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This article is sort of a follow-up to my article on Flexible Loop Syntax. This was reported to my by a coworker who spotted the anomaly. I guess he had done with if the same sort of thing that I had done with for. Read on for a little insight into how the comma operator interacts with the if statement.

My coworker’s claim was that the comma operator acts like the && operator in an if statement. So I tried this out:

if (true, false)
{
	trace("true, false");
}
if (false, true)
{
	trace("false, true");
}

If his claim were correct, neither would be traced. Instead, we get:

false, true

So it seems that only the second operand to the comma operator is being used to decide truth for the if statement. Let’s test that out with some more experiments:

if (true, false, true)
{
	trace("true, false, true");
}
if (false, true, false)
{
	trace("false, true, false");
}
if (false, false, false, false, false, true)
{
	trace("false, false, false, false, false, true");
}
if (true, true, true, true, true, false)
{
	trace("true, true, true, true, true, false");
}

Which prints:

true, false, true
false, false, false, false, false, true

So it seems as though we were correct originally when we said that only the second operand to the comma operator is used to determine truth. With more commas we are essentially just building up one big nested comma operator. Here are rewritten versions of the last two tests to demonstrate this:

if (((((false, false), false), false), false), true)
{
	trace("((((false, false), false), false), false), true");
}
if (((((true, true), true), true), true), false)
{
	trace("((((true, true), true), true), true), false");
}

This, as expected, yields the same results:

((((false, false), false), false), false), true

So maybe the if and for statement syntax is not so flexible after all, but instead it is the comma operator syntax that can be used in unexpected places. If this puzzled my coworker and I, I’m sure it’s puzzled some of you out there. This is that it seems to be an ECMAScript family feature since it applies equally to AS2, AS3, and JavaScript so you’ll probably have to keep this in mind whenever you’re programming for a browser. If there’s anyone out there who has a good use for this feature, feel free to post it in the comments with code formatting blocks like this:

<pre lang="actionscript3" // code </pre>

#1 by Valentin on November 2nd, 2009 · Reply

So, the conclusion is that only the last value is used.

#2 by jackson on November 2nd, 2009 · Reply

Yes, but it’s important to remember that all of the operands are evaluated. Consider this:

var x:int = 0; if (x++, x++, x++) { } trace(x);

Since each x++ is evaluated, the value of x is 3 at the end. The value of x is not 1 like you would expect if you thought that only the last operand was even considered.

#3 by dVyper on November 3rd, 2009 · Reply

What an interesting discovery! Not too useful perhaps (unless you really like obfuscated code) but still cool to know :-)

#4 by Nicolas on November 10th, 2009 · Reply

If statements are processed until a true statement is found, that is why one should put the simpler computations first.

if (simpleComputation() &amp;&amp; complexComputation())

If simpleCompuation turns out to be true, the complexComputation wonâ€™t even be made.

#5 by skyboy on August 6th, 2010 · Reply

i’d like to point out that nicolas is wrong; the && operator evaluates both operands if the first one is true, but only the first if it is false. the || operator on the other hand would do what he said.

but for usefulness of the comma operator, in var-statements

var a:int = 15, b:Number, d:uint = uint(-1);

and if you don’t want to use the braces in a short if-block

if (value) func1(), func2(); // func1 is called, then func2 is called if value is true

though there’s hardly a use for it inside the if-statement

Flexible If Syntax

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