185 lines
8.1 KiB
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185 lines
8.1 KiB
Plaintext
Generated
<!DOCTYPE html>
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<html>
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<head>
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<meta charset="utf-8">
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<title>Go by Example: Sorting by Functions</title>
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<link rel=stylesheet href="site.css">
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</head>
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<script>
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if (e.key == "ArrowLeft") {
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window.location.href = 'sorting';
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window.location.href = 'panic';
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<body>
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<div class="example" id="sorting-by-functions">
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<h2><a href="./">Go by Example</a>: Sorting by Functions</h2>
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<table>
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<tr>
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<td class="docs">
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<p>Sometimes we’ll want to sort a collection by something
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other than its natural order. For example, suppose we
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wanted to sort strings by their length instead of
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alphabetically. Here’s an example of custom sorts
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in Go.</p>
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</td>
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<td class="code empty leading">
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</td>
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</tr>
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<tr>
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<td class="docs">
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</td>
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<td class="code leading">
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<a href="http://play.golang.org/p/Jtxf94x94Hx"><img title="Run code" src="play.png" class="run" /></a><img title="Copy code" src="clipboard.png" class="copy" />
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<div class="highlight"><pre><span class="kn">package</span> <span class="nx">main</span>
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</pre></div>
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</td>
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</tr>
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<tr>
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<td class="docs">
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</td>
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<td class="code leading">
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<div class="highlight"><pre><span class="kn">import</span> <span class="p">(</span>
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<span class="s">"fmt"</span>
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<span class="s">"sort"</span>
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<span class="p">)</span>
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</pre></div>
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</td>
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</tr>
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<tr>
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<td class="docs">
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<p>In order to sort by a custom function in Go, we need a
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corresponding type. Here we’ve created a <code>byLength</code>
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type that is just an alias for the builtin <code>[]string</code>
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type.</p>
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</td>
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<td class="code leading">
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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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</pre></div>
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</td>
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</tr>
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<tr>
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<td class="docs">
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<p>We implement <code>sort.Interface</code> - <code>Len</code>, <code>Less</code>, and
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<code>Swap</code> - on our type so we can use the <code>sort</code> package’s
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generic <code>Sort</code> function. <code>Len</code> and <code>Swap</code>
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will usually be similar across types and <code>Less</code> will
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hold the actual custom sorting logic. In our case we
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want to sort in order of increasing string length, so
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we use <code>len(s[i])</code> and <code>len(s[j])</code> here.</p>
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</td>
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<td class="code leading">
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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>
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<span class="k">return</span> <span class="nb">len</span><span class="p">(</span><span class="nx">s</span><span class="p">)</span>
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<span class="p">}</span>
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<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>
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<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>
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<span class="p">}</span>
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<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>
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<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>
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<span class="p">}</span>
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</pre></div>
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</td>
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</tr>
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<tr>
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<td class="docs">
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<p>With all of this in place, we can now implement our
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custom sort by converting the original <code>fruits</code> slice
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to <code>byLength</code>, and then use <code>sort.Sort</code> on that typed
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slice.</p>
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</td>
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<td class="code">
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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>
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<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>
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<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>
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<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>
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<span class="p">}</span>
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</pre></div>
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</td>
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</tr>
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</table>
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<table>
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<tr>
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<td class="docs">
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<p>Running our program shows a list sorted by string
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length, as desired.</p>
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</td>
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<td class="code leading">
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<div class="highlight"><pre><span class="gp">$</span> go run sorting-by-functions.go
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<span class="go">[kiwi peach banana]</span>
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</pre></div>
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</td>
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</tr>
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<tr>
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<td class="docs">
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<p>By following this same pattern of creating a custom
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type, implementing the three <code>Interface</code> methods on that
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type, and then calling sort.Sort on a collection of that
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custom type, we can sort Go slices by arbitrary
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functions.</p>
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</td>
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<td class="code empty">
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</td>
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</tr>
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</table>
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<p class="next">
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Next example: <a href="panic">Panic</a>.
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</p>
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<p class="footer">
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by <a href="https://markmcgranaghan.com">Mark McGranaghan</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>
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</p>
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</div>
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<script>
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var codeLines = [];
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codeLines.push('');codeLines.push('package main\u000A');codeLines.push('import (\u000A \"fmt\"\u000A \"sort\"\u000A)\u000A');codeLines.push('type byLength []string\u000A');codeLines.push('func (s byLength) Len() int {\u000A return len(s)\u000A}\u000Afunc (s byLength) Swap(i, j int) {\u000A s[i], s[j] = s[j], s[i]\u000A}\u000Afunc (s byLength) Less(i, j int) bool {\u000A return len(s[i]) \x3C len(s[j])\u000A}\u000A');codeLines.push('func main() {\u000A fruits := []string{\"peach\", \"banana\", \"kiwi\"}\u000A sort.Sort(byLength(fruits))\u000A fmt.Println(fruits)\u000A}\u000A');codeLines.push('');codeLines.push('');
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