/gobyexample

Permalink
Find file
gobyexample/public/sorting-by-functions
Fetching contributors…
Cannot retrieve contributors at this time
Raw Blame History
168 lines (136 sloc) 7.47 KB
<!DOCTYPE html>
<html>
<head>
<meta http-eqiv="content-type" content="text/html;charset=utf-8">
<title>Go by Example: Sorting by Functions</title>
<link rel=stylesheet href="site.css">
</head>
<script type="text/javascript">
if (window.location.host == "gobyexample.com") {
var _gaq = _gaq || [];
_gaq.push(['_setAccount', 'UA-34996217-1']);
_gaq.push(['_trackPageview']);
(function() {
var ga = document.createElement('script'); ga.type = 'text/javascript'; ga.async = true;
ga.src = ('https:' == document.location.protocol ? 'https://ssl' : 'http://www') + '.google-analytics.com/ga.js';
var s = document.getElementsByTagName('script')[0]; s.parentNode.insertBefore(ga, s);
})();
}
</script>
<body>
<div class="example" id="sorting-by-functions">
<h2><a href="./">Go by Example</a>: Sorting by Functions</h2>
<table>
<tr>
<td class="docs">
<p>Sometimes we&rsquo;ll want to sort a collection by something
other than its natural order. For example, suppose we
wanted to sort strings by their length instead of
alphabetically. Here&rsquo;s an example of custom sorts sorts
in Go.</p>
</td>
<td class="code empty leading">
</td>
</tr>
<tr>
<td class="docs">
</td>
<td class="code leading">
<div class="highlight"><pre><span class="kn">package</span> <span class="nx">main</span>
</pre></div>
</td>
</tr>
<tr>
<td class="docs">
</td>
<td class="code leading">
<div class="highlight"><pre><span class="kn">import</span> <span class="s">&quot;sort&quot;</span>
<span class="kn">import</span> <span class="s">&quot;fmt&quot;</span>
</pre></div>
</td>
</tr>
<tr>
<td class="docs">
<p>In order to sort by a custom function in Go, we need a
corresponding type. Here we&rsquo;ve created a <code>ByLength</code>
type that is just an alias for the builtin <code>[]string</code>
type.</p>
</td>
<td class="code leading">
<div class="highlight"><pre><span class="kd">type</span> <span class="nx">ByLength</span> <span class="p">[]</span><span class="kt">string</span>
</pre></div>
</td>
</tr>
<tr>
<td class="docs">
<p>We implement <code>sort.Interface</code> - <code>Len</code>, <code>Less</code>, and
<code>Swap</code> - on our type so we can use the <code>sort</code> package&rsquo;s
generic <code>Sort</code> function. <code>Len</code> and <code>Swap</code>
will usually be similar accross types and <code>Less</code> will
hold the actual custom sorting logic. In our case we
want to sort in order of increasing string length, so
we use <code>len(s[i])</code> and <code>len(s[j])</code> here.</p>
</td>
<td class="code leading">
<div class="highlight"><pre><span class="kd">func</span> <span class="p">(</span><span class="nx">s</span> <span class="nx">ByLength</span><span class="p">)</span> <span class="nx">Len</span><span class="p">()</span> <span class="kt">int</span> <span class="p">{</span>
<span class="k">return</span> <span class="nb">len</span><span class="p">(</span><span class="nx">s</span><span class="p">)</span>
<span class="p">}</span>
<span class="kd">func</span> <span class="p">(</span><span class="nx">s</span> <span class="nx">ByLength</span><span class="p">)</span> <span class="nx">Swap</span><span class="p">(</span><span class="nx">i</span><span class="p">,</span> <span class="nx">j</span> <span class="kt">int</span><span class="p">)</span> <span class="p">{</span>
<span class="nx">s</span><span class="p">[</span><span class="nx">i</span><span class="p">],</span> <span class="nx">s</span><span class="p">[</span><span class="nx">j</span><span class="p">]</span> <span class="p">=</span> <span class="nx">s</span><span class="p">[</span><span class="nx">j</span><span class="p">],</span> <span class="nx">s</span><span class="p">[</span><span class="nx">i</span><span class="p">]</span>
<span class="p">}</span>
<span class="kd">func</span> <span class="p">(</span><span class="nx">s</span> <span class="nx">ByLength</span><span class="p">)</span> <span class="nx">Less</span><span class="p">(</span><span class="nx">i</span><span class="p">,</span> <span class="nx">j</span> <span class="kt">int</span><span class="p">)</span> <span class="kt">bool</span> <span class="p">{</span>
<span class="k">return</span> <span class="nb">len</span><span class="p">(</span><span class="nx">s</span><span class="p">[</span><span class="nx">i</span><span class="p">])</span> <span class="p">&lt;</span> <span class="nb">len</span><span class="p">(</span><span class="nx">s</span><span class="p">[</span><span class="nx">j</span><span class="p">])</span>
<span class="p">}</span>
</pre></div>
</td>
</tr>
<tr>
<td class="docs">
<p>With all of this in place, we can now implement our
custom sort by casting the original <code>fruits</code> slice to
<code>ByLength</code>, and then use <code>sort.Sort</code> on that typed
slice.</p>
</td>
<td class="code">
<div class="highlight"><pre><span class="kd">func</span> <span class="nx">main</span><span class="p">()</span> <span class="p">{</span>
<span class="nx">fruits</span> <span class="o">:=</span> <span class="p">[]</span><span class="kt">string</span><span class="p">{</span><span class="s">&quot;peach&quot;</span><span class="p">,</span> <span class="s">&quot;banana&quot;</span><span class="p">,</span> <span class="s">&quot;kiwi&quot;</span><span class="p">}</span>
<span class="nx">sort</span><span class="p">.</span><span class="nx">Sort</span><span class="p">(</span><span class="nx">ByLength</span><span class="p">(</span><span class="nx">fruits</span><span class="p">))</span>
<span class="nx">fmt</span><span class="p">.</span><span class="nx">Println</span><span class="p">(</span><span class="nx">fruits</span><span class="p">)</span>
<span class="p">}</span>
</pre></div>
</td>
</tr>
</table>
<table>
<tr>
<td class="docs">
<p>Running our program shows a list sorted by string
length, as desired.</p>
</td>
<td class="code leading">
<div class="highlight"><pre><span class="gp">$</span> go run sorting-by-functions.go
<span class="go">[kiwi peach banana]</span>
</pre></div>
</td>
</tr>
<tr>
<td class="docs">
<p>By following this same pattern of creating a custom
type, implementing the three <code>Interface</code> methods on that
type, and then calling sort.Sort on a collection of that
custom type, we can sort Go slices by arbitrary
functions.</p>
</td>
<td class="code empty">
</td>
</tr>
</table>
<p class="next">
Next example: <a href="panic">Panic</a>.
</p>
<p class="footer">
<a href="https://twitter.com/gobyexample">@gobyexample</a> | <a href="mailto:mmcgrana@gmail.com">feedback</a> | <a href="https://github.com/mmcgrana/gobyexample/blob/master/examples/sorting-by-functions">source</a> | <a href="https://github.com/mmcgrana/gobyexample#license">license</a>
</p>
</div>
</body>
</html>