Why is the output 0 when using equality inside printf?

Question

#include<stdio.h>
int main()
{
    int var = 10 ; 
    printf ( "\n%d %d",var==10, var = 100) ;
}

Output:

0 100

Inside the printf statement, var==10 evaluates to true but then I'm getting the output as 0. Why does this happen?

Answer 1

You're modifying var inside of the function call. Parameters to a function can be evaluated in any order. In this particular example, var = 100 is being evaluated before var==10, but there's no guarantee the behavior will be the same if you use a different compiler.

Because you're attempting to read and modify a variable in the same expression without a sequence point to separate them, you're invoking undefined behavior.