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<meta name="description" content="遍历 - for-range语法 # for-range语法可以用来遍历数组、指向数组的指针，切片、字符串、映射和通道。
遍历数组 # 当遍历一个数组a时候，循环范围会从0到len(a) -1：
func main() { var a [3]int for i, v := range a { fmt.Println(i, v) } for i, v := range &amp;a { fmt.Println(i, v) } } 遍历切片 # 当遍历一个切片s时候，循环范围会从0到len(s) -1，若切片是nil，则迭代次数是0次：
func main() { a := make([]int, 3) for i, v := range a { fmt.Println(i, v) } a = nil for i, v := range a { fmt.Println(i, v) } } for-range切片时候可以边遍历边append吗？ # 当遍历切片时候，可以边遍历边append操作，这并不会造成死循环。因为遍历之前已经确定了循环范围，遍历操作相当如下伪代码：">
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<meta property="og:description" content="遍历 - for-range语法 # for-range语法可以用来遍历数组、指向数组的指针，切片、字符串、映射和通道。
遍历数组 # 当遍历一个数组a时候，循环范围会从0到len(a) -1：
func main() { var a [3]int for i, v := range a { fmt.Println(i, v) } for i, v := range &amp;a { fmt.Println(i, v) } } 遍历切片 # 当遍历一个切片s时候，循环范围会从0到len(s) -1，若切片是nil，则迭代次数是0次：
func main() { a := make([]int, 3) for i, v := range a { fmt.Println(i, v) } a = nil for i, v := range a { fmt.Println(i, v) } } for-range切片时候可以边遍历边append吗？ # 当遍历切片时候，可以边遍历边append操作，这并不会造成死循环。因为遍历之前已经确定了循环范围，遍历操作相当如下伪代码：" />
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<strong>for-range语法</strong>
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<article class="markdown"><h1 id="遍历---for-range语法">
遍历 - for-range语法
<a class="anchor" href="#%e9%81%8d%e5%8e%86---for-range%e8%af%ad%e6%b3%95">#</a>
</h1>
<p>for-range语法可以用来遍历数组、指向数组的指针，切片、字符串、映射和通道。</p>
<h2 id="遍历数组">
遍历数组
<a class="anchor" href="#%e9%81%8d%e5%8e%86%e6%95%b0%e7%bb%84">#</a>
</h2>
<p>当遍历一个数组a时候，循环范围会从0到len(a) -1：</p>
<div class="highlight"><pre tabindex="0" style="color:#f8f8f2;background-color:#272822;-moz-tab-size:4;-o-tab-size:4;tab-size:4;"><code class="language-go" data-lang="go"><span style="display:flex;"><span><span style="color:#66d9ef">func</span> <span style="color:#a6e22e">main</span>() {
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">var</span> <span style="color:#a6e22e">a</span> [<span style="color:#ae81ff">3</span>]<span style="color:#66d9ef">int</span>
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">for</span> <span style="color:#a6e22e">i</span>, <span style="color:#a6e22e">v</span> <span style="color:#f92672">:=</span> <span style="color:#66d9ef">range</span> <span style="color:#a6e22e">a</span> {
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">fmt</span>.<span style="color:#a6e22e">Println</span>(<span style="color:#a6e22e">i</span>, <span style="color:#a6e22e">v</span>)
</span></span><span style="display:flex;"><span> }
</span></span><span style="display:flex;"><span>
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">for</span> <span style="color:#a6e22e">i</span>, <span style="color:#a6e22e">v</span> <span style="color:#f92672">:=</span> <span style="color:#66d9ef">range</span> <span style="color:#f92672">&amp;</span><span style="color:#a6e22e">a</span> {
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">fmt</span>.<span style="color:#a6e22e">Println</span>(<span style="color:#a6e22e">i</span>, <span style="color:#a6e22e">v</span>)
</span></span><span style="display:flex;"><span> }
</span></span><span style="display:flex;"><span>}
</span></span></code></pre></div><h2 id="遍历切片">
遍历切片
<a class="anchor" href="#%e9%81%8d%e5%8e%86%e5%88%87%e7%89%87">#</a>
</h2>
<p>当遍历一个切片s时候，循环范围会从0到len(s) -1，若切片是nil，则迭代次数是0次：</p>
<div class="highlight"><pre tabindex="0" style="color:#f8f8f2;background-color:#272822;-moz-tab-size:4;-o-tab-size:4;tab-size:4;"><code class="language-go" data-lang="go"><span style="display:flex;"><span><span style="color:#66d9ef">func</span> <span style="color:#a6e22e">main</span>() {
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">a</span> <span style="color:#f92672">:=</span> make([]<span style="color:#66d9ef">int</span>, <span style="color:#ae81ff">3</span>)
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">for</span> <span style="color:#a6e22e">i</span>, <span style="color:#a6e22e">v</span> <span style="color:#f92672">:=</span> <span style="color:#66d9ef">range</span> <span style="color:#a6e22e">a</span> {
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">fmt</span>.<span style="color:#a6e22e">Println</span>(<span style="color:#a6e22e">i</span>, <span style="color:#a6e22e">v</span>)
</span></span><span style="display:flex;"><span> }
</span></span><span style="display:flex;"><span>
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">a</span> = <span style="color:#66d9ef">nil</span>
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">for</span> <span style="color:#a6e22e">i</span>, <span style="color:#a6e22e">v</span> <span style="color:#f92672">:=</span> <span style="color:#66d9ef">range</span> <span style="color:#a6e22e">a</span> {
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">fmt</span>.<span style="color:#a6e22e">Println</span>(<span style="color:#a6e22e">i</span>, <span style="color:#a6e22e">v</span>)
</span></span><span style="display:flex;"><span> }
</span></span><span style="display:flex;"><span>}
</span></span></code></pre></div><h3 id="for-range切片时候可以边遍历边append吗">
for-range切片时候可以边遍历边append吗？
<a class="anchor" href="#for-range%e5%88%87%e7%89%87%e6%97%b6%e5%80%99%e5%8f%af%e4%bb%a5%e8%be%b9%e9%81%8d%e5%8e%86%e8%be%b9append%e5%90%97">#</a>
</h3>
<p>当遍历切片时候，可以边遍历边append操作，这并不会造成死循环。因为遍历之前已经确定了循环范围，遍历操作相当如下伪代码：</p>
<div class="highlight"><pre tabindex="0" style="color:#f8f8f2;background-color:#272822;-moz-tab-size:4;-o-tab-size:4;tab-size:4;"><code class="language-go" data-lang="go"><span style="display:flex;"><span><span style="color:#a6e22e">len_temp</span> <span style="color:#f92672">:=</span> len(<span style="color:#66d9ef">range</span>) <span style="color:#75715e">// 循环上界
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span><span style="color:#a6e22e">range_temp</span> <span style="color:#f92672">:=</span> <span style="color:#66d9ef">range</span>
</span></span><span style="display:flex;"><span><span style="color:#66d9ef">for</span> <span style="color:#a6e22e">index_temp</span> = <span style="color:#ae81ff">0</span>; <span style="color:#a6e22e">index_temp</span> &lt; <span style="color:#a6e22e">len_temp</span>; <span style="color:#a6e22e">index_temp</span><span style="color:#f92672">++</span> {
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">value_temp</span> = <span style="color:#a6e22e">range_temp</span>[<span style="color:#a6e22e">index_temp</span>]
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">index</span> = <span style="color:#a6e22e">index_temp</span>
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">value</span> = <span style="color:#a6e22e">value_temp</span>
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">original</span> <span style="color:#a6e22e">body</span>
</span></span><span style="display:flex;"><span>}
</span></span></code></pre></div><h3 id="for-range切片时候返回的是值拷贝">
for-range切片时候，返回的是值拷贝
<a class="anchor" href="#for-range%e5%88%87%e7%89%87%e6%97%b6%e5%80%99%e8%bf%94%e5%9b%9e%e7%9a%84%e6%98%af%e5%80%bc%e6%8b%b7%e8%b4%9d">#</a>
</h3>
<p>无论遍历数组还是切片，返回都是数组或切片中的值拷贝：</p>
<div class="highlight"><pre tabindex="0" style="color:#f8f8f2;background-color:#272822;-moz-tab-size:4;-o-tab-size:4;tab-size:4;"><code class="language-go" data-lang="go"><span style="display:flex;"><span><span style="color:#66d9ef">func</span> <span style="color:#a6e22e">main</span>() {
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">users</span> <span style="color:#f92672">:=</span> []<span style="color:#a6e22e">User</span>{
</span></span><span style="display:flex;"><span> {
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">Name</span>: <span style="color:#e6db74">&#34;a1&#34;</span>,
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">Age</span>: <span style="color:#ae81ff">100</span>,
</span></span><span style="display:flex;"><span> },
</span></span><span style="display:flex;"><span> {
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">Name</span>: <span style="color:#e6db74">&#34;a2&#34;</span>,
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">Age</span>: <span style="color:#ae81ff">101</span>,
</span></span><span style="display:flex;"><span> },
</span></span><span style="display:flex;"><span> {
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">Name</span>: <span style="color:#e6db74">&#34;a2&#34;</span>,
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">Age</span>: <span style="color:#ae81ff">102</span>,
</span></span><span style="display:flex;"><span> },
</span></span><span style="display:flex;"><span> }
</span></span><span style="display:flex;"><span>
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">fmt</span>.<span style="color:#a6e22e">Println</span>(<span style="color:#e6db74">&#34;before: &#34;</span>, <span style="color:#a6e22e">users</span>)
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">for</span> <span style="color:#a6e22e">_</span>, <span style="color:#a6e22e">v</span> <span style="color:#f92672">:=</span> <span style="color:#66d9ef">range</span> <span style="color:#a6e22e">users</span> {
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">v</span>.<span style="color:#a6e22e">Age</span> = <span style="color:#a6e22e">v</span>.<span style="color:#a6e22e">Age</span> <span style="color:#f92672">+</span> <span style="color:#ae81ff">10</span> <span style="color:#75715e">// 想给users中所有用户年龄增加10岁
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span> }
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">fmt</span>.<span style="color:#a6e22e">Println</span>(<span style="color:#e6db74">&#34;after: &#34;</span>, <span style="color:#a6e22e">users</span>)
</span></span><span style="display:flex;"><span>
</span></span><span style="display:flex;"><span>}
</span></span></code></pre></div><p>执行上面代码，输入以下内容：</p>
<pre tabindex="0"><code>before: [{a1 100} {a2 101} {a2 102}]
after: [{a1 100} {a2 101} {a2 102}]
</code></pre><p>解决办法可以通过索引访问原切片或数组：</p>
<div class="highlight"><pre tabindex="0" style="color:#f8f8f2;background-color:#272822;-moz-tab-size:4;-o-tab-size:4;tab-size:4;"><code class="language-go" data-lang="go"><span style="display:flex;"><span><span style="color:#66d9ef">func</span> <span style="color:#a6e22e">main</span>() {
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">users</span> <span style="color:#f92672">:=</span> []<span style="color:#a6e22e">User</span>{
</span></span><span style="display:flex;"><span> {
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">Name</span>: <span style="color:#e6db74">&#34;a1&#34;</span>,
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">Age</span>: <span style="color:#ae81ff">100</span>,
</span></span><span style="display:flex;"><span> },
</span></span><span style="display:flex;"><span> {
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">Name</span>: <span style="color:#e6db74">&#34;a2&#34;</span>,
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">Age</span>: <span style="color:#ae81ff">101</span>,
</span></span><span style="display:flex;"><span> },
</span></span><span style="display:flex;"><span> {
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">Name</span>: <span style="color:#e6db74">&#34;a2&#34;</span>,
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">Age</span>: <span style="color:#ae81ff">102</span>,
</span></span><span style="display:flex;"><span> },
</span></span><span style="display:flex;"><span> }
</span></span><span style="display:flex;"><span>
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">fmt</span>.<span style="color:#a6e22e">Println</span>(<span style="color:#e6db74">&#34;before: &#34;</span>, <span style="color:#a6e22e">users</span>)
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">for</span> <span style="color:#a6e22e">i</span> <span style="color:#f92672">:=</span> <span style="color:#66d9ef">range</span> <span style="color:#a6e22e">users</span> {
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">users</span>[<span style="color:#a6e22e">i</span>].<span style="color:#a6e22e">Age</span> = <span style="color:#a6e22e">users</span>[<span style="color:#a6e22e">i</span>].<span style="color:#a6e22e">Age</span> <span style="color:#f92672">+</span> <span style="color:#ae81ff">10</span>
</span></span><span style="display:flex;"><span> }
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">fmt</span>.<span style="color:#a6e22e">Println</span>(<span style="color:#e6db74">&#34;after: &#34;</span>, <span style="color:#a6e22e">users</span>)
</span></span><span style="display:flex;"><span>}
</span></span></code></pre></div><h2 id="遍历字符串">
遍历字符串
<a class="anchor" href="#%e9%81%8d%e5%8e%86%e5%ad%97%e7%ac%a6%e4%b8%b2">#</a>
</h2>
<p>当遍历字符串时候，返回的是rune类型，rune类型是int32类型的别名，一个rune就是一个码点(code point)：</p>
<div class="highlight"><pre tabindex="0" style="color:#f8f8f2;background-color:#272822;-moz-tab-size:4;-o-tab-size:4;tab-size:4;"><code class="language-go" data-lang="go"><span style="display:flex;"><span><span style="color:#75715e">// rune is an alias for int32 and is equivalent to int32 in all ways. It is
</span></span></span><span style="display:flex;"><span><span style="color:#75715e">// used, by convention, to distinguish character values from integer values.
</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span><span style="color:#66d9ef">type</span> <span style="color:#66d9ef">rune</span> = <span style="color:#66d9ef">int32</span>
</span></span></code></pre></div><p>由于遍历字符串时候，返回的是码点，所以索引并不总是依次增加1的：</p>
<div class="highlight"><pre tabindex="0" style="color:#f8f8f2;background-color:#272822;-moz-tab-size:4;-o-tab-size:4;tab-size:4;"><code class="language-go" data-lang="go"><span style="display:flex;"><span><span style="color:#66d9ef">func</span> <span style="color:#a6e22e">main</span>() {
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">var</span> <span style="color:#a6e22e">str</span> = <span style="color:#e6db74">&#34;hello，你好&#34;</span>
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">var</span> <span style="color:#a6e22e">buf</span> [<span style="color:#ae81ff">100</span>]<span style="color:#66d9ef">byte</span>
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">for</span> <span style="color:#a6e22e">i</span>, <span style="color:#a6e22e">v</span> <span style="color:#f92672">:=</span> <span style="color:#66d9ef">range</span> <span style="color:#a6e22e">str</span> {
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">vl</span> <span style="color:#f92672">:=</span> <span style="color:#a6e22e">utf8</span>.<span style="color:#a6e22e">RuneLen</span>(<span style="color:#a6e22e">v</span>)
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">si</span> <span style="color:#f92672">:=</span> <span style="color:#a6e22e">i</span> <span style="color:#f92672">+</span> <span style="color:#a6e22e">vl</span>
</span></span><span style="display:flex;"><span> copy(<span style="color:#a6e22e">buf</span>[:], <span style="color:#a6e22e">str</span>[<span style="color:#a6e22e">i</span>:<span style="color:#a6e22e">si</span>])
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">fmt</span>.<span style="color:#a6e22e">Printf</span>(<span style="color:#e6db74">&#34;索引%2d: %q，\t 码点: %#6x，\t 码点转换成字节: %#v\n&#34;</span>, <span style="color:#a6e22e">i</span>, <span style="color:#a6e22e">v</span>, <span style="color:#a6e22e">v</span>, <span style="color:#a6e22e">buf</span>[:<span style="color:#a6e22e">vl</span>])
</span></span><span style="display:flex;"><span> }
</span></span><span style="display:flex;"><span>}
</span></span></code></pre></div><p>执行上面代码将输出以下内容：</p>
<pre tabindex="0"><code>索引 0: &#39;h&#39;， 码点: 0x68， 码点转换成字节: []byte{0x68}
索引 1: &#39;e&#39;， 码点: 0x65， 码点转换成字节: []byte{0x65}
索引 2: &#39;l&#39;， 码点: 0x6c， 码点转换成字节: []byte{0x6c}
索引 3: &#39;l&#39;， 码点: 0x6c， 码点转换成字节: []byte{0x6c}
索引 4: &#39;o&#39;， 码点: 0x6f， 码点转换成字节: []byte{0x6f}
索引 5: &#39;，&#39;， 码点: 0xff0c， 码点转换成字节: []byte{0xef, 0xbc, 0x8c}
索引 8: &#39;你&#39;， 码点: 0x4f60， 码点转换成字节: []byte{0xe4, 0xbd, 0xa0}
索引11: &#39;好&#39;， 码点: 0x597d， 码点转换成字节: []byte{0xe5, 0xa5, 0xbd}
</code></pre><h2 id="遍历映射">
遍历映射
<a class="anchor" href="#%e9%81%8d%e5%8e%86%e6%98%a0%e5%b0%84">#</a>
</h2>
<p>当遍历切片时候，Go语言是不会保证遍历顺序的，为了明确强调这一点，Go语言在实现的时候，故意随机地选择一个桶开始遍历。当映射通道为nil时候，遍历次数为0次。</p>
<div class="highlight"><pre tabindex="0" style="color:#f8f8f2;background-color:#272822;-moz-tab-size:4;-o-tab-size:4;tab-size:4;"><code class="language-go" data-lang="go"><span style="display:flex;"><span><span style="color:#66d9ef">func</span> <span style="color:#a6e22e">main</span>() {
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">m</span> <span style="color:#f92672">:=</span> <span style="color:#66d9ef">map</span>[<span style="color:#66d9ef">int</span>]<span style="color:#66d9ef">int</span>{
</span></span><span style="display:flex;"><span> <span style="color:#ae81ff">1</span>: <span style="color:#ae81ff">10</span>,
</span></span><span style="display:flex;"><span> <span style="color:#ae81ff">2</span>: <span style="color:#ae81ff">20</span>,
</span></span><span style="display:flex;"><span> <span style="color:#ae81ff">3</span>: <span style="color:#ae81ff">30</span>,
</span></span><span style="display:flex;"><span> }
</span></span><span style="display:flex;"><span>
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">for</span> <span style="color:#a6e22e">i</span>, <span style="color:#a6e22e">v</span> <span style="color:#f92672">:=</span> <span style="color:#66d9ef">range</span> <span style="color:#a6e22e">m</span> {
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">fmt</span>.<span style="color:#a6e22e">Println</span>(<span style="color:#a6e22e">i</span>, <span style="color:#a6e22e">v</span>)
</span></span><span style="display:flex;"><span> }
</span></span><span style="display:flex;"><span>
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">m</span> = <span style="color:#66d9ef">nil</span>
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">for</span> <span style="color:#a6e22e">i</span>, <span style="color:#a6e22e">v</span> <span style="color:#f92672">:=</span> <span style="color:#66d9ef">range</span> <span style="color:#a6e22e">m</span> {
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">fmt</span>.<span style="color:#a6e22e">Println</span>(<span style="color:#a6e22e">i</span>, <span style="color:#a6e22e">v</span>)
</span></span><span style="display:flex;"><span> }
</span></span><span style="display:flex;"><span>}
</span></span></code></pre></div><h3 id="for-range映射时候可以边遍历边新增或删除吗">
for-range映射时候可以边遍历，边新增或删除吗？
<a class="anchor" href="#for-range%e6%98%a0%e5%b0%84%e6%97%b6%e5%80%99%e5%8f%af%e4%bb%a5%e8%be%b9%e9%81%8d%e5%8e%86%e8%be%b9%e6%96%b0%e5%a2%9e%e6%88%96%e5%88%a0%e9%99%a4%e5%90%97">#</a>
</h3>
<p>若在一个Goroutine里面边遍历边新增、删除，理论上是可以的，不会触发写检测的，新增的key-value可能会被访问到，也可能不会。</p>
<p>若多个Goroutine中进行遍历、新增、删除操作的话，是不可以的，是可能触发写检测的，然后直接panic。</p>
<h2 id="遍历通道">
遍历通道
<a class="anchor" href="#%e9%81%8d%e5%8e%86%e9%80%9a%e9%81%93">#</a>
</h2>
<p>当遍历通道时，直到通道关闭才会终止，若通道是nil，则会永远阻塞。遍历通道源码分析请见
<a href="/concurrency/channel/#id4">从通道中读取数据</a>。</p>
<h2 id="进一步阅读">
进一步阅读
<a class="anchor" href="#%e8%bf%9b%e4%b8%80%e6%ad%a5%e9%98%85%e8%af%bb">#</a>
</h2>
<ul>
<li>
<a href="https://garbagecollected.org/2017/02/22/go-range-loop-internals/">Go Range Loop Internals</a></li>
<li>
<a href="https://golang.org/ref/spec#For_statements">For statements</a></li>
</ul>
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