Comments on: initializing for loop counter outside of loop in ActionScript

By: william bingham

william bingham — Tue, 08 Jan 2008 00:28:21 +0000

In case you hadn’t realized…
The test condition doesn’t need to be about the initialized variable at all…

for(line=0; !feof(filebuffer); ++line){
linetext = fgets(filebuffer);
echo( line , “\t”, linetext);
}

or for a graphic example

for (sprite.x=1, sprite.y=1; !collision(enemy, sprite); ++sprite.x){
playflyingsound();
if (sprite.x > xmax) {sprite.x = 1;}
}

By: Samuel Neff

Samuel Neff — Wed, 24 Oct 2007 14:24:01 +0000

Here are the examples of why it’s different..

The following code works fine and prints nothing:

var a:Array = [];
var i:int = 0;
for(; i < a.length; i++) {
trace(a[i]);
}

but the modified version accesses an invalid index in the array:

var a:Array = [];
var i:int = 0;
for( ; ; i++) {
trace(a[i]);
if (i == 5) {
break;
}
}

By: Samuel Neff

Samuel Neff — Wed, 24 Oct 2007 14:23:09 +0000

looks like it cut off my examples :(

By: Samuel Neff

Samuel Neff — Wed, 24 Oct 2007 14:20:19 +0000

When you move the conditional outside the “for” and to the end of the inner braces, it’s actually different code now. A for loop will execute both the initialization and the test prior to first iteration so if the initialization results in a failed test, the loop never runs at all. If you move the condition inside the loop to the end, it’s a different loop.

That’s not to say it’s wrong, there are certainly situations where you want to do that, but it’s important to be aware of the difference. Example:

The following code works fine and prints nothing:

var a:Array = [];
var i:int = 0;
for(; i

By: Keith Peters

Keith Peters — Wed, 24 Oct 2007 11:18:43 +0000

gah. stupid less than sign…

trying again.

for(var i=0, j=0; i < 10, j < 9; i++, j++)
{
trace(i, j);
}

By: Keith Peters

Keith Peters — Wed, 24 Oct 2007 11:17:35 +0000

As I recall, in AS2, a for loop would compile into a while loop in byte code. Not sure if that’s true in AS3, but I wouldn’t be surprised. What you are doing here is basically using a for loop as a while loop. Whatever the use case, it might be a better use case for a while loop.

Also, a lot of people don’t know that they can do multiple expressions in the for loop:

for(var i=0, j=0; i

By: Mario Klingemann

Mario Klingemann — Wed, 24 Oct 2007 11:02:34 +0000

Alexandre: Never trust a performance test that you haven’t faked yourself. Or rather - that you haven’t confirmed yourself. I have also attended CÃ©drics session and one of the things I wrote down because it sounded so surprising was the i+=1 being faster than the i++. Unfortunately when I made a quick test for myself the i++ was actually faster. So maybe certain other circumstances have to be right that this works.

By: Alexandre Madurell

Alexandre Madurell — Wed, 24 Oct 2007 08:19:18 +0000

Hey Mike!

Hmmm… why would this be useful?… hmmm… tough question :D

Nah… seriously: Performance comes to mind.

I attended the Optimizing ActionScript 3.0 Performance session at MAX by CÃ©dric Tabin, last week in Barcelona, and he gave some tips I would have never thought of (like i+=1 having better performance than i++ depending on datatype).

Who knows… maybe defining the variable outside the loop has a better performance (let me know if anyone tests it) ;)

Looking forwards to seeing you in Barcelona :)