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<li><em>Go语言圣经读书笔记(挖坑中...): <a href="https://github.com/chai2010/gopl-notes-zh">https://github.com/chai2010/gopl-notes-zh</a></em></li>
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<h2 id="57-可变参数"><a class="header" href="#57-可变参数">5.7. 可变参数</a></h2>
<p>参数数量可变的函数称为可变参数函数。典型的例子就是fmt.Printf和类似函数。Printf首先接收一个必备的参数之后接收任意个数的后续参数。</p>
<p>在声明可变参数函数时,需要在参数列表的最后一个参数类型之前加上省略符号“...”,这表示该函数会接收任意数量的该类型参数。</p>
<p><u><i>gopl.io/ch5/sum</i></u></p>
<pre><code class="language-Go">func sum(vals ...int) int {
total := 0
for _, val := range vals {
total += val
}
return total
}
</code></pre>
<p>sum函数返回任意个int型参数的和。在函数体中vals被看作是类型为[] int的切片。sum可以接收任意数量的int型参数</p>
<pre><code class="language-Go">fmt.Println(sum()) // &quot;0&quot;
fmt.Println(sum(3)) // &quot;3&quot;
fmt.Println(sum(1, 2, 3, 4)) // &quot;10&quot;
</code></pre>
<p>在上面的代码中调用者隐式的创建一个数组并将原始参数复制到数组中再把数组的一个切片作为参数传给被调用函数。如果原始参数已经是切片类型我们该如何传递给sum只需在最后一个参数后加上省略符。下面的代码功能与上个例子中最后一条语句相同。</p>
<pre><code class="language-Go">values := []int{1, 2, 3, 4}
fmt.Println(sum(values...)) // &quot;10&quot;
</code></pre>
<p>虽然在可变参数函数内部,...int 型参数的行为看起来很像切片类型,但实际上,可变参数函数和以切片作为参数的函数是不同的。</p>
<pre><code class="language-Go">func f(...int) {}
func g([]int) {}
fmt.Printf(&quot;%T\n&quot;, f) // &quot;func(...int)&quot;
fmt.Printf(&quot;%T\n&quot;, g) // &quot;func([]int)&quot;
</code></pre>
<p>可变参数函数经常被用于格式化字符串。下面的errorf函数构造了一个以行号开头的经过格式化的错误信息。函数名的后缀f是一种通用的命名规范代表该可变参数函数可以接收Printf风格的格式化字符串。</p>
<pre><code class="language-Go">func errorf(linenum int, format string, args ...interface{}) {
fmt.Fprintf(os.Stderr, &quot;Line %d: &quot;, linenum)
fmt.Fprintf(os.Stderr, format, args...)
fmt.Fprintln(os.Stderr)
}
linenum, name := 12, &quot;count&quot;
errorf(linenum, &quot;undefined: %s&quot;, name) // &quot;Line 12: undefined: count&quot;
</code></pre>
<p>interface{}表示函数的最后一个参数可以接收任意类型我们会在第7章详细介绍。</p>
<p><strong>练习5.15</strong> 编写类似sum的可变参数函数max和min。考虑不传参时max和min该如何处理再编写至少接收1个参数的版本。</p>
<p><strong>练习5.16</strong> 编写多参数版本的strings.Join。</p>
<p><strong>练习5.17</strong> 编写多参数版本的ElementsByTagName函数接收一个HTML结点树以及任意数量的标签名返回与这些标签名匹配的所有元素。下面给出了2个例子</p>
<pre><code class="language-Go">func ElementsByTagName(doc *html.Node, name...string) []*html.Node
images := ElementsByTagName(doc, &quot;img&quot;)
headings := ElementsByTagName(doc, &quot;h1&quot;, &quot;h2&quot;, &quot;h3&quot;, &quot;h4&quot;)
</code></pre>
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