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ColdFusion: Article

All About Arrays: Part 2

All About Arrays: Part 2

In the last issue of CFDJ (December 2002), we looked at arrays, those enormously useful creatures that are underused by some developers. We confined ourselves to one-dimensional arrays - those arrays that look like a single row from a spreadsheet.

When Data Won't Fit into a One-Dimensional Array
But, of course, all data doesn't fit neatly into a single row. Consider, for instance, the case in which we want to keep track of five students who have each taken four tests. We might create a spreadsheet like this to keep track of their test scores:

How, though, do we get that into an array? We can fit those scores into an array only if we can expand our definition of array to mean something more than a single row of values. Luckily we can. We have just such a definition when we explore multidimensional arrays in ColdFusion.

Arrays of Arrays
You can best think of a multidimensional array as an "array of arrays." In the test scores example above, changing the image a bit can be helpful in understanding this concept. For the sake of clarity, I omit the descriptive information that isn't part of the array itself:

Now we have a series of one-dimensional arrays stacked on top of one another. That idea, as we'll see, is key to understanding how to use ColdFusion array functions with multidimensional arrays. But first, let's create this array.

Creating a Two-Dimensional Array
If we only had the first "row" to deal with (the test scores for the first test), creating the array would be easy:

onedee = ArrayNew(1);
onedee[1] = 93;
onedee[2] = 82;
onedee[3] = 99;
onedee[4] = 100;
onedee[5] = 74;

Each number in square brackets (the index) is the number of an associated column. To work with arrays in two dimensions (width and height), we need to use two indices to point at a specific array element. The first index will indicate the row number and the second will indicate its column number. The code for writing the "array of arrays" to keep track of all test scores is just a matter of keeping the row and column numbers straight.

twodee = ArrayNew(2);

twodee[1][1] = 93;
twodee[1][2] = 87;
twodee[1][3] = 99;
twodee[1][4] = 100;
twodee[1][5] = 74;

twodee[2][1] = 89;
twodee[2][2] = 78;
twodee[2][3] = 100;
twodee[2][4] = 82;
twodee[2][5] = 82;

twodee[3][1] = 94;
twodee[3][2] = 84;
twodee[3][3] = 92;
twodee[3][4] = 96;
twodee[3][5] = 84;

twodee[4][1] = 88;
twodee[4][2] = 81;
twodee[4][3] = 100;
twodee[4][4] = 94;
twodee[4][5] = 83;

Accessing Two-Dimensional Array Elements
How did Carla (who's represented by the third column) do on her second test (represented by the second row)? Let's find out:

Carla got a #twodee[2][3]# on her second test.

...and that snippet produces this:
Carla got a 100 on her second test.
How did Ellen do on her first test?

Ellen got a #twodee[1][5]# on her first test.

...which produces: Ellen got a 74 on her first test. Whoops, I guess we shouldn't have asked.

Looping over Two-Dimensional Arrays
Looping over one of these "stacked-up" arrays requires us to use two loops, one nested within the other. Start by looping over the rows and then within this loop, loop over the columns for each row. Here's the code:


That code will produce this screen (see Figure 4):

If you'd like it to look a bit more like a spreadsheet, you can adapt the code so that it places the information in a table:

<table border="thin" bordercolor="silver">

That code produces this screen (see Figure 5):

Working with ColdFusion Array Functions
How do ColdFusion's built-in array functions work on multidimensional arrays? Unfortunately, the ColdFusion documentation does not offer us much guidance on this, so we'll have to put on our lab coats and become scientists. Our first observation should be of the code above that loops over the twodee array. There, we make use of the ArrayLen() function.

The first time we use ArrayLen() is in the outer loop:

<cfloop from="1" to="#ArrayLen(twodee)#" index="rowNumber">

When we use the formulation ArrayLen(array_name), where array_name is a multidimensional array, ColdFusion returns the number of rows in the array, so ArrayLen(twodee) returns "4". But if array_name represents a one-dimensional array, ColdFusion returns the number of columns.

We need to get both rows and columns. Rows, we see, is quite simple - just provide a two-dimensional array to the ArrayLen() function. But to get columns, we must pass ArrayLen() a one-dimensional array - something we don't have. Or do we? Look again at Figure 3. I said that a multidimensional array can be thought of as a series of one-dimensional arrays and one-dimensional arrays are just what we need.

Looked at this way, twodee is a set of four stacked, one-dimensional arrays. We tell ColdFusion that we want to access one of these by providing the array name and an index representing the row number, like this: twodee[1]. That particular formulation represents the first row in our multidimensional array. What number will ArrayLen(twodee[1]) display?

If you said, "five", give yourself full marks. If you said, "four", you might not have seen that, by providing the index along with the array name, we are pointing at the first row, which is a one-dimensional array.

Most of the ColdFusion array functions are designed for one-dimensional arrays and can therefore be used by pointing to a one-dimensional array within a multidimensional array. I've taken the following material from the "ColdFusion Foundations" class I teach, as it illustrates the differences in operation of an array function depending on the type of array. Each of the illustrations assumes the existence of A, a one-dimensional array, and B, a two-dimensional array.

ArrayAppend(A, 'x')
ArrayAppend(B, 'x') Error

ArrayAppend(B[2], 'x')

ArrayAvg(A) 2.5
ArrayAvg(B) Error
ArrayAvg(B[2]) 5

ArrayClear(A) Yes
ArrayClear(B) Yes
ArrayClear(B[2]) Yes

ArrayDeleteAt(A, 2)

ArrayDeleteAt(B, 2)

ArrayDeleteAt(B[2], 2)

ArrayInsertAt(A, 3, 'x')
ArrayInsertAt(B, 2, 'x') Error

ArrayInsertAt(B[2], 2, 'x')

ArrayMax(A) 4
ArrayMax(B) Error
ArrayMax(B[2]) 8

ArrayMin(A) 1
ArrayMin(B) Error
ArrayMin(B[2]) 2

ArrayPrepend(A, 'x')
ArrayPrepend(B, 'x') Error

ArrayPrepend(B[2], 'x')

ArraySet(A, 1, 4, 2)
ArraySet(B, 1, 4, 2) Error

ArraySet(B[2], 1, 4, 2)

ArraySort (B, 'numeric', 'desc') Error

(B[2], 'numeric', 'desc')

ArraySum(A) 10
ArraySum(B) Error
ArraySum(B[2]) 20

ArraySwap(A, 3, 2)

ArraySwap(B, 3, 2)

ArraySwap(B[2], 3, 2)

ArrayToList(A) 1,2,3,4
ArrayToList(B) Error
ArrayToList(B[2], '|') 2|4|6|8

Once arrays are explained, their mystery vanishes and they become just what they were invented to be - extraordinarily helpful constructs that can actually make our code simpler. For more on arrays - including arrays of more than two dimensions - and for some exercises in arrays, go to halhelms.com and test your knowledge of arrays with this month's test/exercise.

More Stories By Hal Helms

Hal Helms is a well-known speaker/writer/strategist on software development issues. He holds training sessions on Java, ColdFusion, and software development processes. He authors a popular monthly newsletter series. For more information, contact him at hal (at) halhelms.com or see his website, www.halhelms.com.

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Most Recent Comments
TenTonJim 02/09/07 05:06:15 PM EST

Great tutorial on the arrays - I am still having trouble though. Say in your example... How would you display a name of the test across the top and the student names down the left column? I have been struggling with this one. Thanks!

Jim S.
Jacksonville, FL USA

Hal Helms 02/13/03 12:03:00 AM EST

Glad to hear that, Mike. They really are very simple if you get started with them in the right way.

Mike Elgers 01/15/03 10:15:00 PM EST

I've never quite understood arrays before and stayed away from them. After reading Hal's article, I think they sound really useful. Well written; I learned a lot.

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