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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-zh.html">译者序</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" class="active"><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-hidden="true">4.6.</strong> 文本和HTML模板</a></li></ol></li><li class="chapter-item expanded "><a href="../ch5/ch5.html"><strong aria-hidden="true">5.</strong> 函数</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../ch5/ch5-01.html"><strong aria-hidden="true">5.1.</strong> 函数声明</a></li><li class="chapter-item expanded "><a href="../ch5/ch5-02.html"><strong aria-hidden="true">5.2.</strong> 递归</a></li><li class="chapter-item expanded "><a href="../ch5/ch5-03.html"><strong aria-hidden="true">5.3.</strong> 多返回值</a></li><li class="chapter-item expanded "><a href="../ch5/ch5-04.html"><strong aria-hidden="true">5.4.</strong> 错误</a></li><li class="chapter-item expanded "><a href="../ch5/ch5-05.html"><strong aria-hidden="true">5.5.</strong> 函数值</a></li><li class="chapter-item expanded "><a href="../ch5/ch5-06.html"><strong aria-hidden="true">5.6.</strong> 匿名函数</a></li><li class="chapter-item expanded "><a href="../ch5/ch5-07.html"><strong aria-hidden="true">5.7.</strong> 可变参数</a></li><li class="chapter-item expanded "><a href="../ch5/ch5-08.html"><strong aria-hidden="true">5.8.</strong> Deferred函数</a></li><li class="chapter-item expanded "><a href="../ch5/ch5-09.html"><strong aria-hidden="true">5.9.</strong> Panic异常</a></li><li class="chapter-item expanded "><a href="../ch5/ch5-10.html"><strong aria-hidden="true">5.10.</strong> Recover捕获异常</a></li></ol></li><li class="chapter-item expanded "><a href="../ch6/ch6.html"><strong aria-hidden="true">6.</strong> 方法</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../ch6/ch6-01.html"><strong aria-hidden="true">6.1.</strong> 方法声明</a></li><li class="chapter-item expanded "><a href="../ch6/ch6-02.html"><strong aria-hidden="true">6.2.</strong> 基于指针对象的方法</a></li><li class="chapter-item expanded "><a href="../ch6/ch6-03.html"><strong aria-hidden="true">6.3.</strong> 通过嵌入结构体来扩展类型</a></li><li class="chapter-item expanded "><a href="../ch6/ch6-04.html"><strong aria-hidden="true">6.4.</strong> 方法值和方法表达式</a></li><li class="chapter-item expanded "><a href="../ch6/ch6-05.html"><strong aria-hidden="true">6.5.</strong> 示例: Bit数组</a></li><li class="chapter-item expanded "><a href="../ch6/ch6-06.html"><strong aria-hidden="true">6.6.</strong> 封装</a></li></ol></li><li class="chapter-item expanded "><a href="../ch7/ch7.html"><strong aria-hidden="true">7.</strong> 接口</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../ch7/ch7-01.html"><strong aria-hidden="true">7.1.</strong> 接口是合约</a></li><li class="chapter-item expanded "><a href="../ch7/ch7-02.html"><strong aria-hidden="true">7.2.</strong> 接口类型</a></li><li class="chapter-item expanded "><a href="../ch7/ch7-03.html"><strong aria-hidden="true">7.3.</strong> 实现接口的条件</a></li><li class="chapter-item expanded "><a href="../ch7/ch7-04.html"><strong aria-hidden="true">7.4.</strong> flag.Value接口</a></li><li class="chapter-item expanded "><a href="../ch7/ch7-05.html"><strong aria-hidden="true">7.5.</strong> 接口值</a></li><li class="chapter-item expanded "><a href="../ch7/ch7-06.html"><strong aria-hidden="true">7.6.</strong> sort.Interface接口</a></li><li class="chapter-item expanded "><a href="../ch7/ch7-07.html"><strong aria-hidden="true">7.7.</strong> http.Handler接口</a></li><li class="chapter-item expanded "><a href="../ch7/ch7-08.html"><strong aria-hidden="true">7.8.</strong> error接口</a></li><li class="chapter-item expanded "><a href="../ch7/ch7-09.html"><strong aria-hidden="true">7.9.</strong> 示例: 表达式求值</a></li><li class="chapter-item expanded "><a href="../ch7/ch7-10.html"><strong aria-hidden="true">7.10.</strong> 类型断言</a></li><li class="chapter-item expanded "><a href="../ch7/ch7-11.html"><strong aria-hidden="true">7.11.</strong> 基于类型断言识别错误类型</a></li><li class="chapter-item expanded "><a href="../ch7/ch7-12.html"><strong aria-hidden="true">7.12.</strong> 通过类型断言查询接口</a></li><li class="chapter-item expanded "><a href="../ch7/ch7-13.html"><strong aria-hidden="true">7.13.</strong> 类型分支</a></li><li class="chapter-item expanded "><a href="../ch7/ch7-14.html"><strong aria-hidden="true">7.14.</strong> 示例: 基于标记的XML解码</a></li><li class="chapter-item expanded "><a href="../ch7/ch7-15.html"><strong aria-hidden="true">7.15.</strong> 补充几点</a></li></ol></li><li class="chapter-item expanded "><a href="../ch8/ch8.html"><strong aria-hidden="true">8.</strong> Goroutines和Channels</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../ch8/ch8-01.html"><strong aria-hidden="true">8.1.</strong> Goroutines</a></li><li class="chapter-item expanded "><a href="../ch8/ch8-02.html"><strong aria-hidden="true">8.2.</strong> 示例: 并发的Clock服务</a></li><li class="chapter-item expanded "><a href="../ch8/ch8-03.html"><strong aria-hidden="true">8.3.</strong> 示例: 并发的Echo服务</a></li><li class="chapter-item expanded "><a href="../ch8/ch8-04.html"><strong aria-hidden="true">8.4.</strong> Channels</a></li><li class="chapter-item expanded "><a href="../ch8/ch8-05.html"><strong aria-hidden="true">8.5.</strong> 并发的循环</a></li><li class="chapter-item expanded "><a href="../ch8/ch8-06.html"><strong aria-hidden="true">8.6.</strong> 示例: 并发的Web爬虫</a></li><li class="chapter-item expanded "><a href="../ch8/ch8-07.html"><strong aria-hidden="true">8.7.</strong> 基于select的多路复用</a></li><li class="chapter-item expanded "><a href="../ch8/ch8-08.html"><strong aria-hidden="true">8.8.</strong> 示例: 并发的目录遍历</a></li><li class="chapter-item expanded "><a href="../ch8/ch8-09.html"><strong aria-hidden="true">8.9.</strong> 并发的退出</a></li><li class="chapter-item expanded "><a href="../ch8/ch8-10.html"><strong aria-hidden="true">8.10.</strong> 示例: 聊天服务</a></li></ol></li><li class="chapter-item expanded "><a href="../ch9/ch9.html"><strong aria-hidden="true">9.</strong> 基于共享变量的并发</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../ch9/ch9-01.html"><strong aria-hidden="true">9.1.</strong> 竞争条件</a></li><li class="chapter-item expanded "><a href="../ch9/ch9-02.html"><strong aria-hidden="true">9.2.</strong> sync.Mutex互斥锁</a></li><li class="chapter-item expanded "><a href="../ch9/ch9-03.html"><strong aria-hidden="true">9.3.</strong> sync.RWMutex读写锁</a></li><li class="chapter-item expanded "><a href="../ch9/ch9-04.html"><strong aria-hidden="true">9.4.</strong> 内存同步</a></li><li class="chapter-item expanded "><a href="../ch9/ch9-05.html"><strong aria-hidden="true">9.5.</strong> sync.Once惰性初始化</a></li><li class="chapter-item expanded "><a href="../ch9/ch9-06.html"><strong aria-hidden="true">9.6.</strong> 竞争条件检测</a></li><li class="chapter-item expanded "><a href="../ch9/ch9-07.html"><strong aria-hidden="true">9.7.</strong> 示例: 并发的非阻塞缓存</a></li><li class="chapter-item expanded "><a href="../ch9/ch9-08.html"><strong aria-hidden="true">9.8.</strong> Goroutines和线程</a></li></ol></li><li class="chapter-item expanded "><a href="../ch10/ch10.html"><strong aria-hidden="true">10.</strong> 包和工具</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../ch10/ch10-01.html"><strong aria-hidden="true">10.1.</strong> 包简介</a></li><li class="chapter-item expanded "><a href="../ch10/ch10-02.html"><strong aria-hidden="true">10.2.</strong> 导入路径</a></li><li class="chapter-item expanded "><a href="../ch10/ch10-03.html"><strong aria-hidden="true">10.3.</strong> 包声明</a></li><li class="chapter-item expanded "><a href="../ch10/ch10-04.html"><strong aria-hidden="true">10.4.</strong> 导入声明</a></li><li class="chapter-item expanded "><a href="../ch10/ch10-05.html"><strong aria-hidden="true">10.5.</strong> 包的匿名导入</a></li><li class="chapter-item expanded "><a href="../ch10/ch10-06.html"><strong aria-hidden="true">10.6.</strong> 包和命名</a></li><li class="chapter-item expanded "><a href="../ch10/ch10-07.html"><strong aria-hidden="true">10.7.</strong> 工具</a></li></ol></li><li class="chapter-item expanded "><a href="../ch11/ch11.html"><strong aria-hidden="true">11.</strong> 测试</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../ch11/ch11-01.html"><strong aria-hidden="true">11.1.</strong> go test</a></li><li class="chapter-item expanded "><a href="../ch11/ch11-02.html"><strong aria-hidden="true">11.2.</strong> 测试函数</a></li><li class="chapter-item expanded "><a href="../ch11/ch11-03.html"><strong aria-hidden="true">11.3.</strong> 测试覆盖率</a></li><li class="chapter-item expanded "><a href="../ch11/ch11-04.html"><strong aria-hidden="true">11.4.</strong> 基准测试</a></li><li class="chapter-item expanded "><a href="../ch11/ch11-05.html"><strong aria-hidden="true">11.5.</strong> 剖析</a></li><li class="chapter-item expanded "><a href="../ch11/ch11-06.html"><strong aria-hidden="true">11.6.</strong> 示例函数</a></li></ol></li><li class="chapter-item expanded "><a href="../ch12/ch12.html"><strong aria-hidden="true">12.</strong> 反射</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../ch12/ch12-01.html"><strong aria-hidden="true">12.1.</strong> 为何需要反射?</a></li><li class="chapter-item expanded "><a href="../ch12/ch12-02.html"><strong aria-hidden="true">12.2.</strong> reflect.Type和reflect.Value</a></li><li class="chapter-item expanded "><a href="../ch12/ch12-03.html"><strong aria-hidden="true">12.3.</strong> Display递归打印</a></li><li class="chapter-item expanded "><a href="../ch12/ch12-04.html"><strong aria-hidden="true">12.4.</strong> 示例: 编码S表达式</a></li><li class="chapter-item expanded "><a href="../ch12/ch12-05.html"><strong aria-hidden="true">12.5.</strong> 通过reflect.Value修改值</a></li><li class="chapter-item expanded "><a href="../ch12/ch12-06.html"><strong aria-hidden="true">12.6.</strong> 示例: 解码S表达式</a></li><li class="chapter-item expanded "><a href="../ch12/ch12-07.html"><strong aria-hidden="true">12.7.</strong> 获取结构体字段标签</a></li><li class="chapter-item expanded "><a href="../ch12/ch12-08.html"><strong aria-hidden="true">12.8.</strong> 显示一个类型的方法集</a></li><li class="chapter-item expanded "><a href="../ch12/ch12-09.html"><strong aria-hidden="true">12.9.</strong> 几点忠告</a></li></ol></li><li class="chapter-item expanded "><a href="../ch13/ch13.html"><strong aria-hidden="true">13.</strong> 底层编程</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../ch13/ch13-01.html"><strong aria-hidden="true">13.1.</strong> unsafe.Sizeof, Alignof 和 Offsetof</a></li><li class="chapter-item expanded "><a href="../ch13/ch13-02.html"><strong aria-hidden="true">13.2.</strong> unsafe.Pointer</a></li><li class="chapter-item expanded "><a href="../ch13/ch13-03.html"><strong aria-hidden="true">13.3.</strong> 示例: 深度相等判断</a></li><li class="chapter-item expanded "><a href="../ch13/ch13-04.html"><strong aria-hidden="true">13.4.</strong> 通过cgo调用C代码</a></li><li class="chapter-item expanded "><a href="../ch13/ch13-05.html"><strong aria-hidden="true">13.5.</strong> 几点忠告</a></li></ol></li><li class="chapter-item expanded "><a href="../appendix/appendix.html"><strong aria-hidden="true">14.</strong> 附录</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../appendix/appendix-a-errata.html"><strong aria-hidden="true">14.1.</strong> 附录A：原文勘误</a></li><li class="chapter-item expanded "><a href="../appendix/appendix-b-author.html"><strong aria-hidden="true">14.2.</strong> 附录B：作者译者</a></li><li class="chapter-item expanded "><a href="../appendix/appendix-c-cpoyright.html"><strong aria-hidden="true">14.3.</strong> 附录C：译文授权</a></li><li class="chapter-item expanded "><a href="../appendix/appendix-d-translations.html"><strong aria-hidden="true">14.4.</strong> 附录D：其它语言</a></li></ol></li></ol> </div>
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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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<li><em>凹语言(Go实现, 面向WASM设计): <a href="https://github.com/wa-lang/wa">https://github.com/wa-lang/wa</a></em></li>
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<hr>
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<h2 id="31-整型"><a class="header" href="#31-整型">3.1. 整型</a></h2>
|
||
<p>Go语言的数值类型包括几种不同大小的整数、浮点数和复数。每种数值类型都决定了对应的大小范围和是否支持正负符号。让我们先从整数类型开始介绍。</p>
|
||
<p>Go语言同时提供了有符号和无符号类型的整数运算。这里有int8、int16、int32和int64四种截然不同大小的有符号整数类型，分别对应8、16、32、64bit大小的有符号整数，与此对应的是uint8、uint16、uint32和uint64四种无符号整数类型。</p>
|
||
<p>这里还有两种一般对应特定CPU平台机器字大小的有符号和无符号整数int和uint；其中int是应用最广泛的数值类型。这两种类型都有同样的大小，32或64bit，但是我们不能对此做任何的假设；因为不同的编译器即使在相同的硬件平台上可能产生不同的大小。</p>
|
||
<p>Unicode字符rune类型是和int32等价的类型，通常用于表示一个Unicode码点。这两个名称可以互换使用。同样byte也是uint8类型的等价类型，byte类型一般用于强调数值是一个原始的数据而不是一个小的整数。</p>
|
||
<p>最后，还有一种无符号的整数类型uintptr，没有指定具体的bit大小但是足以容纳指针。uintptr类型只有在底层编程时才需要，特别是Go语言和C语言函数库或操作系统接口相交互的地方。我们将在第十三章的unsafe包相关部分看到类似的例子。</p>
|
||
<p>不管它们的具体大小，int、uint和uintptr是不同类型的兄弟类型。其中int和int32也是不同的类型，即使int的大小也是32bit，在需要将int当作int32类型的地方需要一个显式的类型转换操作，反之亦然。</p>
|
||
<p>其中有符号整数采用2的补码形式表示，也就是最高bit位用来表示符号位，一个n-bit的有符号数的值域是从-2<sup>n-1</sup>到2<sup>n-1</sup>-1。无符号整数的所有bit位都用于表示非负数，值域是0到2<sup>n</sup>-1。例如，int8类型整数的值域是从-128到127，而uint8类型整数的值域是从0到255。</p>
|
||
<p>下面是Go语言中关于算术运算、逻辑运算和比较运算的二元运算符，它们按照优先级递减的顺序排列：</p>
|
||
<pre><code>* / % << >> & &^
|
||
+ - | ^
|
||
== != < <= > >=
|
||
&&
|
||
||
|
||
</code></pre>
|
||
<p>二元运算符有五种优先级。在同一个优先级，使用左优先结合规则，但是使用括号可以明确优先顺序，使用括号也可以用于提升优先级，例如<code>mask & (1 << 28)</code>。</p>
|
||
<p>对于上表中前两行的运算符，例如+运算符还有一个与赋值相结合的对应运算符+=，可以用于简化赋值语句。</p>
|
||
<p>算术运算符<code>+</code>、<code>-</code>、<code>*</code>和<code>/</code>可以适用于整数、浮点数和复数，但是取模运算符%仅用于整数间的运算。对于不同编程语言，%取模运算的行为可能并不相同。在Go语言中，%取模运算符的符号和被取模数的符号总是一致的，因此<code>-5%3</code>和<code>-5%-3</code>结果都是-2。除法运算符<code>/</code>的行为则依赖于操作数是否全为整数，比如<code>5.0/4.0</code>的结果是1.25，但是5/4的结果是1，因为整数除法会向着0方向截断余数。</p>
|
||
<p>一个算术运算的结果，不管是有符号或者是无符号的，如果需要更多的bit位才能正确表示的话，就说明计算结果是溢出了。超出的高位的bit位部分将被丢弃。如果原始的数值是有符号类型，而且最左边的bit位是1的话，那么最终结果可能是负的，例如int8的例子：</p>
|
||
<pre><code class="language-Go">var u uint8 = 255
|
||
fmt.Println(u, u+1, u*u) // "255 0 1"
|
||
|
||
var i int8 = 127
|
||
fmt.Println(i, i+1, i*i) // "127 -128 1"
|
||
</code></pre>
|
||
<p>两个相同的整数类型可以使用下面的二元比较运算符进行比较；比较表达式的结果是布尔类型。</p>
|
||
<pre><code>== 等于
|
||
!= 不等于
|
||
< 小于
|
||
<= 小于等于
|
||
> 大于
|
||
>= 大于等于
|
||
</code></pre>
|
||
<p>事实上，布尔型、数字类型和字符串等基本类型都是可比较的，也就是说两个相同类型的值可以用==和!=进行比较。此外，整数、浮点数和字符串可以根据比较结果排序。许多其它类型的值可能是不可比较的，因此也就可能是不可排序的。对于我们遇到的每种类型，我们需要保证规则的一致性。</p>
|
||
<p>这里是一元的加法和减法运算符：</p>
|
||
<pre><code>+ 一元加法（无效果）
|
||
- 负数
|
||
</code></pre>
|
||
<p>对于整数，+x是0+x的简写，-x则是0-x的简写；对于浮点数和复数，+x就是x，-x则是x 的负数。</p>
|
||
<p>Go语言还提供了以下的bit位操作运算符，前面4个操作运算符并不区分是有符号还是无符号数：</p>
|
||
<pre><code>& 位运算 AND
|
||
| 位运算 OR
|
||
^ 位运算 XOR
|
||
&^ 位清空（AND NOT）
|
||
<< 左移
|
||
>> 右移
|
||
</code></pre>
|
||
<p>位操作运算符<code>^</code>作为二元运算符时是按位异或（XOR），当用作一元运算符时表示按位取反；也就是说，它返回一个每个bit位都取反的数。位操作运算符<code>&^</code>用于按位置零（AND NOT）：如果对应y中bit位为1的话，表达式<code>z = x &^ y</code>结果z的对应的bit位为0，否则z对应的bit位等于x相应的bit位的值。</p>
|
||
<p>下面的代码演示了如何使用位操作解释uint8类型值的8个独立的bit位。它使用了Printf函数的%b参数打印二进制格式的数字；其中%08b中08表示打印至少8个字符宽度，不足的前缀部分用0填充。</p>
|
||
<pre><code class="language-Go">var x uint8 = 1<<1 | 1<<5
|
||
var y uint8 = 1<<1 | 1<<2
|
||
|
||
fmt.Printf("%08b\n", x) // "00100010", the set {1, 5}
|
||
fmt.Printf("%08b\n", y) // "00000110", the set {1, 2}
|
||
|
||
fmt.Printf("%08b\n", x&y) // "00000010", the intersection {1}
|
||
fmt.Printf("%08b\n", x|y) // "00100110", the union {1, 2, 5}
|
||
fmt.Printf("%08b\n", x^y) // "00100100", the symmetric difference {2, 5}
|
||
fmt.Printf("%08b\n", x&^y) // "00100000", the difference {5}
|
||
|
||
for i := uint(0); i < 8; i++ {
|
||
if x&(1<<i) != 0 { // membership test
|
||
fmt.Println(i) // "1", "5"
|
||
}
|
||
}
|
||
|
||
fmt.Printf("%08b\n", x<<1) // "01000100", the set {2, 6}
|
||
fmt.Printf("%08b\n", x>>1) // "00010001", the set {0, 4}
|
||
</code></pre>
|
||
<p>（6.5节给出了一个可以远大于一个字节的整数集的实现。）</p>
|
||
<p>在<code>x<<n</code>和<code>x>>n</code>移位运算中，决定了移位操作的bit数部分必须是无符号数；被操作的x可以是有符号数或无符号数。算术上，一个<code>x<<n</code>左移运算等价于乘以$2^n$，一个<code>x>>n</code>右移运算等价于除以$2^n$。</p>
|
||
<p>左移运算用零填充右边空缺的bit位，无符号数的右移运算也是用0填充左边空缺的bit位，但是有符号数的右移运算会用符号位的值填充左边空缺的bit位。因为这个原因，最好用无符号运算，这样你可以将整数完全当作一个bit位模式处理。</p>
|
||
<p>尽管Go语言提供了无符号数的运算，但即使数值本身不可能出现负数，我们还是倾向于使用有符号的int类型，就像数组的长度那样，虽然使用uint无符号类型似乎是一个更合理的选择。事实上，内置的len函数返回一个有符号的int，我们可以像下面例子那样处理逆序循环。</p>
|
||
<pre><code class="language-Go">medals := []string{"gold", "silver", "bronze"}
|
||
for i := len(medals) - 1; i >= 0; i-- {
|
||
fmt.Println(medals[i]) // "bronze", "silver", "gold"
|
||
}
|
||
</code></pre>
|
||
<p>另一个选择对于上面的例子来说将是灾难性的。如果len函数返回一个无符号数，那么i也将是无符号的uint类型，然后条件<code>i >= 0</code>则永远为真。在三次迭代之后，也就是<code>i == 0</code>时，i--语句将不会产生-1，而是变成一个uint类型的最大值（可能是$2^64-1$），然后medals[i]表达式运行时将发生panic异常（§5.9），也就是试图访问一个slice范围以外的元素。</p>
|
||
<p>出于这个原因，无符号数往往只有在位运算或其它特殊的运算场景才会使用，就像bit集合、分析二进制文件格式或者是哈希和加密操作等。它们通常并不用于仅仅是表达非负数量的场合。</p>
|
||
<p>一般来说，需要一个显式的转换将一个值从一种类型转化为另一种类型，并且算术和逻辑运算的二元操作中必须是相同的类型。虽然这偶尔会导致需要很长的表达式，但是它消除了所有和类型相关的问题，而且也使得程序容易理解。</p>
|
||
<p>在很多场景，会遇到类似下面代码的常见的错误：</p>
|
||
<pre><code class="language-Go">var apples int32 = 1
|
||
var oranges int16 = 2
|
||
var compote int = apples + oranges // compile error
|
||
</code></pre>
|
||
<p>当尝试编译这三个语句时，将产生一个错误信息：</p>
|
||
<pre><code>invalid operation: apples + oranges (mismatched types int32 and int16)
|
||
</code></pre>
|
||
<p>这种类型不匹配的问题可以有几种不同的方法修复，最常见方法是将它们都显式转型为一个常见类型：</p>
|
||
<pre><code class="language-Go">var compote = int(apples) + int(oranges)
|
||
</code></pre>
|
||
<p>如2.5节所述，对于每种类型T，如果转换允许的话，类型转换操作T(x)将x转换为T类型。许多整数之间的相互转换并不会改变数值；它们只是告诉编译器如何解释这个值。但是对于将一个大尺寸的整数类型转为一个小尺寸的整数类型，或者是将一个浮点数转为整数，可能会改变数值或丢失精度：</p>
|
||
<pre><code class="language-Go">f := 3.141 // a float64
|
||
i := int(f)
|
||
fmt.Println(f, i) // "3.141 3"
|
||
f = 1.99
|
||
fmt.Println(int(f)) // "1"
|
||
</code></pre>
|
||
<p>浮点数到整数的转换将丢失任何小数部分，然后向数轴零方向截断。你应该避免对可能会超出目标类型表示范围的数值做类型转换，因为截断的行为可能依赖于具体的实现：</p>
|
||
<pre><code class="language-Go">f := 1e100 // a float64
|
||
i := int(f) // 结果依赖于具体实现
|
||
</code></pre>
|
||
<p>任何大小的整数字面值都可以用以0开始的八进制格式书写，例如0666；或用以0x或0X开头的十六进制格式书写，例如0xdeadbeef。十六进制数字可以用大写或小写字母。如今八进制数据通常用于POSIX操作系统上的文件访问权限标志，十六进制数字则更强调数字值的bit位模式。</p>
|
||
<p>当使用fmt包打印一个数值时，我们可以用%d、%o或%x参数控制输出的进制格式，就像下面的例子：</p>
|
||
<pre><code class="language-Go">o := 0666
|
||
fmt.Printf("%d %[1]o %#[1]o\n", o) // "438 666 0666"
|
||
x := int64(0xdeadbeef)
|
||
fmt.Printf("%d %[1]x %#[1]x %#[1]X\n", x)
|
||
// Output:
|
||
// 3735928559 deadbeef 0xdeadbeef 0XDEADBEEF
|
||
</code></pre>
|
||
<p>请注意fmt的两个使用技巧。通常Printf格式化字符串包含多个%参数时将会包含对应相同数量的额外操作数，但是%之后的<code>[1]</code>副词告诉Printf函数再次使用第一个操作数。第二，%后的<code>#</code>副词告诉Printf在用%o、%x或%X输出时生成0、0x或0X前缀。</p>
|
||
<p>字符面值通过一对单引号直接包含对应字符。最简单的例子是ASCII中类似'a'写法的字符面值，但是我们也可以通过转义的数值来表示任意的Unicode码点对应的字符，马上将会看到这样的例子。</p>
|
||
<p>字符使用<code>%c</code>参数打印，或者是用<code>%q</code>参数打印带单引号的字符：</p>
|
||
<pre><code class="language-Go">ascii := 'a'
|
||
unicode := '国'
|
||
newline := '\n'
|
||
fmt.Printf("%d %[1]c %[1]q\n", ascii) // "97 a 'a'"
|
||
fmt.Printf("%d %[1]c %[1]q\n", unicode) // "22269 国 '国'"
|
||
fmt.Printf("%d %[1]q\n", newline) // "10 '\n'"
|
||
</code></pre>
|
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