how to test for equivalence of several variables in C

Question

Say I wanted to test not just one, but several variables for equivalence in an if statement:

if(x1==x2==y1==y2){
    printf("Input values shouldn't be equal!");
}

But this doesn't seem to work. What other approach can do this?

Answer 1

if (x1 == x2 && x2 == y1 && y1 == y2) { ... }

The result of the expression a == b will be an integer value of either 0 or 1. the == operator is left-associative, so the expression a == b == c will be evaluated as (a == b) == c; that is, the result of a == b (0 or 1) will be compared against the value of c. So in the code below

if (a == b == c) { ... }

the expression will only evaluate to true if a == b and c == 1 or a != b and c == 0.

Answer 2

if (x1 == x2 && x1 == y1 && x1 == y2)
{
  printf("Input values shouldn't be equal!");
}

Answer 3

if( x1 == x2 && x2 == y1 && y1 == y2 ) { ... }