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<div class="example" id="sorting-by-functions">
<h2><a href="./">Go by Example</a>: Sorting by Functions</h2>
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<p>Sometimes we’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’s an example of custom sorts sorts
in Go.</p>
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<a href="http://play.golang.org/p/M06NADP985"><img title="Run code" src="play.png" class="run" /></a>
<div class="highlight"><pre><span class="kn">package</span> <span class="nx">main</span>
</pre></div>
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<div class="highlight"><pre><span class="kn">import</span> <span class="s">"sort"</span>
<span class="kn">import</span> <span class="s">"fmt"</span>
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<p>In order to sort by a custom function in Go, we need a
corresponding type. Here we’ve created a <code>ByLength</code>
type that is just an alias for the builtin <code>[]string</code>
type.</p>
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<div class="highlight"><pre><span class="kd">type</span> <span class="nx">ByLength</span> <span class="p">[]</span><span class="kt">string</span>
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<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’s
generic <code>Sort</code> function. <code>Len</code> and <code>Swap</code>
will usually be similar across 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>
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<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"><</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>
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<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>
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<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">"peach"</span><span class="p">,</span> <span class="s">"banana"</span><span class="p">,</span> <span class="s">"kiwi"</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>
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<p>Running our program shows a list sorted by string
length, as desired.</p>
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<div class="highlight"><pre><span class="gp">$</span> go run sorting-by-functions.go
<span class="go">[kiwi peach banana]</span>
</pre></div>
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<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>
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<p class="next">
Next example: <a href="panic">Panic</a>.
</p>
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