Introduction to C-Programming As seen in ENGS 20

printf() statements are a quick and easy way to trace the path of execution of a program.

But...

It can be tedious to insert and delete printf statements when code is being debugged. (After all, we don’t want debug statements flashing by all the time.)

Of course you can comment out (/* */) the printf() statements after code is debugged. But it can still be a bit tedious (yet, it’s my preferred method...).

There is another way however:

You can conditionally include debugging lines (such as printf() statements) by instructing the preprocessor to either include them or to not do so. Any line that starts with a hash mark (#) is meant for and interpreted by the preprocessor. In fact, we have already seen the preprocessor at work with our #include statements! Therefore, when using preprocessor directives to conditionally include debug statements then it is the preprocessor that does the work!

 

#define DEBUG
. . . 
#ifdef DEBUG printf("No idea what's wrong.");
#else printf("All good!");
#endif

You could even be fancier and allow for different levels of debugging:

#define DEBUG 1
#include <stdio.h>
int main(void)
{
  int N, i, sum;
  i = 1;
  #if (DEBUG >=2)
    printf("It's a beautiful day! ");
  #elif (DEBUG >=1)
    printf("It's sunny! ");
  #elif (DEBUG >=0)
    printf("Not too bad! ");
  #else
    printf("Huh...! ");
  #endif
}