208 lines
10 KiB
Plaintext
Generated
208 lines
10 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: Switch</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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onkeydown = (e) => {
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if (e.key == "ArrowLeft") {
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window.location.href = 'if-else';
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if (e.key == "ArrowRight") {
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window.location.href = 'arrays';
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<body>
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<div class="example" id="switch">
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<h2><a href="./">Go by Example</a>: Switch</h2>
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<table>
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<tr>
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<td class="docs">
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<p><em>Switch statements</em> express conditionals across many
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branches.</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/qVDqWoUQ6AI"><img title="Run code" src="play.png" class="run" /></a><img title="Copy code" src="clipboard.png" class="copy" />
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<pre class="chroma"><span class="kn">package</span> <span class="nx">main</span>
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</pre>
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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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<pre class="chroma"><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">"time"</span>
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<span class="p">)</span>
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</pre>
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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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<pre class="chroma"><span class="kd">func</span> <span class="nf">main</span><span class="p">()</span> <span class="p">{</span>
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</pre>
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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>Here’s a basic <code>switch</code>.</p>
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</td>
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<td class="code leading">
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<pre class="chroma">
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<span class="nx">i</span> <span class="o">:=</span> <span class="mi">2</span>
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<span class="nx">fmt</span><span class="p">.</span><span class="nf">Print</span><span class="p">(</span><span class="s">"Write "</span><span class="p">,</span> <span class="nx">i</span><span class="p">,</span> <span class="s">" as "</span><span class="p">)</span>
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<span class="k">switch</span> <span class="nx">i</span> <span class="p">{</span>
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<span class="k">case</span> <span class="mi">1</span><span class="p">:</span>
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<span class="nx">fmt</span><span class="p">.</span><span class="nf">Println</span><span class="p">(</span><span class="s">"one"</span><span class="p">)</span>
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<span class="k">case</span> <span class="mi">2</span><span class="p">:</span>
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<span class="nx">fmt</span><span class="p">.</span><span class="nf">Println</span><span class="p">(</span><span class="s">"two"</span><span class="p">)</span>
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<span class="k">case</span> <span class="mi">3</span><span class="p">:</span>
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<span class="nx">fmt</span><span class="p">.</span><span class="nf">Println</span><span class="p">(</span><span class="s">"three"</span><span class="p">)</span>
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<span class="p">}</span>
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</pre>
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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>You can use commas to separate multiple expressions
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in the same <code>case</code> statement. We use the optional
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<code>default</code> case in this example as well.</p>
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</td>
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<td class="code leading">
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<pre class="chroma">
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<span class="k">switch</span> <span class="nx">time</span><span class="p">.</span><span class="nf">Now</span><span class="p">().</span><span class="nf">Weekday</span><span class="p">()</span> <span class="p">{</span>
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<span class="k">case</span> <span class="nx">time</span><span class="p">.</span><span class="nx">Saturday</span><span class="p">,</span> <span class="nx">time</span><span class="p">.</span><span class="nx">Sunday</span><span class="p">:</span>
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<span class="nx">fmt</span><span class="p">.</span><span class="nf">Println</span><span class="p">(</span><span class="s">"It's the weekend"</span><span class="p">)</span>
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<span class="k">default</span><span class="p">:</span>
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<span class="nx">fmt</span><span class="p">.</span><span class="nf">Println</span><span class="p">(</span><span class="s">"It's a weekday"</span><span class="p">)</span>
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<span class="p">}</span>
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</pre>
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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><code>switch</code> without an expression is an alternate way
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to express if/else logic. Here we also show how the
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<code>case</code> expressions can be non-constants.</p>
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</td>
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<td class="code leading">
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<pre class="chroma">
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<span class="nx">t</span> <span class="o">:=</span> <span class="nx">time</span><span class="p">.</span><span class="nf">Now</span><span class="p">()</span>
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<span class="k">switch</span> <span class="p">{</span>
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<span class="k">case</span> <span class="nx">t</span><span class="p">.</span><span class="nf">Hour</span><span class="p">()</span> <span class="p"><</span> <span class="mi">12</span><span class="p">:</span>
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<span class="nx">fmt</span><span class="p">.</span><span class="nf">Println</span><span class="p">(</span><span class="s">"It's before noon"</span><span class="p">)</span>
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<span class="k">default</span><span class="p">:</span>
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<span class="nx">fmt</span><span class="p">.</span><span class="nf">Println</span><span class="p">(</span><span class="s">"It's after noon"</span><span class="p">)</span>
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<span class="p">}</span>
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</pre>
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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>A type <code>switch</code> compares types instead of values. You
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can use this to discover the type of an interface
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value. In this example, the variable <code>t</code> will have the
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type corresponding to its clause.</p>
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</td>
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<td class="code">
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<pre class="chroma">
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<span class="nx">whatAmI</span> <span class="o">:=</span> <span class="kd">func</span><span class="p">(</span><span class="nx">i</span> <span class="kd">interface</span><span class="p">{})</span> <span class="p">{</span>
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<span class="k">switch</span> <span class="nx">t</span> <span class="o">:=</span> <span class="nx">i</span><span class="p">.(</span><span class="kd">type</span><span class="p">)</span> <span class="p">{</span>
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<span class="k">case</span> <span class="kt">bool</span><span class="p">:</span>
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<span class="nx">fmt</span><span class="p">.</span><span class="nf">Println</span><span class="p">(</span><span class="s">"I'm a bool"</span><span class="p">)</span>
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<span class="k">case</span> <span class="kt">int</span><span class="p">:</span>
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<span class="nx">fmt</span><span class="p">.</span><span class="nf">Println</span><span class="p">(</span><span class="s">"I'm an int"</span><span class="p">)</span>
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<span class="k">default</span><span class="p">:</span>
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<span class="nx">fmt</span><span class="p">.</span><span class="nf">Printf</span><span class="p">(</span><span class="s">"Don't know type %T\n"</span><span class="p">,</span> <span class="nx">t</span><span class="p">)</span>
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<span class="p">}</span>
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<span class="p">}</span>
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<span class="nf">whatAmI</span><span class="p">(</span><span class="kc">true</span><span class="p">)</span>
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<span class="nf">whatAmI</span><span class="p">(</span><span class="mi">1</span><span class="p">)</span>
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<span class="nf">whatAmI</span><span class="p">(</span><span class="s">"hey"</span><span class="p">)</span>
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<span class="p">}</span>
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</pre>
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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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</td>
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<td class="code">
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<pre class="chroma"><span class="gp">$</span> go run switch.go
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<span class="go">Write 2 as two
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</span><span class="go">It's a weekday
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</span><span class="go">It's after noon
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</span><span class="go">I'm a bool
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</span><span class="go">I'm an int
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</span><span class="go">Don't know type string</span></pre>
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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="arrays">Arrays</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> and <a href="https://eli.thegreenplace.net">Eli Bendersky</a> | <a href="https://github.com/mmcgrana/gobyexample">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 \"time\"\u000A)\u000A');codeLines.push('func main() {\u000A');codeLines.push(' i :\u003D 2\u000A fmt.Print(\"Write \", i, \" as \")\u000A switch i {\u000A case 1:\u000A fmt.Println(\"one\")\u000A case 2:\u000A fmt.Println(\"two\")\u000A case 3:\u000A fmt.Println(\"three\")\u000A }\u000A');codeLines.push(' switch time.Now().Weekday() {\u000A case time.Saturday, time.Sunday:\u000A fmt.Println(\"It\'s the weekend\")\u000A default:\u000A fmt.Println(\"It\'s a weekday\")\u000A }\u000A');codeLines.push(' t :\u003D time.Now()\u000A switch {\u000A case t.Hour() \u003C 12:\u000A fmt.Println(\"It\'s before noon\")\u000A default:\u000A fmt.Println(\"It\'s after noon\")\u000A }\u000A');codeLines.push(' whatAmI :\u003D func(i interface{}) {\u000A switch t :\u003D i.(type) {\u000A case bool:\u000A fmt.Println(\"I\'m a bool\")\u000A case int:\u000A fmt.Println(\"I\'m an int\")\u000A default:\u000A fmt.Printf(\"Don\'t know type %T\\n\", t)\u000A }\u000A }\u000A whatAmI(true)\u000A whatAmI(1)\u000A whatAmI(\"hey\")\u000A}\u000A');codeLines.push('');
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