This results in while(i < = 3), which is a syntax error.

The correct way to write the macro is:

#define NUM 3

To define a real constant, use the const keyword.