Expert: Joseph Moore - 11/9/2009

Question
Hi, Joseph, I've come across this program the other day, and I just can't get the answer for this one, please help.

Here is the program, it asks me what would the output be:
#include <stdio.h>

void print_letter2(void);

int ctr;
char letter1 = 'X';
char letter2 = '=';

int main(void) {
 for(ctr = 0; ctr < 10; ctr++) {
    printf("%c", letter1);
    print_letter2();
 }   
 return 0;
}

void print_letter2(void) {
 for(ctr = 0; ctr < 2; ctr++)
   printf("%c", letter2);
}

At first, I simply thought it would print X== 10 times on the same line. However, the answer is that X== will keep printing forever, and it does seem to do that except I don't know how it happened.

Answer
Hi, Angela.

The problem is that you have declared a global variable called ctr and used this in both loops.  The code execution will go something like this:

   ctr = 0 (start of first loop)
   printf X (body of first loop)
   ctr = 0 (start of second loop)
   printf = (body of second loop)
   ctr = 1 (increment of second loop)
   printf = (body of second loop)
   ctr = 2 (increment of second loop)
   ctr = 3 (increment of first loop)
   printf X (body of first loop)
   ctr = 0 (start of second loop)
   ... see we've hit the infinite loop part here

If you use separate counter variables for each loop, then you would see the desired behavior.

Let me know if you have further questions! :)