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< ol class = "chapter" > < li class = "chapter-item expanded affix " > < a href = "../index.html" > Go语言圣经< / a > < / li > < li class = "chapter-item expanded affix " > < a href = "../preface.html" > 前言< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1.html" > < strong aria-hidden = "true" > 1.< / strong > 入门< / a > < / li > < li > < ol class = "section" > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-01.html" > < strong aria-hidden = "true" > 1.1.< / strong > Hello, World< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-02.html" > < strong aria-hidden = "true" > 1.2.< / strong > 命令行参数< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-03.html" > < strong aria-hidden = "true" > 1.3.< / strong > 查找重复的行< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-04.html" > < strong aria-hidden = "true" > 1.4.< / strong > GIF动画< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-05.html" > < strong aria-hidden = "true" > 1.5.< / strong > 获取URL< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-06.html" > < strong aria-hidden = "true" > 1.6.< / strong > 并发获取多个URL< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-07.html" > < strong aria-hidden = "true" > 1.7.< / strong > Web服务< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-08.html" > < strong aria-hidden = "true" > 1.8.< / strong > 本章要点< / a > < / li > < / ol > < / li > < li class = "chapter-item expanded " > < a href = "../ch2/ch2.html" > < strong aria-hidden = "true" > 2.< / strong > 程序结构< / a > < / li > < li > < ol class = "section" > < li class = "chapter-item expanded " > < a href = "../ch2/ch2-01.html" > < strong aria-hidden = "true" > 2.1.< / strong > 命名< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch2/ch2-02.html" > < strong aria-hidden = "true" > 2.2.< / strong > 声明< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch2/ch2-03.html" > < strong aria-hidden = "true" > 2.3.< / strong > 变量< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch2/ch2-04.html" > < strong aria-hidden = "true" > 2.4.< / strong > 赋值< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch2/ch2-05.html" > < strong aria-hidden = "true" > 2.5.< / strong > 类型< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch2/ch2-06.html" > < strong aria-hidden = "true" > 2.6.< / strong > 包和文件< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch2/ch2-07.html" > < strong aria-hidden = "true" > 2.7.< / strong > 作用域< / a > < / li > < / ol > < / li > < li class = "chapter-item expanded " > < a href = "../ch3/ch3.html" > < strong aria-hidden = "true" > 3.< / strong > 基础数据类型< / a > < / li > < li > < ol class = "section" > < li class = "chapter-item expanded " > < a href = "../ch3/ch3-01.html" > < strong aria-hidden = "true" > 3.1.< / strong > 整型< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch3/ch3-02.html" > < strong aria-hidden = "true" > 3.2.< / strong > 浮点数< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch3/ch3-03.html" > < strong aria-hidden = "true" > 3.3.< / strong > 复数< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch3/ch3-04.html" > < strong aria-hidden = "true" > 3.4.< / strong > 布尔型< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch3/ch3-05.html" > < strong aria-hidden = "true" > 3.5.< / strong > 字符串< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch3/ch3-06.html" > < strong aria-hidden = "true" > 3.6.< / strong > 常量< / a > < / li > < / ol > < / li > < li class = "chapter-item expanded " > < a href = "../ch4/ch4.html" > < strong aria-hidden = "true" > 4.< / strong > 复合数据类型< / a > < / li > < li > < ol class = "section" > < li class = "chapter-item expanded " > < a href = "../ch4/ch4-01.html" > < strong aria-hidden = "true" > 4.1.< / strong > 数组< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch4/ch4-02.html" > < strong aria-hidden = "true" > 4.2.< / strong > Slice< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch4/ch4-03.html" > < strong aria-hidden = "true" > 4.3.< / strong > Map< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch4/ch4-04.html" > < strong aria-hidden = "true" > 4.4.< / strong > 结构体< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch4/ch4-05.html" > < strong aria-hidden = "true" > 4.5.< / strong > JSON< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch4/ch4-06.html" > < strong aria-hidd
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2022-08-24 10:48:15 +00:00
< 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 >
2022-08-04 06:59:33 +00:00
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< h2 id = "55-函数值" > < a class = "header" href = "#55-函数值" > 5.5. 函数值< / a > < / h2 >
< p > 在Go中， （ ） ： ， ， ， ， （ ） ： < / p >
< pre > < code class = "language-Go" > func square(n int) int { return n * n }
func negative(n int) int { return -n }
func product(m, n int) int { return m * n }
f := square
fmt.Println(f(3)) // " 9"
f = negative
fmt.Println(f(3)) // " -3"
fmt.Printf(" %T\n" , f) // " func(int) int"
f = product // compile error: can't assign func(int, int) int to func(int) int
< / code > < / pre >
< p > 函数类型的零值是nil。调用值为nil的函数值会引起panic错误： < / p >
< pre > < code class = "language-Go" > var f func(int) int
f(3) // 此处f的值为nil, 会引起panic错误
< / code > < / pre >
< p > 函数值可以与nil比较： < / p >
< pre > < code class = "language-Go" > var f func(int) int
if f != nil {
f(3)
}
< / code > < / pre >
< p > 但是函数值之间是不可比较的， < / p >
< p > 函数值使得我们不仅仅可以通过数据来参数化函数， ， < / p >
< pre > < code class = "language-Go" > func add1(r rune) rune { return r + 1 }
fmt.Println(strings.Map(add1, " HAL-9000" )) // " IBM.:111"
fmt.Println(strings.Map(add1, " VMS" )) // " WNT"
fmt.Println(strings.Map(add1, " Admix" )) // " Benjy"
< / code > < / pre >
< p > 5.2节的findLinks函数使用了辅助函数visit， ， ， ， < / p >
< p > < u > < i > gopl.io/ch5/outline2< / i > < / u > < / p >
< pre > < code class = "language-Go" > // forEachNode针对每个结点x，
// pre和post都是可选的。
// 遍历孩子结点之前，
// 遍历孩子结点之后，
func forEachNode(n *html.Node, pre, post func(n *html.Node)) {
if pre != nil {
pre(n)
}
for c := n.FirstChild; c != nil; c = c.NextSibling {
forEachNode(c, pre, post)
}
if post != nil {
post(n)
}
}
< / code > < / pre >
< p > 该函数接收2个函数作为参数， ， < code > < b> ...< /b> < / code > ： < / p >
< pre > < code class = "language-Go" > var depth int
func startElement(n *html.Node) {
if n.Type == html.ElementNode {
fmt.Printf(" %*s< %s> \n" , depth*2, " " , n.Data)
depth++
}
}
func endElement(n *html.Node) {
if n.Type == html.ElementNode {
depth--
fmt.Printf(" %*s< /%s> \n" , depth*2, " " , n.Data)
}
}
< / code > < / pre >
< p > 上面的代码利用fmt.Printf的一个小技巧控制输出的缩进。< code > %*s< / code > 中的< code > *< / code > 会在字符串之前填充一些空格。在例子中，每次输出会先填充< code > depth*2< / code > 数量的空格，再输出" " ， < / p >
< p > 如果我们像下面这样调用forEachNode： < / p >
< pre > < code class = "language-Go" > forEachNode(doc, startElement, endElement)
< / code > < / pre >
< p > 与之前的outline程序相比， ： < / p >
< pre > < code > $ go build gopl.io/ch5/outline2
$ ./outline2 http://gopl.io
< html>
< head>
< meta>
< /meta>
< title>
< /title>
< style>
< /style>
< /head>
< body>
< table>
< tbody>
< tr>
< td>
< a>
< img>
< /img>
...
< / code > < / pre >
< p > < strong > 练习 5.7： < / strong > 完善startElement和endElement函数， ： ， （ < a href='...'> ）。使用简略格式输出没有孩子结点的元素（即用< code > < img/> < / code > 代替< code > < img> < /img> < / code > ） ， （ ） < / p >
< p > < strong > 练习 5.8： < / strong > 修改pre和post函数， ， ， ， ， < / p >
< pre > < code class = "language-Go" > func ElementByID(doc *html.Node, id string) *html.Node
< / code > < / pre >
< p > < strong > 练习 5.9： < / strong > 编写函数expand， " foo" 替换为f(" foo" )的返回值。< / p >
< pre > < code class = "language-Go" > func expand(s string, f func(string) string) string
< / code > < / pre >
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