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<li><em>KusonStack一站式可编程配置技术栈(Go): <a href="https://github.com/KusionStack/kusion">https://github.com/KusionStack/kusion</a></em></li>
<li><em>KCL 配置编程语言(Rust): <a href="https://github.com/KusionStack/KCLVM">https://github.com/KusionStack/KCLVM</a></em></li>
<li><em>凹语言™: <a href="https://github.com/wa-lang/wa">https://github.com/wa-lang/wa</a></em></li>
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<hr>
<h2 id="52-递归"><a class="header" href="#52-递归">5.2. 递归</a></h2>
<p>函数可以是递归的这意味着函数可以直接或间接的调用自身。对许多问题而言递归是一种强有力的技术例如处理递归的数据结构。在4.4节我们通过遍历二叉树来实现简单的插入排序在本章节我们再次使用它来处理HTML文件。</p>
<p>下文的示例代码使用了非标准包 golang.org/x/net/html 解析HTML。golang.org/x/... 目录下存储了一些由Go团队设计、维护对网络编程、国际化文件处理、移动平台、图像处理、加密解密、开发者工具提供支持的扩展包。未将这些扩展包加入到标准库原因有二一是部分包仍在开发中二是对大多数Go语言的开发者而言扩展包提供的功能很少被使用。</p>
<p>例子中调用golang.org/x/net/html的部分api如下所示。html.Parse函数读入一组bytes解析后返回html.Node类型的HTML页面树状结构根节点。HTML拥有很多类型的结点如text文本、commnets注释类型在下面的例子中我们 只关注&lt; name key='value' &gt;形式的结点。</p>
<p><u><i>golang.org/x/net/html</i></u></p>
<pre><code class="language-Go">package html
type Node struct {
Type NodeType
Data string
Attr []Attribute
FirstChild, NextSibling *Node
}
type NodeType int32
const (
ErrorNode NodeType = iota
TextNode
DocumentNode
ElementNode
CommentNode
DoctypeNode
)
type Attribute struct {
Key, Val string
}
func Parse(r io.Reader) (*Node, error)
</code></pre>
<p>main函数解析HTML标准输入通过递归函数visit获得links链接并打印出这些links</p>
<p><u><i>gopl.io/ch5/findlinks1</i></u></p>
<pre><code class="language-Go">// Findlinks1 prints the links in an HTML document read from standard input.
package main
import (
&quot;fmt&quot;
&quot;os&quot;
&quot;golang.org/x/net/html&quot;
)
func main() {
doc, err := html.Parse(os.Stdin)
if err != nil {
fmt.Fprintf(os.Stderr, &quot;findlinks1: %v\n&quot;, err)
os.Exit(1)
}
for _, link := range visit(nil, doc) {
fmt.Println(link)
}
}
</code></pre>
<p>visit函数遍历HTML的节点树从每一个anchor元素的href属性获得link,将这些links存入字符串数组中并返回这个字符串数组。</p>
<pre><code class="language-Go">// visit appends to links each link found in n and returns the result.
func visit(links []string, n *html.Node) []string {
if n.Type == html.ElementNode &amp;&amp; n.Data == &quot;a&quot; {
for _, a := range n.Attr {
if a.Key == &quot;href&quot; {
links = append(links, a.Val)
}
}
}
for c := n.FirstChild; c != nil; c = c.NextSibling {
links = visit(links, c)
}
return links
}
</code></pre>
<p>为了遍历结点n的所有后代结点每次遇到n的孩子结点时visit递归的调用自身。这些孩子结点存放在FirstChild链表中。</p>
<p>让我们以Go的主页golang.org作为目标运行findlinks。我们以fetch1.5章的输出作为findlinks的输入。下面的输出做了简化处理。</p>
<pre><code>$ go build gopl.io/ch1/fetch
$ go build gopl.io/ch5/findlinks1
$ ./fetch https://golang.org | ./findlinks1
#
/doc/
/pkg/
/help/
/blog/
http://play.golang.org/
//tour.golang.org/
https://golang.org/dl/
//blog.golang.org/
/LICENSE
/doc/tos.html
http://www.google.com/intl/en/policies/privacy/
</code></pre>
<p>注意在页面中出现的链接格式,在之后我们会介绍如何将这些链接,根据根路径( https://golang.org 生成可以直接访问的url。</p>
<p>在函数outline中我们通过递归的方式遍历整个HTML结点树并输出树的结构。在outline内部每遇到一个HTML元素标签就将其入栈并输出。</p>
<p><u><i>gopl.io/ch5/outline</i></u></p>
<pre><code class="language-Go">func main() {
doc, err := html.Parse(os.Stdin)
if err != nil {
fmt.Fprintf(os.Stderr, &quot;outline: %v\n&quot;, err)
os.Exit(1)
}
outline(nil, doc)
}
func outline(stack []string, n *html.Node) {
if n.Type == html.ElementNode {
stack = append(stack, n.Data) // push tag
fmt.Println(stack)
}
for c := n.FirstChild; c != nil; c = c.NextSibling {
outline(stack, c)
}
}
</code></pre>
<p>有一点值得注意outline有入栈操作但没有相对应的出栈操作。当outline调用自身时被调用者接收的是stack的拷贝。被调用者对stack的元素追加操作修改的是stack的拷贝其可能会修改slice底层的数组甚至是申请一块新的内存空间进行扩容但这个过程并不会修改调用方的stack。因此当函数返回时调用方的stack与其调用自身之前完全一致。</p>
<p>下面是 https://golang.org 页面的简要结构:</p>
<pre><code>$ go build gopl.io/ch5/outline
$ ./fetch https://golang.org | ./outline
[html]
[html head]
[html head meta]
[html head title]
[html head link]
[html body]
[html body div]
[html body div]
[html body div div]
[html body div div form]
[html body div div form div]
[html body div div form div a]
...
</code></pre>
<p>正如你在上面实验中所见大部分HTML页面只需几层递归就能被处理但仍然有些页面需要深层次的递归。</p>
<p>大部分编程语言使用固定大小的函数调用栈常见的大小从64KB到2MB不等。固定大小栈会限制递归的深度当你用递归处理大量数据时需要避免栈溢出除此之外还会导致安全性问题。与此相反Go语言使用可变栈栈的大小按需增加初始时很小。这使得我们使用递归时不必考虑溢出和安全问题。</p>
<p><strong>练习 5.1</strong> 修改findlinks代码中遍历n.FirstChild链表的部分将循环调用visit改成递归调用。</p>
<p><strong>练习 5.2</strong> 编写函数记录在HTML树中出现的同名元素的次数。</p>
<p><strong>练习 5.3</strong> 编写函数输出所有text结点的内容。注意不要访问<code>&lt;script&gt;</code><code>&lt;style&gt;</code>元素,因为这些元素对浏览者是不可见的。</p>
<p><strong>练习 5.4</strong> 扩展visit函数使其能够处理其他类型的结点如images、scripts和style sheets。</p>
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