Precedence in C operators == and ( = )

Question

I have to analyze what some code in C does, and I have a doubt about what happens in a certain line. The code is

#define PRINTX printf("%d\n", x)

void problem() { 
    int x = 2, y, z;
    x *= 3 + 2; PRINTX;
    x *= y = z = 4; PRINTX;
    x = y == z; PRINTX;
    x == (y = z); PRINTX; // This line is the problem
} 

This code snippet prints the resulting numbers:

10
40
1
1  // This result **

the problem is that I'm still trying to figure out why does the last line prints out x = 1, when the operation is x == (y = z). I'm having trouble finding out what that 1 means and the precedence of the operations. Hope someone can help me! :)

Answer 1

Nothing in the last statement changes the value of x, so its value remains unchanged.

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Parens were used to override precedence, forcing the = to be the operand of the ==.

An operator's operands must necessarily be evaluated before the operator itself, so we know the following:

• y is evaluated at some point before the =.
• z is evaluated at some point before the =.
• x is evaluated at some point before the ==.
• = is evaluated at some point before ==.

That's it. All of these are valid orders:

• z y = x ==
• y z = x ==
• x y z = ==
• etc.

But whenever x, y and z are evaluated, we can count on the following happening:

1. = assigns the value of z (currently 4) to y and returns it.
2. == compares the value of x (currently 1) with the value returned by = (4). Since they're different, == returns 0 (which isn't used by anything).

As you see, nothing changed x, so it still has the value it previously had (1).