JavaScript And DHTML Cookbook [Electronic resources] نسخه متنی

3.2 Creating a Multidimensional Array

NN 3, IE 4

3.2.1 Problem

You want to consolidate data in an
array construction of two dimensionssuch as a tableor
even more dimensions.

3.2.2 Solution

Create an array
of arrays. As an example, consider the following small table of
regional sales data:

To place the data portions of this table into an array that has three
items (one for each region row), in which each item contains an array
of four nested items (sales figures for each quarter column for that
region), you can use a variety of array-creation syntaxes. A
comparatively long version creates each of the nested arrays first,
and then assigns those nested arrays to the outer array:

var eastArray = new Array(2300, 3105, 2909, 4800);
var centralArray = new Array(1800, 1940, 2470, 4350);
var westArray = new Array(900, 1200, 1923, 3810);
var salesArray = new Array(eastArray, centralArray, westArray);

The most compact array creation approach is to use the bracket
shortcuts exclusively:

var salesArray = [[2300, 3105, 2909, 4800], 
[1800, 1940, 2470, 4350], 
[900, 1200, 1923, 3810]];

To access any nested item within
salesArray, use a double index. For example, to
reach the first item (East Q1), the reference is:

salesArray[0][0]

There are no commas or other symbols allowed between the bracketed
index values in this kind of reference. The first index applies to
the first-level array, while the second applies to the nested arrays.
Therefore, to reach the Central region's Q3 sales,
the reference is:

salesArray[1][2]

You may read and write to these multidimensional array items just
like any other array items.

3.2.3 Discussion

There is no practical limit to the number of nesting levels you can
create for a multidimensional array. For each dimension, lengthen the
reference to the most deeply nested items with another bracketed
index value. See Recipe 3.4 for using loops to inspect every item in
a deeply nested array.

One potential problem with using a multidimensional array is that you
may lose track of what a particular entry represents. When you look
at the array creation examples just shown, the numbers lose their
contextual meaning with respect to region or quarter. Their position
in the two-dimensional array is all that the numbers know about. It
is up to the rest of your scripts to keep the relationships between
the data points and their meanings straight. In many cases, you may
be better served by creating an array of custom objects. The objects
can contain properties that provide labels and context for the raw
data. See Recipe 3.8 and Recipe 3.9 for additional thoughts on the issue.

3.2.4 See Also

Recipe 3.4 to see how to iterate through simple and multidimensional
arrays; Recipe 3.8 for using an array of objects in place of a
multidimensional array; Recipe 3.9 for a custom object implementation
of the sales example and how to create a simulated hash table to
speed access to a particular entry.