Associative Array Access
by
Jesse Marangoni
(TheCanadian),
reviewed by
Kyle Murray (Krilnon) | 1 March 2011
Have questions? Discuss this Flash / ActionScript
tutorial with others on the forums.
I see a lot of people on this forum struggling with this
fairly basic concept so I figured I’d write a quick little
tutorial about it. Let’s start with the basics and break
down what associative array referencing even means.
An
associative array is a very basic way of organizing data
(aka a data structure) that associates a key with a value.
Let’s start off with creating an associative array using the
Object class:
- var
o:Object
= {};
- o.prop
= "Hello
world";
In this example, o is the associative array,
prop is the
key and Hello World is the value. I’m sure we’ve all seen
something like that before. Well, every object in AS can be
thought of as an associative array in this way. Consider the
well known MovieClip, for example, and how we’d set its x
location to 100. That code might look like this:
- var
mc:MovieClip
= new
MovieClip();
- mc.x
= 100;
Here mc would be the associative array,
x would be the
key and 100 would be the value. If you look at any code you
can probably see this simple data structure in action.
Notice the notation AS uses to access associative arrays.
Commonly called dot notation, the basic syntax is
associativeArray.key = value. Yes, I realize how basic this
is for most of you but it’s important for you to understand
this in the context of associative arrays. We’ll come back
to this in a bit.
Right now let’s talk about the Array class you’ve come to
know and love in ActionScript. For the sake of this
tutorial, instances of the Array class will be called
Arrays
and associative arrays will be called arrays (notice the
capitalisation). The Array class associates a numerical
index with a value versus the key->value pairing of
associative arrays. The fundamental principal of the Array
class is that the values are stored at equally spaced
locations in memory and that location is computed from the
index referencing it. This implies for the most part that
the indices of an Array are numeric and sequential. There
are exceptions such as when the indices are not sequential
but all this jazz about memory addresses isn’t really
important within the scope of this tutorial. The point I’m
trying to make is that, while there are differences between
Arrays and arrays, Arrays are just associative arrays with
numbers as the keys and a collection of methods to
manipulate them. Thinking of them as the same will allow us
to draw a comparison in how the indices of Arrays and the
keys of arrays are referenced.
Remember earlier we used dot
syntax to reference keys of arrays? Well theoretically we
could use dot syntax to reference indices of Arrays as well:
- var
a:Array
= [];
- a.1
=
"Hello";
- a.2
=
"World";
Don’t actually try that because it won’t work, but that is
because identifiers cannot start with numbers due to
compiler restrictions and not because Arrays and arrays are
distinct entities. Another issue dot notation presents is
that it allows no way to create dynamic keys (keys dependent
on values generated at run-time). To deal with the need to
access numerical and dynamic keys we have the array access
operator, which are square brackets: [ and ].
Note: Operator Overloading
As a quick aside and to hopefully avoid some confusion
let’s talk about quickly about operator overloading . This
means that an operator behaves differently depending on what
context it is used in and the type of its operands. You
probably know that the square brackets are also used to
construct new Arrays, as in the last code example. Too often
I have seen people trying to use this operator to reference
arrays but in fact they are creating new Arrays. When this
operator is not prefixed by operand and there is either
zero, one, or a comma delimited list of values between the
brackets, a new Array is being constructed and that is not
what we are trying to do here. When the operator is prefixed
by an object reference and, between the brackets, includes a
single value that is or can be resolved to a string, that is
when it is accessing an array.
So to solve our previous, non-functioning Array example,
we’ll use array access instead of dot notation:
- var
a:Array
= [];
- a[1]
=
"Hello";
- a[2]
=
"World";
I’m sure you’ve all seen that before as it’s the
standard/only way to create key->value pairs with Arrays.
Let’s extrapolate this syntax: Since Arrays are a type of
associative array, this notation will also work with any
other associative array. Let’s rewrite the very first
example using array instead of dot access. The dot notation
is shown here commented out for reference.
- /* var o:Object = {};
- o.prop = "Hello world";*/
- var
o:Object
= {};
- o["prop"]
=
"Hello world";
You can also use this to reference objects that are
multiple levels deep and dot and array notation can be used
interchangeably:
- var
o:Object
= {};
- o["a"]
= {};
//same as o.a = {};
- o["a"]["b"]
=
"Hello World";
- //or
- o.a.b
=
"Hello World";
- //or
- o.a["b"]
=
"Hello World";
- //or
- o["a"].b
=
"Hello World";
The last four examples of assigning Hello World to a key
will all do the same thing and that’s the main idea I want
you to take away from this tutorial. There’s nothing
particularly magical about array access which I often feel
is what some of you think and is where a lot of your
problems are coming from. Array access is just another way
of accessing associative arrays alongside dot notation and
it really is that simple. The difference is that array
access resolves the entire expression within the brackets to
a dynamic string and is interpreted by the virtual machine
at run-time whereas dot notation uses keys interpreted by
the compiler when your movie is published.
The fact that array notation uses strings also means that
it can be used to create the aforementioned dynamic
references. A common example of this is using the iterator
of a loop in a variable name:
- var
o:Object
= {};
- o.prop1
= "one";
- o.prop2
= "two";
- o.prop3
=
"three";
- for(var
i:uint
= 1;
i <
4; i++)
{
- trace(o["prop"
+
i]);
//one, two, three
- }
In each iteration of the loop, the value of i is
concatenated with the string prop to produce the keys prop1,
prop2, prop3 and their values are referenced using array
access.
Finally I’ll mention a common mistake I see people make
and that is trying to use dot notation inside of array
access, which will not work. For example:
- associativeArray["a.b.c"
+ i];
//wrong , although this can be
resolved using the eval function in AS2
- associativeArray.a.b["c"
+ i];
//right
Well that was incredibly long winded for such a simple
concept but I’ve seen a lot of unnecessary confusion with
this and hopefully I cleared it up for some of you.
Hopefully none of you will ever have to ask a question about
this ever again (kidding) but if you must please feel free
to use the
forums and someone there will probably explain it
better than I could here.
 |
Jesse
Marangoni (TheCanadian) |
 |
Kyle
Murray (Krilnon) |
|
