JacksonDunstan.com

package
{
	import flash.display.*;
	import flash.utils.*;
	import flash.text.*;
 
	public class ArrayVsDictionary extends Sprite
	{
		private var __logger:TextField = new TextField();
		private function row(...cols): void
		{
			__logger.appendText(cols.join(",")+"\n");
		}
 
		public function ArrayVsDictionary()
		{
			stage.align = StageAlign.TOP_LEFT;
			stage.scaleMode = StageScaleMode.NO_SCALE;
 
			__logger.autoSize = TextFieldAutoSize.LEFT;
			addChild(__logger);
 
			init();
		}
 
		private function init(): void
		{
			var beforeTime:int;
			var afterTime:int;
			var i:int;
			var REPS:int = 10000000;
			var array:Array = new Array();
			var dictionary:Dictionary = new Dictionary();
			var j:int;
 
			row("Class", "Time");
 
			beforeTime = getTimer();
			for (i = REPS-1; i >= 0; --i)
			{
				array[i] = i;
			}
			afterTime = getTimer();
			row("Array (write)", (afterTime-beforeTime));
 
			beforeTime = getTimer();
			for (i = REPS-1; i >= 0; --i)
			{
				dictionary[i] = i;
			}
			afterTime = getTimer();
			row("Dictionary (write)", (afterTime-beforeTime));
 
			beforeTime = getTimer();
			for (i = REPS-1; i >= 0; --i)
			{
				j = array[i];
			}
			afterTime = getTimer();
			row("Array (read)", (afterTime-beforeTime));
 
			beforeTime = getTimer();
			for (i = REPS-1; i >= 0; --i)
			{
				j = dictionary[i];
			}
			afterTime = getTimer();
			row("Dictionary (read)", (afterTime-beforeTime));
		}
	}
}

Comments

• #1 by Joran on September 10th, 2012 · Reply

  One thing that I’ve noticed using arrays like this was a really weird and annoying bug in flash that forced me to use a dictionary.
  What happened was this: I had an array that I used as a map to set items in. Let’s start with this situation:
  

  
  [0] object
  [1] null
  [2] object
  [3] object
  [4] null
  

  
  Then fore some reason, when I deleted the value for index 2 (to make it null) this happened:
  

  
  delete (theArray[2]);
  
  [0] object
  [1] undefined
  [2] null
  [3] object
  [4] null
  

  Index 1 became undefined for no reason at all. Problem was that my for each loop broke because undefined is a value it iterates trough.
  Come to think about it, I should have used if(value == undefined) instead of switching to a dictionary…

  • #2 by Pavel on September 10th, 2012 · Reply

    @Joran

    full example please?

    *damn it, missed the “Reply”*

• #3 by Pavel on September 10th, 2012 · Reply

  @Joran

  full example please?

• #4 by skyboy on September 25th, 2012 · Reply

  Though, Adobe’s Arrays are a bit stupid when migrating between sparse/dense. Once you pass a threshold to qualify the Array as sparse, it isn’t converted back to dense unless you set/reduce its length to 0 (special code handles this, resetting the entire object). Accidentally making an Array sparse permanently impacts performance of that Array.

  • #5 by jackson on September 25th, 2012 · Reply

    Sounds disastrous. Any idea where the threshold is? Any tests to show the effect?

    • #6 by skyboy on September 25th, 2012 · Reply

      (denseLen > MIN_SPARSE_INDEX && denseLen > ((denseUsed+1)<<2)) || int32_t(denseLen) < 0;

      The minimum sparse index is defined as 32. If there are 2^32 elements or more, any index is not an int, or less than half of the dense array is actually occupied, from the first non-undefined element to the last non-undefined element (starting index for the dense array portion can be higher than 0); nulls count, then it is converted to sparse.

      One possible test would be to define an array of 1000 elements, set each to null, then starting at index 1 and going to index 600 use the delete statement on each element. Iterating over the array several times in each state should show the difference between sparse/dense.

      http://hg.mozilla.org/tamarin-redux/file/3fc566f3a8b3/core/ArrayObject.cpp#l112

      • #7 by skyboy on September 25th, 2012 · Reply

        Er. My bad, when less than 1/4th of the dense portion is occupied. Indexes from 1 to 800.