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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" class = "active" > < 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" > < s
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< li > < em > KusonStack一站式可编程配置技术栈: < a href = "https://github.com/KusionStack/kusion" > https://github.com/KusionStack/kusion< / 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 = "13-查找重复的行" > < a class = "header" href = "#13-查找重复的行" > 1.3. 查找重复的行< / a > < / h2 >
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< p > 对文件做拷贝、打印、搜索、排序、统计或类似事情的程序都有一个差不多的程序结构：一个处理输入的循环，在每个元素上执行计算处理，在处理的同时或最后产生输出。我们会展示一个名为 < code > dup< / code > 的程序的三个版本；灵感来自于 Unix 的 < code > uniq< / code > 命令，其寻找相邻的重复行。该程序使用的结构和包是个参考范例，可以方便地修改。< / p >
< p > < code > dup< / code > 的第一个版本打印标准输入中多次出现的行，以重复次数开头。该程序将引入 < code > if< / code > 语句，< code > map< / code > 数据类型以及 < code > bufio< / code > 包。< / p >
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< p > < u > < i > gopl.io/ch1/dup1< / i > < / u > < / p >
< pre > < code class = "language-go" > // Dup1 prints the text of each line that appears more than
// once in the standard input, preceded by its count.
package main
import (
" bufio"
" fmt"
" os"
)
func main() {
counts := make(map[string]int)
input := bufio.NewScanner(os.Stdin)
for input.Scan() {
counts[input.Text()]++
}
// NOTE: ignoring potential errors from input.Err()
for line, n := range counts {
if n > 1 {
fmt.Printf(" %d\t%s\n" , n, line)
}
}
}
< / code > < / pre >
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< p > 正如 < code > for< / code > 循环一样，< code > if< / code > 语句条件两边也不加括号，但是主体部分需要加。< code > if< / code > 语句的 < code > else< / code > 部分是可选的，在 < code > if< / code > 的条件为 < code > false< / code > 时执行。< / p >
< p > < strong > map< / strong > 存储了键/值（ ） ， ， ， < code > ==< / code > 运算符比较，最常见的例子是字符串；值则可以是任意类型。这个例子中的键是字符串，值是整数。内置函数 < code > make< / code > 创建空 < code > map< / code > ， ， < code > map< / code > 。< / p >
< p > （译注：从功能和实现上说，< code > Go< / code > 的 < code > map< / code > 类似于 < code > Java< / code > 语言中的 < code > HashMap< / code > ， < code > dict< / code > ， < code > Lua< / code > 语言中的 < code > table< / code > ，通常使用 < code > hash< / code > 实现。遗憾的是，对于该词的翻译并不统一，数学界术语为< em > 映射< / em > ，而计算机界众说纷纭莫衷一是。为了防止对读者造成误解，保留不译。）< / p >
< p > 每次 < code > dup< / code > 读取一行输入，该行被当做键存入 < code > map< / code > ，其对应的值递增。< code > counts[input.Text()]++< / code > 语句等价下面两句：< / p >
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< pre > < code class = "language-go" > line := input.Text()
counts[line] = counts[line] + 1
< / code > < / pre >
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< p > < code > map< / code > 中不含某个键时不用担心，首次读到新行时，等号右边的表达式 < code > counts[line]< / code > 的值将被计算为其类型的零值，对于 < code > int< / code > 即 < code > 0< / code > 。< / p >
< p > 为了打印结果，我们使用了基于 < code > range< / code > 的循环，并在 < code > counts< / code > 这个 < code > map< / code > 上迭代。跟之前类似，每次迭代得到两个结果，键和其在 < code > map< / code > 中对应的值。< code > map< / code > 的迭代顺序并不确定，从实践来看，该顺序随机，每次运行都会变化。这种设计是有意为之的，因为能防止程序依赖特定遍历顺序，而这是无法保证的。（译注：具体可以参见这里< a href = "https://stackoverflow.com/questions/11853396/google-go-lang-assignment-order" > https://stackoverflow.com/questions/11853396/google-go-lang-assignment-order< / a > ） < / p >
< p > 继续来看 < code > bufio< / code > 包，它使处理输入和输出方便又高效。< code > Scanner< / code > 类型是该包最有用的特性之一，它读取输入并将其拆成行或单词；通常是处理行形式的输入最简单的方法。< / p >
< p > 程序使用短变量声明创建 < code > bufio.Scanner< / code > 类型的变量 < code > input< / code > 。< / p >
< pre > < code class = "language-go" > input := bufio.NewScanner(os.Stdin)
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< / code > < / pre >
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< p > 该变量从程序的标准输入中读取内容。每次调用 < code > input.Scan()< / code > ，即读入下一行，并移除行末的换行符；读取的内容可以调用 < code > input.Text()< / code > 得到。< code > Scan< / code > 函数在读到一行时返回 < code > true< / code > ，不再有输入时返回 < code > false< / code > 。< / p >
< p > 类似于 C 或其它语言里的 < code > printf< / code > 函数，< code > fmt.Printf< / code > 函数对一些表达式产生格式化输出。该函数的首个参数是个格式字符串， （ ） ， ， < code > %d< / code > 表示以十进制形式打印一个整型操作数，而 < code > %s< / code > 则表示把字符串型操作数的值展开。< / p >
< p > < code > Printf< / code > 有一大堆这种转换， < em > 动词（ ） < / em > 。下面的表格虽然远不是完整的规范，但展示了可用的很多特性：< / p >
< pre > < code class = "language-text" > %d 十进制整数
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%x, %o, %b 十六进制，八进制，二进制整数。
%f, %g, %e 浮点数： 3.141593 3.141592653589793 3.141593e+00
%t 布尔：
%c 字符（ ）
%s 字符串
%q 带双引号的字符串" abc" 或带单引号的字符'c'
%v 变量的自然形式（ ）
%T 变量的类型
%% 字面上的百分号标志（无操作数）
< / code > < / pre >
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< p > < code > dup1< / code > 的格式字符串中还含有制表符< code > \t< / code > 和换行符< code > \n< / code > 。字符串字面上可能含有这些代表不可见字符的< strong > 转义字符（ ） < / strong > 。默认情况下，< code > Printf< / code > 不会换行。按照惯例，以字母 < code > f< / code > 结尾的格式化函数，如 < code > log.Printf< / code > 和 < code > fmt.Errorf< / code > ，都采用 < code > fmt.Printf< / code > 的格式化准则。而以 < code > ln< / code > 结尾的格式化函数，则遵循 < code > Println< / code > 的方式，以跟 < code > %v< / code > 差不多的方式格式化参数，并在最后添加一个换行符。（译注：后缀 < code > f< / code > 指 < code > format< / code > ， < code > ln< / code > 指 < code > line< / code > 。）< / p >
< p > 很多程序要么从标准输入中读取数据，如上面的例子所示，要么从一系列具名文件中读取数据。< code > dup< / code > 程序的下个版本读取标准输入或是使用 < code > os.Open< / code > 打开各个具名文件，并操作它们。< / p >
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< p > < u > < i > gopl.io/ch1/dup2< / i > < / u > < / p >
< pre > < code class = "language-go" > // Dup2 prints the count and text of lines that appear more than once
// in the input. It reads from stdin or from a list of named files.
package main
import (
" bufio"
" fmt"
" os"
)
func main() {
counts := make(map[string]int)
files := os.Args[1:]
if len(files) == 0 {
countLines(os.Stdin, counts)
} else {
for _, arg := range files {
f, err := os.Open(arg)
if err != nil {
fmt.Fprintf(os.Stderr, " dup2: %v\n" , err)
continue
}
countLines(f, counts)
f.Close()
}
}
for line, n := range counts {
if n > 1 {
fmt.Printf(" %d\t%s\n" , n, line)
}
}
}
func countLines(f *os.File, counts map[string]int) {
input := bufio.NewScanner(f)
for input.Scan() {
counts[input.Text()]++
}
// NOTE: ignoring potential errors from input.Err()
}
< / code > < / pre >
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< p > < code > os.Open< / code > 函数返回两个值。第一个值是被打开的文件（< code > *os.File< / code > ），其后被 < code > Scanner< / code > 读取。< / p >
< p > < code > os.Open< / code > 返回的第二个值是内置 < code > error< / code > 类型的值。如果 < code > err< / code > 等于内置值< code > nil< / code > （译注：相当于其它语言里的 < code > NULL< / code > ），那么文件被成功打开。读取文件，直到文件结束，然后调用 < code > Close< / code > 关闭该文件，并释放占用的所有资源。相反的话，如果 < code > err< / code > 的值不是 < code > nil< / code > ，说明打开文件时出错了。这种情况下，错误值描述了所遇到的问题。我们的错误处理非常简单，只是使用 < code > Fprintf< / code > 与表示任意类型默认格式值的动词 < code > %v< / code > ，向标准错误流打印一条信息，然后 < code > dup< / code > 继续处理下一个文件；< code > continue< / code > 语句直接跳到 < code > for< / code > 循环的下个迭代开始执行。< / p >
< p > 为了使示例代码保持合理的大小，本书开始的一些示例有意简化了错误处理，显而易见的是，应该检查 < code > os.Open< / code > 返回的错误值，然而，使用 < code > input.Scan< / code > 读取文件过程中， ， < / p >
< p > 注意 < code > countLines< / code > 函数在其声明前被调用。函数和包级别的变量（ ） ， （ ： ） < / p >
< p > < code > map< / code > 是一个由 < code > make< / code > 函数创建的数据结构的引用。< code > map< / code > 作为参数传递给某函数时， （ ， ） ， < code > map< / code > 底层数据结构的任何修改，调用者函数都可以通过持有的 < code > map< / code > 引用看到。在我们的例子中，< code > countLines< / code > 函数向 < code > counts< / code > 插入的值，也会被 < code > main< / code > 函数看到。（译注：类似于 C++ 里的引用传递，实际上指针是另一个指针了，但内部存的值指向同一块内存）< / p >
< p > < code > dup< / code > 的前两个版本以" 流”模式读取输入，并根据需要拆分成多个行。理论上，这些程序可以处理任意数量的输入数据。还有另一个方法，就是一口气把全部输入数据读到内存中，一次分割为多行，然后处理它们。下面这个版本，< code > dup3< / code > ，就是这么操作的。这个例子引入了 < code > ReadFile< / code > 函数（来自于< code > io/ioutil< / code > 包），其读取指定文件的全部内容，< code > strings.Split< / code > 函数把字符串分割成子串的切片。（< code > Split< / code > 的作用与前文提到的 < code > strings.Join< / code > 相反。）< / p >
< p > 我们略微简化了 < code > dup3< / code > 。首先，由于 < code > ReadFile< / code > 函数需要文件名作为参数，因此只读指定文件，不读标准输入。其次，由于行计数代码只在一处用到，故将其移回 < code > main< / code > 函数。< / p >
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< p > < u > < i > gopl.io/ch1/dup3< / i > < / u > < / p >
< pre > < code class = "language-go" > package main
import (
" fmt"
" io/ioutil"
" os"
" strings"
)
func main() {
counts := make(map[string]int)
for _, filename := range os.Args[1:] {
data, err := ioutil.ReadFile(filename)
if err != nil {
fmt.Fprintf(os.Stderr, " dup3: %v\n" , err)
continue
}
for _, line := range strings.Split(string(data), " \n" ) {
counts[line]++
}
}
for line, n := range counts {
if n > 1 {
fmt.Printf(" %d\t%s\n" , n, line)
}
}
}
< / code > < / pre >
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< p > < code > ReadFile< / code > 函数返回一个字节切片（ ） ， < code > string< / code > ，才能用 < code > strings.Split< / code > 分割。我们会在3.5.4 节详细讲解字符串和字节切片。< / p >
< p > 实现上，< code > bufio.Scanner< / code > 、< code > ioutil.ReadFile< / code > 和 < code > ioutil.WriteFile< / code > 都使用 < code > *os.File< / code > 的 < code > Read< / code > 和 < code > Write< / code > 方法， ， （ ） （ ） ， < code > bufio< / code > 和 < code > io/ioutil< / code > 包中所提供的那些，用起来要容易点。< / p >
< hr / >
< p > < strong > 练习 1.4： < / strong > 修改 < code > dup2< / code > ，出现重复的行时打印文件名称。< / p >
2022-08-04 06:59:33 +00:00
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