Unsure what this while statement is doing

Question

So, I'm doing homework. I've encountered something I haven't seen before and cannot find a decent explanation of what it does. Basically,

Object object;
...
while((value1, value2) = function(object)) {
    object.foo(value1, value2);
}

The (value1, value2) in the while statement really throws me. Any ideas?

You'll find answers by searching for "comma operator". [Here's one](http://stackoverflow.com/q/54142/1016716) — Mr Lister, Jul 06 '13 at 05:05
Haven't tried. This was part of a specification. Not sure what to make of it — lambgrp425, Jul 06 '13 at 05:06
THe others answered what it does. I just want to comment- never do this. Its confusing and brings no value, there's cleaner ways to write it. This is basically a stupid C++ tricks question. — Gabe Sechan, Jul 06 '13 at 05:10
@GabeSechan: I agree with everything you said. But I suspect it may be a failed translation from another language where multiple values can be returned. If the original code has `return 1,2;` inside `function()` one may think that you can call the function like `(a,b) = function();`. — Martin York, Jul 06 '13 at 05:14

score 1 · Accepted Answer · answered Jul 06 '13 at 05:05

Its a comma operator.

The result of the comma operator is the last value (the others are evaluated and discarded).

while((value1, value2) = function(object)) {
    object.foo(value1, value2);
}

If value1 is just a variable and not an expression then it is equivalent too:

while(value2 = function(object)) {
    object.foo(value1, value2);
}

If value1 is an expression then it is evaluated each time around the loop. The result is discarded, but if the expression has side effects these will take effect.

Unsure what this while statement is doing

1 Answers1