Decrementing and incrementing a variable with an if condition produces unexpected result in C language

Question

I'm running this code from the command prompt. The output of this code is 0 but I was expecting it to output 1. Can you explain how this code is actually compiled and runs?

#include <stdio.h>
int main() {
    int x;
    x = 1;
    if (--x && ++x) {
        printf("Great");
    }
    printf("%d\n", x);
    return 0;
}

Answer 1

--x decrements the value of x before using the value of the variable. So you start with x=1, then do an if clause

if (--x

which decrements x and then inspects the value. The value is now 0. You haven't put any other boolean test in there, in C if (0) equates to false. So it doesn't execute the second part of the conditional (which would incremenet x), it simply skips the 'if' clause and goes to printf and return 0.

If you change this to something like:

if (--x >= 0 && ++x >= 0)

Or:

if (x-- && x++)

Because x-- uses the variable before decrementing it, so the if will test x when its value is 1, before decrementing it. Then it'll do what you want.