Question

I have stumbled upon a piece of code that generates some interesting results while debugging someone else’s program.

I have created a small program to illustrate this behavior:

#include <stdio.h>

int main()
{
        char* word = "foobar"; int i, iterator = 0;
        for (i = 0; i < 6; i++ && iterator++)
          printf("%c", word[iterator]);
        return 0;
}

I know that this is not the right way to print a string. This is for demonstration purpose only.

Here I expected the output to be “foobar”, obviously, but instead it is “ffooba”. Basically it reads the first character twice, as if the first time iterator++ is executed nothing happens.

Can anyone explain why this happens?

Answer 1

The thing is iterator++ actually isn’t executed the first time. The ++ operator returns the current value of a variable and then increments it, so the first time through, i++ will be equal to 0. && short-circuits, so iterator++ is not executed the first time.

To fix this, you could use the comma operator which unconditionally evaluates both, rather than the short-circuiting &&.