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<h2><a href="./">Go by Example 中文版</a>: Map</h2>
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<p><em>map</em> 是 Go 内建的<a href="http://zh.wikipedia.org/wiki/关联数组">关联数据类型</a>
（在一些其他的语言中也被称为 <em>哈希(hash)</em> 或者 <em>字典(dict)</em> ）。</p>
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<a href="http://play.golang.org/p/pcmVt5kka8w"><img title="Run code" src="play.png" class="run" /></a><img title="Copy code" src="clipboard.png" class="copy" />
<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">"fmt"</span>
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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>要创建一个空 map，需要使用内建函数 <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>
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<p>使用典型的 <code>make[key] = val</code> 语法来设置键值对。</p>
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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>
<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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<p>打印 map。例如，使用 <code>fmt.Println</code> 打印一个 map，会输出它所有的键值对。</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">"map:"</span><span class="p">,</span> <span class="nx">m</span><span class="p">)</span>
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<p>使用 <code>name[key]</code> 来获取一个键的值。</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">"k1"</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">"v1: "</span><span class="p">,</span> <span class="nx">v1</span><span class="p">)</span>
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<p>内建函数 <code>len</code> 可以返回一个 map 的键值对数量。</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">"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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<p>内建函数 <code>delete</code> 可以从一个 map 中移除键值对。</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">"k2"</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">"map:"</span><span class="p">,</span> <span class="nx">m</span><span class="p">)</span>
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<p>当从一个 map 中取值时，还有可以选择是否接收的第二个返回值，该值表明了 map 中是否存在这个键。
这可以用来消除 <code>键不存在</code> 和 <code>键的值为零值</code> 产生的歧义，
例如 <code>0</code> 和 <code>""</code>。这里我们不需要值，所以用 <em>空白标识符(blank identifier)</em> _ 将其忽略。</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">"k2"</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">"prs:"</span><span class="p">,</span> <span class="nx">prs</span><span class="p">)</span>
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<p>你也可以通过右边的语法在一行代码中声明并初始化一个新的 map。</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">"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>
<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>
<span class="p">}</span>
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<p>注意，使用 <code>fmt.Println</code> 打印一个 map 的时候，
是以 <code>map[k:v k:v]</code> 的格式输出的。</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>
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下一个例子: <a href="range">Range 遍历</a>
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