221 lines
9.1 KiB
Plaintext
221 lines
9.1 KiB
Plaintext
<!DOCTYPE html>
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<meta charset="utf-8">
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<title>Go by Example: Maps</title>
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<link rel=stylesheet href="site.css">
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<div class="example" id="maps">
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<h2><a href="./">Go by Example</a>: Maps</h2>
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<table>
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<tr>
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<td class="docs">
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<p><em>Maps</em> are Go’s built-in <a href="http://en.wikipedia.org/wiki/Associative_array">associative data type</a>
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(sometimes called <em>hashes</em> or <em>dicts</em> in other languages).</p>
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</td>
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<td class="code empty leading">
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<td class="docs">
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<td class="code leading">
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<a href="http://play.golang.org/p/U67R66Oab8r"><img title="Run code" class="run" src="data:image/png;base64,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" /></a>
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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 class="docs">
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<td class="code leading">
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<div class="highlight"><pre><span class="kn">import</span> <span class="s">"fmt"</span>
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</pre></div>
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<td class="code leading">
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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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</pre></div>
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</td>
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<td class="docs">
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<p>To create an empty map, use the builtin <code>make</code>:
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<code>make(map[key-type]val-type)</code>.</p>
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</td>
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<td class="code leading">
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<div class="highlight"><pre> <span class="nx">m</span> <span class="o">:=</span> <span class="nb">make</span><span class="p">(</span><span class="kd">map</span><span class="p">[</span><span class="kt">string</span><span class="p">]</span><span class="kt">int</span><span class="p">)</span>
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</pre></div>
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<td class="docs">
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<p>Set key/value pairs using typical <code>name[key] = val</code>
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syntax.</p>
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<td class="code leading">
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<div class="highlight"><pre> <span class="nx">m</span><span class="p">[</span><span class="s">"k1"</span><span class="p">]</span> <span class="p">=</span> <span class="mi">7</span>
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<span class="nx">m</span><span class="p">[</span><span class="s">"k2"</span><span class="p">]</span> <span class="p">=</span> <span class="mi">13</span>
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</pre></div>
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</td>
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</tr>
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<td class="docs">
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<p>Printing a map with e.g. <code>fmt.Println</code> will show all of
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its key/value pairs.</p>
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</td>
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<td class="code leading">
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<div class="highlight"><pre> <span class="nx">fmt</span><span class="p">.</span><span class="nx">Println</span><span class="p">(</span><span class="s">"map:"</span><span class="p">,</span> <span class="nx">m</span><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>Get a value for a key with <code>name[key]</code>.</p>
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</td>
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<td class="code leading">
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<div class="highlight"><pre> <span class="nx">v1</span> <span class="o">:=</span> <span class="nx">m</span><span class="p">[</span><span class="s">"k1"</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="s">"v1: "</span><span class="p">,</span> <span class="nx">v1</span><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>The builtin <code>len</code> returns the number of key/value
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pairs when called on a map.</p>
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</td>
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<td class="code leading">
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<div class="highlight"><pre> <span class="nx">fmt</span><span class="p">.</span><span class="nx">Println</span><span class="p">(</span><span class="s">"len:"</span><span class="p">,</span> <span class="nb">len</span><span class="p">(</span><span class="nx">m</span><span class="p">))</span>
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</pre></div>
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</td>
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</tr>
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<td class="docs">
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<p>The builtin <code>delete</code> removes key/value pairs from
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a map.</p>
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</td>
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<td class="code leading">
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<div class="highlight"><pre> <span class="nb">delete</span><span class="p">(</span><span class="nx">m</span><span class="p">,</span> <span class="s">"k2"</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="s">"map:"</span><span class="p">,</span> <span class="nx">m</span><span class="p">)</span>
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</pre></div>
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</td>
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</tr>
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<td class="docs">
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<p>The optional second return value when getting a
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value from a map indicates if the key was present
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in the map. This can be used to disambiguate
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between missing keys and keys with zero values
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like <code>0</code> or <code>""</code>. Here we didn’t need the value
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itself, so we ignored it with the <em>blank identifier</em>
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<code>_</code>.</p>
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</td>
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<td class="code leading">
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<div class="highlight"><pre> <span class="nx">_</span><span class="p">,</span> <span class="nx">prs</span> <span class="o">:=</span> <span class="nx">m</span><span class="p">[</span><span class="s">"k2"</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="s">"prs:"</span><span class="p">,</span> <span class="nx">prs</span><span class="p">)</span>
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</pre></div>
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</td>
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<td class="docs">
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<p>You can also declare and initialize a new map in
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the same line with this syntax.</p>
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</td>
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<td class="code">
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<div class="highlight"><pre> <span class="nx">n</span> <span class="o">:=</span> <span class="kd">map</span><span class="p">[</span><span class="kt">string</span><span class="p">]</span><span class="kt">int</span><span class="p">{</span><span class="s">"foo"</span><span class="p">:</span> <span class="mi">1</span><span class="p">,</span> <span class="s">"bar"</span><span class="p">:</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="nx">Println</span><span class="p">(</span><span class="s">"map:"</span><span class="p">,</span> <span class="nx">n</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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</table>
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<table>
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<td class="docs">
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<p>Note that maps appear in the form <code>map[k:v k:v]</code> when
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printed with <code>fmt.Println</code>.</p>
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</td>
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<td class="code">
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<div class="highlight"><pre><span class="gp">$</span> go run maps.go
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<span class="go">map: map[k1:7 k2:13]</span>
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<span class="go">v1: 7</span>
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<span class="go">len: 2</span>
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<span class="go">map: map[k1:7]</span>
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<span class="go">prs: false</span>
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<span class="go">map: map[bar:2 foo:1]</span>
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</pre></div>
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</table>
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<p class="next">
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Next example: <a href="range">Range</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/maps">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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</body>
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</html>
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