109 lines
3.7 KiB
HTML
109 lines
3.7 KiB
HTML
---
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layout: page
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category-page: scripts
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category-title: Scripting
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tags: array variable many index selector
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author: Marco Tereh
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title: Arrays
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previous-page: pages/scripts/parameter-expansion.html
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next-page: while-loop.html
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---
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There is a special kind of <a href="variables.html">variable</a>, called an array.<br>
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While variables hold a single value, arrays hold many. They could be called a list of variables.
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The simplest way to create an array is using this format:
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{% highlight bash %}
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names=("Anna" "Bob" "Clarice" "Dee")
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{% endhighlight %}
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This will create an array, called <code>names</code>, which holds 4 values.
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The values in the array can be accessed using the syntax
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{% highlight bash %}
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${names[SELECTOR]}
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{% endhighlight %}
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where SELECTOR is the selector associated to the desired element. There are two kinds of
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arrays, depending on what kind of selector is used.<br>
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<code>names</code> is an <i>indexed</i> array, meaning the selector is a number.
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Since we didn't specify any when we defined it, <code>names</code>' selectors start at
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0 and count up from there.<br>
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Thus we can access the string <code>"Anna"</code> using <code>names[0]</code>.
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In the same way we use <code>names[1]</code> to access <code>"Bob"</code> and so on.<br>
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If we want to specify a particular index for our values when creating the array
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we can do it like this:
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{% highlight bash %}
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names=([1]="Anna" [2]="Bob" [5]="Ernie" [12]="Luigi")
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{% endhighlight %}
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As you might have guessed, this will create an array with <code>"Anna"</code> at index 1,
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<code>"Bob"</code> at index 2, <code>"Ernie"</code> at index 3 and so on.<br>
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If our indices are all sequential and we just want to change the starting index, we can use
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{% highlight bash %}
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names=([12]="Luigi" "Mario" "Nate")
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{% endhighlight %}
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Which will create an array with <i>"Luigi"</i> at index 12,
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<i>"Mario"</i> at index 13 and <i>"Nate"</i> at index 14.<br>
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An indexed array's selector can also be negative, which will start counting from the end,
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so <i>${names[-1]}</i> means <i>"Nate"</i>, <code>${names[-3]}</code> is
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<i>"Luigi"</i> and so on.<br>
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The other kind of array is an <i>associative</i> array.
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In an associative array, the values are not mapped to a number but to some other string.<br>
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Here's an example of an associative array:
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{% highlight bash %}
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colors=([white]="#FFFFFF" [black]="#000000" [red]="#FF0000" [yellow]="#00FFFF")
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{% endhighlight %}
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In this case it is not possible to omit the selector as there is no logical word
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that "follows" white. To access its members, we use the same syntax but write a string
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instead of a number as the selector.<br>
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Arrays, both kinds, can also be modified after creation. We can use
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{% highlight bash %}
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colors[blue]="#0000FF"
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{% endhighlight %}
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to add an element to the array (or modify it if it exists already) and
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{% highlight bash %}
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unset colors[black]
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{% endhighlight %}
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to remove one.<br>
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The <code>@</code> and <code>*</code> selectors are special - they cannot be written to but
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reading from them will return <b>all</b> the values in the array, separated by spaces.
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Similarly, <code>${!colors[@]}</code> is a list of
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all the selectors (or indices) that were assigned a value.<br>
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<a href="parameter_expansion.html">Parameter Expansion</a> works on indexed arrays as well,
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much like it works on strings, but it manipulates the members of the array instead of characters.
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For example:
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{% highlight bash %}
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${names[@]: 13}
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{% endhighlight %}
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is equivalent to <code>"Mario" "Luigi"</code>, while
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{% highlight bash %}
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${#names[@]}
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{% endhighlight %}
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counts the number of elements in the array, in this case 3.<br><br>
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Further reading: <a href="http://www.gnu.org/software/bash/manual/bashref.html#Arrays">
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the bash reference manual</a>
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