A strange way AS3 manages arrays

December 26, 2008 by Emanuele Feronato
Filed Under Actionscript 3, Flash • 18 Comments

Today I was porting my Sudoku creator/solver with PHP in AS3 when I found this strange behavior.

I am calling it “strange behavior” because it’s not properly a bug… let’s say an issue.

Look at this script and its comments…

package {
	import flash.display.Sprite;
	public class wtf extends Sprite {
		public function wtf() {
			var my_array = new Array(1,2,3);
			var my_saved_array = new Array();
			trace(my_array);
			// 1,2,3 ... of course
			my_saved_array.push(my_array);
			trace(my_saved_array);
			// 1,2,3 ... of course
			my_array[2] = 4;
			trace(my_array);
			// 1,2,4 ... of course
			trace(my_saved_array);
			// 1,2,4 !!!! WHY? WHY?
			my_saved_array.push(my_array.slice())
			trace(my_saved_array);
			// 1,2,4,1,2,4 ... dirty trick...
			my_array[2] = 5;
			trace(my_saved_array);
			// 1,2,5,1,2,4 ... yeah!
		}
	}
}

I wanted to save various instances of my_array in my_saved_array in order to make some backtracking, when I found push() does not save the array content but the array itself, so if I change some my_array values, the pushed array will also change.

You can prevent it by saving a copy of your original array like I did with slice().

This does not happen neither with Php nor, as far as I remember, with C

What I know, I had an hard time debugging the script I was writing…

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18 Responses to “A strange way AS3 manages arrays”

Keith Peters on December 27th, 2008 12:14 am

that’s exactly the expected behavior and i don’t know how else you would expect it to work. arrays and other complex objects are always handled by reference. that’s how you get two dimensional arrays.

are you expecting it to push all the elements of the array as individual elements? i think your traces may be confusing you. for example, after line 11, the length of my_saved_array is 1. at the end, it is 2. my_saved_array contains two references to two separate arrays. one in element 0, one in element 1. but when you trace them, it can appear that you are tracing out one big array.
Emanuele Feronato on December 27th, 2008 12:27 am

I was expecting 1,2,3 at line 16 and 1,2,3,1,2,4 at line 22

Php works this way.
Viktor Hesselbom on December 27th, 2008 12:35 am

You can’t really blame ActionScript for this one. PHP is full of these, imo, stupid design choices.

In ActionScript you always pass the reference of an object if it’s not a string, number or boolean. While it’s pretty stupid that you can’t pass a reference to any of these 3 types at least it’s easy to know what will happen. In PHP you mostly have to guess.
Kelvin Luck on December 27th, 2008 12:40 am

But after you run:

my_saved_array.push(my_array);

Then my_saved_array is:

[my_array]

i.e. it is an array and the first item in it is a reference to my_array… That is exactly what array.push is meant to do:

http://livedocs.adobe.com/flash/9.0/ActionScriptLangRefV3/Array.html#push()

It looks like you want to just copy the contents of my_array to my_saved_array? The easiest way to do that would be:

var my_saved_array:Array = my_array.concat();

If you there was already stuff in my_saved_array then you could do:

my_saved_array = my_array.concat(my_saved_array);
brindy on December 27th, 2008 12:52 am

If I understand what you’re trying to do (keep a copy of an array at a certain point) I believe the recommended approach is to use the concat function:

http://livedocs.adobe.com/flex/3/langref/Array.html#concat()
Ink on December 27th, 2008 12:57 am

The hell arrays.Linked lists all the way.
Sameer on December 27th, 2008 1:21 am

You’re assuming AS to automatically convert datatype of my_array when you are pushing to in my_saved_array and split it into pieces. That won’t happen.

By tracing my_saved_array, you get all elements in as ‘String’. And the first element, my_array, is also converted to string and so looks like 1,2,3 and later when you change value at position two, it traces 1,2,4.

I hope you my text here makes sense to you.

Happy Holidays to you and family.

Best.
Sam
Sameer on December 27th, 2008 1:23 am

To add to my above comment, what you want to achieve is possible with Array.concat() method. It takes another array and joins them. In this case, my_saved_array.concat(my_array);
space on December 27th, 2008 1:31 am

That is a normal behavior in many languages. All you are doing is copying the pointer to the array, and not the array itself. It’s the same in AS2. And it’s the same for objects too.

To copy the contents you need to do it manually, like using slice, but even in that case, if your array, contains arrays, those will only be pointers to the original array.

So you need to make a copy function that nests through it. You can make a function for doing that for both objects and arrays. There are actually several examples you can find if you google it.
Daniel Rodriguez on December 27th, 2008 2:00 am

That remembers me a day I almost died with a project in java, it happens something similar.
Jason Reid on December 27th, 2008 5:51 am

Yeah, this is really a PHP thing. Most languages (Javascript included) work more like the way that AS3 does.

C doesn’t have built-in push / pop operators on it’s arrays, so its behavior would depend on whatever libraries you were using for list management.
colbycheeze on December 27th, 2008 5:57 am

I had this confusing issue once before, and it took a while to figure it out.

I thin it is actually much better this way however, so I am glad it does it.

I do wish you could pass primitives as reference like in C++ however…
Michael Kaufman on December 27th, 2008 9:57 am

Exactly, since you made a custom object you are sharing a reference. Really has nothing to do with Arrays and just the way AS handles variable assignments for anything except String, Boolean, Number, int, and uint. I did not know PHP treats assignments differently. Good to know. Thanks :)

[code]
var aNum:uint = 0
var bNum:uint = aNum

trace (aNum) //0
trace (bNum) //0

aNum = 2
trace (aNum) //2
trace (bNum) //0
[/code]
vitaLee on December 28th, 2008 12:24 am

the only problem here was that you approached this situation with wrong expectations.
your php background experience didn’t help you here.
i’ve once felt in this kind of situation and then i found out about the concat method.
HEAD UP !!! :)
Michael Boucher on December 28th, 2008 12:32 am

With PHP you explicitly tell a variable to pass by reference. If I remember correctly this is done as follows:

// Changes to $myArr2 no affect on $myArr1
$myArr2 = $myArr1;

// Changes to $myArr3 affect $myArr1
$myArr3 = &$myArr1;

Agreed that concat() should be used for “copying” an array and breaking the reference. Can’t find the posting with some speed tests but concat() proved to be the fastest method.

Have to somewhat disagree that PHP offers a poor design decision on how it assigns complex types. It’s all a matter of the “default” for both languages, PHP doesn’t lack the ability to assign by reference, it just isn’t the default behavior.

Best,
-michael
mark on December 28th, 2008 8:32 pm

lol… public function wtf(){
Mauro on April 7th, 2009 6:45 pm

ok he was just showing something he thought was curious, why are you guy blaming him?

remember, not even a PROgrammer knows all tricks!

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Sudoku creator/solver with AS3 : Emanuele Feronato on December 27th, 2008 3:10 pm

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