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ColdFusion: Article

Playing with Arrays

A powerful tool in the hands of a skilled coder

When I found out this month's issue would be a "back to basics" issue, I was torn between several topics that I hope are of interest to every CFML developer. I settled on the array, which is a powerful tool in the hands of a skilled coder. Just to make sure everyone's on board we'll start with the assumption that we need to explore the nature of an array first.

Every programming language has simple datatypes and complex datatypes. Simple datatypes are things like strings and numbers - datatypes that store a single value. Complex datatypes are things like arrays and structures (which are also called associative arrays) - datatypes that can store more than one simple value. An array is a complex datatype that stores values in a series of elements. Each element is addressed by a number known as its index.

That's the programming definition of an array. To get a handle on the concept, I find it easiest to think of an array as a container with several pockets like an egg carton. To put something in the carton, we have to say something like "Put an egg in pocket one of the egg carton." In CFML that statement looks like this:

<cfset eggCarton[1] = "egg">

or if we're working inside a <cfscript> block:

eggCarton[1] = "egg";

Similarly, if we want to find out what's in the fifth pocket of the egg carton, we could output it to the browser like this:

<cfoutput>#eggCarton[5]#</cfoutput>

The egg carton example is an array of one dimension. That means it only has one index number - the number of the pocket we need. But we can also have more complex situations. For example, we could have a set of egg cartons. In that case we have to specify not only which pocket we need, but which carton as well. To do this, we use a second index number. So the sixth pocket of the second egg carton could be referenced as eggCartons[2][6] (the plural eggCartons is arbitrary; it just helps to remember the purpose of this particular array). This new array has two dimensions indicated by the two index numbers used. We can think of the first dimension as "carton" and the second dimension as "pocket."

This brings us around to the syntax for creating an array in ColdFusion. Before we can put anything in the array, we have to create a variable with the array datatype. The ArrayNew() function handles the job, and it takes one argument: the number of dimensions we want in the array. CFML will accept a value of 1, 2, or 3 for the number of dimensions. Here are the statements we would use to create the arrays used in the two egg carton examples we've seen so far:

<cfset eggCarton = ArrayNew(1)>
<cfset eggCartons = ArrayNew(2)>

The ArrayNew() function creates an array, but it doesn't store any values in the array's elements. That job remains for us to do.

Even though ColdFusion only allows us to create up to three dimensions with the ArrayNew() function, the number of dimensions isn't really restricted to three. Each extra dimension is really another layer of arrays, so it's possible to manually construct arrays of more than three dimensions. That's a discussion for another article, but here's a bit of code to give you a hint; we could create the eggCartons array this way (assuming we want four cartons):

<cfset eggCartons = ArrayNew(1)>
<cfset ArraySet(eggCartons,1,4,ArrayNew(1))>

The second line in this example is equivalent to the following loop:

<cfloop from="1" to="4" index="i">
    <cfset eggCartons[i] = ArrayNew(1)>
</cfloop>

Based on this example, we can extrapolate the same technique to build an array of as many dimensions as desired. In practical use, though, arrays of more than three dimensions are generally reserved for esoteric uses.

In some programming languages, an array's elements must all be the same datatype such as integer or alphanumeric. This is not the case where ColdFusion is concerned. We can store any datatype in any element of an array, even mixing datatypes in the array. We can have arrays of structures or arrays of arrays (as seen in the code above), or arrays of CFCs. Again, that's a little more than we have room to cover in this article.

However, we do want to do something useful with arrays and take advantage of a few of the array functions in CFML, so let's get to some more practical code. Card games seem to be increasingly popular these days, especially poker, so let's say we want to cook up a way to store a deck of cards, shuffle them, and deal out hands.

The first thing we want to do is create, in code, a model of a deck of cards. Listing 1 shows one way to approach the job with CardsArray.cfm. A quick note on the listings in this article: they all use a Fusedoc at the top of the file to describe the expectations for the code. Even though this code isn't being written for a Fusebox application, the Fusedocs help communicate our intentions in designing the code.

So the responsibilities for CardsArray.cfm are to create an array then populate it with 52 members representing the cards of a standard deck. Line 20 sets up the array then we start the code to get it populated.

Line 21 sets a variable with a list of the suits we want in the deck. Lines 24-34 contain code, including a nested loop, to store card values in the deck array. The outer loop uses the list of suits and the variable aSuit to hold the value of each element of the list as it loops through. For each value in the list of suits, the inner loop stores the number cards (2-10).

The inner loop uses the variable i to store the value of its index. This is common practice when using counting loops - i is an abbreviation for index. When counting loops are nested, it's common practice to use sequential letters starting with i for the indices. So two nested counting loops are often seen using the indices i and j.

The ArrayAppend() function is used to add a new element to the deckArray. ArrayAppend() is very useful in that we don't need to know the size of the array to add an element - we just specify the array's name and the value to append. Once the inner loop has run, we use four more ArrayAppend() statements to add the face cards and aces for the suit. That's it - the deckArray array has been created and populated. However, there's no code in CardsArray.cfm to display the array, so how do we know it worked properly? Since we might want to reuse this routine, we don't want to display its output here. So we'll use a test harness template.

The test harness is named testCardsArray.cfm, and is shown in Listing 2. All it does is use <cfinclude> for the CardsArray.cfm template then <cfdump> to show the deckArray variable. The output is shown in Figure 1 (the cfdump output has been broken up to save print space).

Now that we have a deck of cards, we're ready to start playing. So we sit down at the card table and I pick up the deck and start dealing. What's that you say? You want me to shuffle the cards? Oh, all right, if you insist. Some people are such sticklers for details.

Listing 3 shows ShuffleDeck.cfm. Its job is to take deckArray as created by CardsArray.cfm and produce a shuffled copy of it named shuffledDeckArray. Line 24 creates the copy then lines 27-31 take care of the shuffle.

There have been many articles written over the years on the subject of sorting and shuffling. There are probably as many shuffle algorithms available in the programming world as there are opinions on any given topic. Fortunately for us, there's a function in CFML that makes shuffling easy. That function is ArraySwap(). As its name implies, it swaps two specified elements of an array. So to do our shuffle, we just pick two random indexes between 1 and 52 using the RandRange() function and then swap those two elements in the deck using the ArraySwap() function.


More Stories By Jeff Peters

Jeff Peters works for Open Source Data Integration Software company XAware.

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