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Associative arrays

An associative array, sometimes called a hash or map, uses keys instead of a numeric index to organize stored values. Each key in an associative array is a unique string that is used to access a stored value. An associative array is an instance of the Object class, which means that each key corresponds to a property name. Associative arrays are unordered collections of key and value pairs. Your code should not expect the keys of an associative array to be in a specific order.

ActionScript 3.0 also includes an advanced type of associative array called a dictionary. Dictionaries, which are instances of the Dictionary class in the flash.utils package, use keys that can be of any data type. In other words, dictionary keys are not limited to values of type String.

Associative arrays with string keys​

There are two ways to create associative arrays in ActionScript 3.0. The first way is to use an Object instance. By using an Object instance you can initialize your array with an object literal. An instance of the Object class, also called a generic object, is functionally identical to an associative array. Each property name of the generic object serves as the key that provides access to a stored value.

The following example creates an associative array named monitorInfo, using an object literal to initialize the array with two key and value pairs:

var monitorInfo:Object = {type:"Flat Panel", resolution:"1600 x 1200"};
trace(monitorInfo["type"], monitorInfo["resolution"]);
// output: Flat Panel 1600 x 1200

If you do not need to initialize the array at declaration time, you can use the Object constructor to create the array, as follows:

var monitorInfo:Object = new Object();

After the array is created using either an object literal or the Object class constructor, you can add new values to the array using either the array access ([]) operator or the dot operator (.). The following example adds two new values to monitorArray:

monitorInfo["aspect ratio"] = "16:10"; // bad form, do not use spaces
monitorInfo.colors = "16.7 million";
trace(monitorInfo["aspect ratio"], monitorInfo.colors);
// output: 16:10 16.7 million

Note that the key named aspect ratio contains a space character. This is possible with the array access ( []) operator, but generates an error if attempted with the dot operator. Using spaces in your key names is not recommended.

The second way to create an associative array is to use the Array constructor (or the constructor of any dynamic class) and then use either the array access ([]) operator or the dot operator (.) to add key and value pairs to the array. If you declare your associative array to be of type Array, you cannot use an object literal to initialize the array. The following example creates an associative array named monitorInfo using the Array constructor and adds a key called type and a key called resolution, along with their values:

var monitorInfo:Array = new Array();
monitorInfo["type"] = "Flat Panel";
monitorInfo["resolution"] = "1600 x 1200";
trace(monitorInfo["type"], monitorInfo["resolution"]);
// output: Flat Panel 1600 x 1200

There is no advantage in using the Array constructor to create an associative array. You cannot use the Array.length property or any of the methods of the Array class with associative arrays, even if you use the Array constructor or the Array data type. The use of the Array constructor is best left for the creation of indexed arrays.

Associative arrays with object keys (Dictionaries)​

You can use the Dictionary class to create an associative array that uses objects for keys rather than strings. Such arrays are sometimes called dictionaries, hashes, or maps. For example, consider an application that determines the location of a Sprite object based on its association with a specific container. You can use a Dictionary object to map each Sprite object to a container.

The following code creates three instances of the Sprite class that serve as keys for the Dictionary object. Each key is assigned a value of either GroupA or GroupB. The values can be of any data type, but in this example both GroupA and GroupB are instances of the Object class. Subsequently, you can access the value associated with each key with the array access ([]) operator, as shown in the following code:

import flash.display.Sprite;
import flash.utils.Dictionary;

var groupMap:Dictionary = new Dictionary();

// objects to use as keys
var spr1:Sprite = new Sprite();
var spr2:Sprite = new Sprite();
var spr3:Sprite = new Sprite();

// objects to use as values
var groupA:Object = new Object();
var groupB:Object = new Object();

// Create new key-value pairs in dictionary.
groupMap[spr1] = groupA;
groupMap[spr2] = groupB;
groupMap[spr3] = groupB;

if (groupMap[spr1] == groupA)
{
    trace("spr1 is in groupA");
}
if (groupMap[spr2] == groupB)
{
    trace("spr2 is in groupB");
}
if (groupMap[spr3] == groupB)
{
    trace("spr3 is in groupB");
}

Iterating with object keys​

You can iterate through the contents of a Dictionary object with either a for..in loop or a for each..in loop. A for..in loop allows you to iterate based on the keys, whereas a for each..in loop allows you to iterate based on the values associated with each key.

Use the for..in loop for direct access to the object keys of a Dictionary object. You can also access the values of the Dictionary object with the array access ([]) operator. The following code uses the previous example of the groupMap dictionary to show how to iterate through a Dictionary object with the for..in loop:

for (var key:Object in groupMap)
{
    trace(key, groupMap[key]);
}
/* output:
[object Sprite] [object Object]
[object Sprite] [object Object]
[object Sprite] [object Object]
*/

Use the for each..in loop for direct access to the values of a Dictionary object. The following code also uses the groupMap dictionary to show how to iterate through a Dictionary object with the for each..in loop:

for each (var item:Object in groupMap)
{
    trace(item);
}
/* output:
[object Object]
[object Object]
[object Object]
*/

Object keys and memory management​

Adobe® Flash® Player and Adobe® AIR™ use a garbage collection system to recover memory that is no longer used. When an object has no references pointing to it, the object becomes eligible for garbage collection, and the memory is recovered the next time the garbage collection system executes. For example, the following code creates a new object and assigns a reference to the object to the variable myObject:

var myObject:Object = new Object();

As long as any reference to the object exists, the garbage collection system will not recover the memory that the object occupies. If the value of myObject is changed such that it points to a different object or is set to the value null, the memory occupied by the original object becomes eligible for garbage collection, but only if there are no other references to the original object.

If you use myObject as a key in a Dictionary object, you are creating another reference to the original object. For example, the following code creates two references to an object—the myObject variable, and the key in the myMap object:

import flash.utils.Dictionary;

var myObject:Object = new Object();
var myMap:Dictionary = new Dictionary();
myMap[myObject] = "foo";

To make the object referenced by myObject eligible for garbage collection, you must remove all references to it. In this case, you must change the value of myObject and delete the myObject key from myMap, as shown in the following code:

myObject = null;
delete myMap[myObject];

Alternatively, you can use the useWeakReference parameter of the Dictionary constructor to make all of the dictionary keys weak references. The garbage collection system ignores weak references, which means that an object that has only weak references is eligible for garbage collection. For example, in the following code, you do not need to delete the myObject key from myMap in order to make the object eligible for garbage collection:

import flash.utils.Dictionary;

var myObject:Object = new Object();
var myMap:Dictionary = new Dictionary(true);
myMap[myObject] = "foo";
myObject = null; // Make object eligible for garbage collection.