Why does swapping values with XOR fail when using this compound form?

Question

I found this code to swap two numbers without using a third variable, using the XOR ^ operator.

Code:

int i = 25; int j = 36; j ^= i;        i ^= j; j ^= i;  Console.WriteLine("i:" + i + " j:" + j);  //numbers Swapped correctly //Output: i:36 j:25 

Now I changed the above code to this equivalent code.

My Code:

int i = 25; int j = 36;  j ^= i ^= j ^= i;   // I have changed to this equivalent (???).  Console.WriteLine("i:" + i + " j:" + j);  //Not Swapped correctly             //Output: i:36 j:0 

Now, I want to know, Why does my code give incorrect output?

Answer 1

EDIT: Okay, got it.

The first point to make is that obviously you shouldn't use this code anyway. However, when you expand it, it becomes equivalent to:

j = j ^ (i = i ^ (j = j ^ i)); 

(If we were using a more complicated expression such as foo.bar++ ^= i, it would be important that the ++ was only evaluated once, but here I believe it's simpler.)

Now, the order of evaluation of the operands is always left to right, so to start with we get:

j = 36 ^ (i = i ^ (j = j ^ i)); 

This (above) is the most important step. We've ended up with 36 as the LHS for the XOR operation which is executed last. The LHS is not "the value of j after the RHS has been evaluated".

The evaluation of the RHS of the ^ involves the "one level nested" expression, so it becomes:

j = 36 ^ (i = 25 ^ (j = j ^ i)); 

Then looking at the deepest level of nesting, we can substitute both i and j:

j = 36 ^ (i = 25 ^ (j = 25 ^ 36)); 

... which becomes

j = 36 ^ (i = 25 ^ (j = 61)); 

The assignment to j in the RHS occurs first, but the result is then overwritten at the end anyway, so we can ignore that - there are no further evaluations of j before the final assignment:

j = 36 ^ (i = 25 ^ 61); 

This is now equivalent to:

i = 25 ^ 61; j = 36 ^ (i = 25 ^ 61); 

Or:

i = 36; j = 36 ^ 36; 

Which becomes:

i = 36; j = 0; 

I think that's all correct, and it gets to the right answer... apologies to Eric Lippert if some of the details about evaluation order are slightly off :(