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<title>Go by Example: Maps</title>
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<div class="example" id="maps">
<h2><a href="./">Go by Example</a>: Maps</h2>
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<p><em>Maps</em> are Go&rsquo;s built-in <a href="http://en.wikipedia.org/wiki/Associative_array">associative data type</a>
(sometimes called <em>hashes</em> or <em>dicts</em> in other languages).</p>
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<a href="http://play.golang.org/p/E6cGoiKqka"><img title="Run code" src="play.png" class="run" /></a>
<div class="highlight"><pre><span class="kn">package</span> <span class="nx">main</span>
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<div class="highlight"><pre><span class="kn">import</span> <span class="s">&quot;fmt&quot;</span>
</pre></div>
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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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<p>To create an empty map, use the builtin <code>make</code>:
<code>make(map[key-type]val-type)</code>.</p>
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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>
</pre></div>
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<p>Set key/value pairs using typical <code>name[key] = val</code>
syntax.</p>
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<div class="highlight"><pre> <span class="nx">m</span><span class="p">[</span><span class="s">&quot;k1&quot;</span><span class="p">]</span> <span class="p">=</span> <span class="mi">7</span>
<span class="nx">m</span><span class="p">[</span><span class="s">&quot;k2&quot;</span><span class="p">]</span> <span class="p">=</span> <span class="mi">13</span>
</pre></div>
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<p>Printing a map with e.g. <code>Println</code> will show all of
its key/value pairs.</p>
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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">&quot;map:&quot;</span><span class="p">,</span> <span class="nx">m</span><span class="p">)</span>
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<p>Get a value for a key with <code>name[key]</code>.</p>
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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">&quot;k1&quot;</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="s">&quot;v1: &quot;</span><span class="p">,</span> <span class="nx">v1</span><span class="p">)</span>
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<p>The builtin <code>len</code> returns the number of key/value
pairs when called on a map.</p>
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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">&quot;len:&quot;</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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<p>The builtin <code>delete</code> removes key/value pairs from
a map.</p>
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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">&quot;k2&quot;</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="s">&quot;map:&quot;</span><span class="p">,</span> <span class="nx">m</span><span class="p">)</span>
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<p>The optional second return value when getting a
value from a map indicates if the key was present
in the map. This can be used to disambiguate
between missing keys and keys with zero values
like <code>0</code> or <code>&quot;&quot;</code>. Here we didn&rsquo;t need the value
itself, so we ignored it with the <em>blank identifier</em>
<code>_</code>.</p>
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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">&quot;k2&quot;</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="s">&quot;prs:&quot;</span><span class="p">,</span> <span class="nx">prs</span><span class="p">)</span>
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<p>You can also declare and initialize a new map in
the same line with this syntax.</p>
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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">&quot;foo&quot;</span><span class="p">:</span> <span class="mi">1</span><span class="p">,</span> <span class="s">&quot;bar&quot;</span><span class="p">:</span> <span class="mi">2</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="s">&quot;map:&quot;</span><span class="p">,</span> <span class="nx">n</span><span class="p">)</span>
<span class="p">}</span>
</pre></div>
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<p>Note that maps appear in the form <code>map[k:v k:v]</code> when
printed with <code>fmt.Println</code>.</p>
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<div class="highlight"><pre><span class="gp">$</span> go run maps.go
<span class="go">map: map[k1:7 k2:13]</span>
<span class="go">v1: 7</span>
<span class="go">len: 2</span>
<span class="go">map: map[k1:7]</span>
<span class="go">prs: false</span>
<span class="go">map: map[foo:1 bar:2]</span>
</pre></div>
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<p class="next">
Next example: <a href="range">Range</a>.
</p>
<p class="footer">
by <a href="https://twitter.com/mmcgrana">@mmcgrana</a> | <a href="mailto:mmcgrana@gmail.com">feedback</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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