Confusion about two simple character counting programs in C

Question

The goal is to write a simple character counting program in C. If written like this:

#include <stdio.h>

main()
{
   long nc;

   for(nc = 0; getchar() != EOF; ++nc)
   printf("%ld\n", nc);
}

the last number listed in its output will be the correct number of characters. However, if written like this:

#include <stdio.h>

main()
{
   long nc;

   nc = 0;
   while(getchar() != EOF)
   {
      ++nc;
      printf("%ld\n", nc);
   }
}

the last number in its output will always be larger by one than the true number of characters in the input string. I learned that this is because pressing Enter after inputting the desired string introduces a newline character which gets counted and produces the error. To eliminate that error is trivial, but my question is why doesn't the same problem occur in the program written above?

For example, the first program will correctly output 0 if ran without input. The second one however will output 1 in the same scenario. Why the difference?

If there is no input, it shouldn't print anything at all because the first call to `getchar()` will return `EOF` in that case, and so it will never enter the loop body. I suspect you may have a single newline character in the input. — Ian Abbott, Jan 21 '19 at 13:25

score 2 · Accepted Answer · answered Jan 21 '19 at 13:20

All numbers are larger by one in the second loop, not just the last.

Your for-loop is equivalent to

nc = 0;
while (getchar() != EOF)
{
    printf("%ld\n", nc);
    ++nc;
}

As you can see, it increments after printing, and your while increments before printing.

score 1 · Answer 2 · answered Jan 21 '19 at 13:43

The difference is due to the order in which the statements are executed in both the scenario.

In case of for loop, the order is:

         (1)           (2)        (4)
       |----|  |---------------|   |
   for(nc = 0; getchar() != EOF; ++nc)
       printf("%ld\n", nc);  --------------- (3)

and in case of while loop, the order is:

   nc = 0;     ------------------- (1)
   while(getchar() != EOF)  --------------- (2)
   {
      ++nc;  -------------------- (3)
      printf("%ld\n", nc);  ------------ (4)
   }

So, in case of for loop the ++nc is last executed in every iteration i.e. after printf() and in case of while loop the ++nc is executed before the printf() in every iteration. Hence, you are observing the difference.

Confusion about two simple character counting programs in C

2 Answers2