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<i class="fa fa-check"></i> <b>1.</b> 欢迎使用 Swift
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<i class="fa fa-check"></i> <b>1.2.</b> Swift 初见
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<i class="fa fa-check"></i> <b>2.</b> Swift 教程
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<i class="fa fa-check"></i> <b>2.1.</b> 基础部分
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<i class="fa fa-check"></i> <b>2.2.</b> 基本操作符
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<i class="fa fa-check"></i> <b>2.3.</b> 字符串和字符
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<i class="fa fa-check"></i> <b>2.4.</b> 集合类型
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</a>
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<a href="../chapter2/05_Control_Flow.html">
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<i class="fa fa-check"></i> <b>2.5.</b> 控制流
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</a>
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<li class="chapter " data-level="2.6" data-path="chapter2/06_Functions.html">
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<a href="../chapter2/06_Functions.html">
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<i class="fa fa-check"></i> <b>2.6.</b> 函数
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</a>
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<li class="chapter " data-level="2.7" data-path="chapter2/07_Closures.html">
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<a href="../chapter2/07_Closures.html">
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<i class="fa fa-check"></i> <b>2.7.</b> 闭包
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</a>
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<li class="chapter " data-level="2.8" data-path="chapter2/08_Enumerations.html">
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<a href="../chapter2/08_Enumerations.html">
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<i class="fa fa-check"></i> <b>2.8.</b> 枚举
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</a>
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<li class="chapter " data-level="2.9" data-path="chapter2/09_Classes_and_Structures.html">
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<a href="../chapter2/09_Classes_and_Structures.html">
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<i class="fa fa-check"></i> <b>2.9.</b> 类和结构体
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</a>
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</li>
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<li class="chapter " data-level="2.10" data-path="chapter2/10_Properties.html">
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<a href="../chapter2/10_Properties.html">
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<i class="fa fa-check"></i> <b>2.10.</b> 属性
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</a>
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</li>
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<li class="chapter " data-level="2.11" data-path="chapter2/11_Methods.html">
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<a href="../chapter2/11_Methods.html">
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<i class="fa fa-check"></i> <b>2.11.</b> 方法
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</a>
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</li>
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<li class="chapter " data-level="2.12" data-path="chapter2/12_Subscripts.html">
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<a href="../chapter2/12_Subscripts.html">
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<i class="fa fa-check"></i> <b>2.12.</b> 下标
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</a>
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</li>
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<li class="chapter " data-level="2.13" data-path="chapter2/13_Inheritance.html">
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<a href="../chapter2/13_Inheritance.html">
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<i class="fa fa-check"></i> <b>2.13.</b> 继承
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</a>
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</li>
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<li class="chapter " data-level="2.14" data-path="chapter2/14_Initialization.html">
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<a href="../chapter2/14_Initialization.html">
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<i class="fa fa-check"></i> <b>2.14.</b> 构造函数
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</a>
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</li>
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<li class="chapter " data-level="2.15" data-path="chapter2/15_Deinitialization.html">
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<a href="../chapter2/15_Deinitialization.html">
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<i class="fa fa-check"></i> <b>2.15.</b> 析构函数
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</a>
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</li>
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<li class="chapter " data-level="2.16" data-path="chapter2/16_Automatic_Reference_Counting.html">
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<a href="../chapter2/16_Automatic_Reference_Counting.html">
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<i class="fa fa-check"></i> <b>2.16.</b> 自动引用计数
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</a>
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|
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|
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</li>
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|
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<li class="chapter " data-level="2.17" data-path="chapter2/17_Optional_Chaining.html">
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<a href="../chapter2/17_Optional_Chaining.html">
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<i class="fa fa-check"></i> <b>2.17.</b> 可选链
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||
</a>
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</li>
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<li class="chapter " data-level="2.18" data-path="chapter2/18_Type_Casting.html">
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<a href="../chapter2/18_Type_Casting.html">
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<i class="fa fa-check"></i> <b>2.18.</b> 类型检查
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</a>
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</li>
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<li class="chapter " data-level="2.19" data-path="chapter2/19_Nested_Types.html">
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<a href="../chapter2/19_Nested_Types.html">
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<i class="fa fa-check"></i> <b>2.19.</b> 嵌套类型
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</a>
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||
|
||
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</li>
|
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|
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<li class="chapter " data-level="2.20" data-path="chapter2/20_Extensions.html">
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<a href="../chapter2/20_Extensions.html">
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<i class="fa fa-check"></i> <b>2.20.</b> 扩展
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</a>
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||
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</li>
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<li class="chapter " data-level="2.21" data-path="chapter2/21_Protocols.html">
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<a href="../chapter2/21_Protocols.html">
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<i class="fa fa-check"></i> <b>2.21.</b> 接口
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||
</a>
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||
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</li>
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<li class="chapter " data-level="2.22" data-path="chapter2/22_Generics.html">
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<a href="../chapter2/22_Generics.html">
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<i class="fa fa-check"></i> <b>2.22.</b> 泛型
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</a>
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</li>
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<li class="chapter " data-level="2.23" data-path="chapter2/23_Advanced_Operators.html">
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<a href="../chapter2/23_Advanced_Operators.html">
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<i class="fa fa-check"></i> <b>2.23.</b> 高级操作符
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</a>
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</li>
|
||
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</ul>
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</li>
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<li class="chapter " data-level="3" data-path="chapter3/chapter3.html">
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<a href="../chapter3/chapter3.html">
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<i class="fa fa-check"></i> <b>3.</b> 语言参考
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</a>
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<ul class="articles">
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<li class="chapter " data-level="3.1" data-path="chapter3/01_About_the_Language_Reference.html">
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<a href="../chapter3/01_About_the_Language_Reference.html">
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<i class="fa fa-check"></i> <b>3.1.</b> 关于语言参考
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</a>
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|
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</li>
|
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|
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<li class="chapter " data-level="3.2" data-path="chapter3/02_Lexical_Structure.html">
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<a href="../chapter3/02_Lexical_Structure.html">
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<i class="fa fa-check"></i> <b>3.2.</b> 词法结构
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</a>
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</li>
|
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<li class="chapter " data-level="3.3" data-path="chapter3/03_Types.html">
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<a href="../chapter3/03_Types.html">
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<i class="fa fa-check"></i> <b>3.3.</b> 类型
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||
</a>
|
||
|
||
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</li>
|
||
|
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<li class="chapter " data-level="3.4" data-path="chapter3/04_Expressions.html">
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<a href="../chapter3/04_Expressions.html">
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<i class="fa fa-check"></i> <b>3.4.</b> 表达式
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||
</a>
|
||
|
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|
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</li>
|
||
|
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<li class="chapter " data-level="3.5" data-path="chapter3/05_Declarations.html">
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|
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<a href="../chapter3/05_Declarations.html">
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<i class="fa fa-check"></i> <b>3.5.</b> 声明
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</a>
|
||
|
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|
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</li>
|
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|
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<li class="chapter " data-level="3.6" data-path="chapter3/06_Attributes.html">
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<a href="../chapter3/06_Attributes.html">
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<i class="fa fa-check"></i> <b>3.6.</b> 属性
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||
</a>
|
||
|
||
|
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</li>
|
||
|
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<li class="chapter " data-level="3.7" data-path="chapter3/07_Patterns.html">
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|
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<a href="../chapter3/07_Patterns.html">
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<i class="fa fa-check"></i> <b>3.7.</b> 模式
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</a>
|
||
|
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|
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</li>
|
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|
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<li class="chapter " data-level="3.8" data-path="chapter3/08_Generic_Parameters_and_Arguments.html">
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<a href="../chapter3/08_Generic_Parameters_and_Arguments.html">
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<i class="fa fa-check"></i> <b>3.8.</b> 泛型参数
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</a>
|
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|
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|
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</li>
|
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|
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<li class="chapter " data-level="3.9" data-path="chapter3/09_Summary_of_the_Grammar.html">
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|
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<a href="../chapter3/09_Summary_of_the_Grammar.html">
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<i class="fa fa-check"></i> <b>3.9.</b> 语法总结
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</a>
|
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|
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|
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</li>
|
||
|
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<div class="page-inner">
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<section class="normal" id="section-gitbook_9">
|
||
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||
<h1 id="-">基础部分</h1>
|
||
<p>Swift 是 iOS 和 OS X 应用开发的一门新语言。然而，如果你有 C 或者 Objective-C 开发经验的话，你会发现 Swift 的很多内容都是你熟悉的。</p>
|
||
<p>Swift 的类型是在 C 和 Objective-C 的基础上提出的，<code>Int</code>是整型；<code>Double</code>和<code>Float</code>是浮点型；<code>Bool</code>是布尔型；<code>String</code>是字符串。Swift 还有两个有用的集合类型，<code>Array</code>和<code>Dictionary</code>，详情参见<code>集合类型(待添加链接)</code>。</p>
|
||
<p>就像 C 语言一样，Swift 使用变量来进行存储并通过变量名来关联值。在 Swift 中，值不可变的变量有着广泛的应用，它们就是常量，而且比 C 语言的常量更强大。在 Swift 中，如果你要处理的值不需要改变，那使用常量可以让你的代码更加安全并且更好地表达你的意图。</p>
|
||
<p>除了我们熟悉的类型，Swift 还增加了 Objective-C 中没有的类型比如元组（Tuple）。元组可以让你创建或者传递一组数据，比如作为函数的返回值时，你可以用一个元组可以返回多个值。</p>
|
||
<p>Swift 还增加了可选（Optional）类型，用于处理值缺失的情况。可选表示“那儿有一个值，并且它等于 x ”或者“那儿没有值”。可选有点像在 Objective-C 中使用<code>nil</code>，但是它可以用在任何类型上，不仅仅是类。可选类型比 Objective-C 中的<code>nil</code>指针更加安全也更具表现力，它是 Swift 许多强大特性的重要组成部分。</p>
|
||
<p>Swift 是一个类型安全的语言，可选就是一个很好的例子。Swift 可以让你清楚地知道值的类型。如果你的代码期望得到一个<code>String</code>，类型安全会阻止你不小心传入一个<code>Int</code>。你可以在开发阶段尽早发现并修正错误。</p>
|
||
<h2 id="-">常量和变量</h2>
|
||
<p>常量和变量把一个名字（比如<code>maximumNumberOfLoginAttempts</code>或者<code>welcomeMessage</code>）和一个指定类型的值（比如数字<code>10</code>或者字符串<code>Hello</code>）联系起来。常量的值一旦设定就不能改变，而变量的值可以随意更改。</p>
|
||
<h3 id="-">声明常量和变量</h3>
|
||
<p>常量和变量必须在使用前声明，用<code>let</code>来声明常量，用<code>var</code>来声明变量。下面的例子展示了如何用常量和变量来记录用户尝试登录的次数：</p>
|
||
<pre><code>let maximumNumberOfLoginAttempts = 10
|
||
var currentLoginAttempt = 0
|
||
</code></pre><p>这两行代码可以被理解为：</p>
|
||
<p>“声明一个名字是<code>maximumNumberOfLoginAttempts</code>的新常量，并给它一个值<code>10</code>。然后，声明一个名字是<code>currentLoginAttempt</code>的变量并将它的值初始化为0.”</p>
|
||
<p>在这个例子中，允许的最大尝试登录次数被声明为一个常量，因为这个值不会改变。当前尝试登录次数被声明为一个变量，因为每次尝试登录失败的时候都需要增加这个值。</p>
|
||
<p>你可以在一行中声明多个常量或者多个变量，用逗号隔开：</p>
|
||
<pre><code>var x = 0.0, y = 0.0, z = 0.0
|
||
</code></pre><blockquote>
|
||
<p>注意：如果你的代码中有不需要改变的值，最好将它声明为常量。只将需要改变的值声明为变量。</p>
|
||
</blockquote>
|
||
<h3 id="-">类型标注</h3>
|
||
<p>当你声明常量或者变量的时候可以加上类型标注，说明常量或者变量中要存储的值的类型。如果要添加类型标注，在常量或者变量名后面加上一个冒号和空格，然后加上类型名称。</p>
|
||
<p>这个例子给<code>welcomeMessage</code>变量添加了类型标注，表示这个变量可以存储<code>String</code>类型的值：</p>
|
||
<pre><code>var welcomeMessage: String
|
||
</code></pre><p>声明中的冒号代表着“是...类型”，所以这行代码可以被理解为：：</p>
|
||
<p>“声明一个类型为<code>String</code>，名字为<code>welcomeMessage</code>的变量。”</p>
|
||
<p>“类型为<code>String</code>”的意思是“可以存储任意<code>String</code>类型的值。”</p>
|
||
<p><code>welcomeMessage</code>变量现在可以被设置成任意字符串：</p>
|
||
<pre><code>welcomeMessage = "Hello"
|
||
</code></pre><blockquote>
|
||
<p>注意：一般来说你很少需要写类型标注。如果你在声明常量或者变量的时候赋了一个初始值，Swift可以推断出这个常量或者变量的类型，详情参见<code>类型安全和类型推断(待添加链接)</code>。在上面的例子中，没有给<code>welcomeMessage</code>赋初始值，所以添加了一个类型标注。</p>
|
||
</blockquote>
|
||
<h3 id="-">常量和变量的命名</h3>
|
||
<p>你可以用任何你喜欢的字符作为常量和变量名，包括Unicode字符：</p>
|
||
<pre><code> let π = 3.14159
|
||
let 你好 = "你好世界"
|
||
let 🐶🐮 = "dogcow"
|
||
</code></pre><p>常量与变量名不能包含数学符号，箭头，保留的(或者非法的)Unicode码位，连线与制表符。尽管常量与变量名中可以包含数字，但是它们不能以数字打头。</p>
|
||
<p>一旦你将常量或者变量声明为确定的类型，你就不能使用相同的名字再次进行声明，或者以改变其存储的值为其他类型。同时，你也不能将常量与变量进行互转。</p>
|
||
<blockquote>
|
||
<p>注意：如果你需要使用与Swift保留关键字相同的名称作为常量或者变量名，你可以使用反引号(`)将关键字围住的方式将其作为名字使用。无论如何，你应当避免使用关键字作为常量或变量名，除非你别无选择。</p>
|
||
</blockquote>
|
||
<p>你可以更改现有的变量值为其他同类型的值，在下面的例子中，<code>friendlyWelcome</code>的值从<code>"Hello!"</code>改为了<code>"Bonjour!"</code>:</p>
|
||
<pre><code> var friendlyWelcome = "Hello!"
|
||
friendlyWelcome = "Bonjour!"
|
||
// friendlyWelcome is now "Bonjour!"
|
||
</code></pre><p>和变量不一样，常量的值一旦被确定以后就不能更改了。尝试这样做会在编译时报错：</p>
|
||
<pre><code> let languageName = "Swift"
|
||
languageName = "Swift++"
|
||
// this is a compile-time error - languageName cannot be changed
|
||
</code></pre><h3 id="-">输出常量和变量</h3>
|
||
<p>你可以用<code>println</code>函数来输出当前常量或变量的值:</p>
|
||
<pre><code> println(friendlyWelcome)
|
||
// prints "Bonjour!"
|
||
</code></pre><p><code>println</code>是一个用来输出的全局函数，输出的内容会在最后带换行。如果你用Xcode，<code>println</code>将会输出内容到“console”面板上。(另一种函数叫<code>print</code>，唯一区别是在输出内容最后不会加入换行。)</p>
|
||
<p><code>println</code>函数输出传入的<code>String</code>值：</p>
|
||
<pre><code> println("This is a string")
|
||
// prints "This is a string"
|
||
</code></pre><p>像Cocoa里的<code>NSLog</code>函数一样，<code>println</code>函数可以输出更复杂的信息。这些信息可以包含当前常量和变量的值。</p>
|
||
<p>Swift用字符串插值（string interpolation）的方式把常量名或者变量名当做占位符加入到长字符串中，Swift会用当前常量或变量的值替换这些占位符。将常量或变量名放入反斜杠符加一对圆括号中<code>"\()"</code>：</p>
|
||
<pre><code> println("The current value of friendlyWelcome is \(friendlyWelcome)")
|
||
// prints "The current value of friendlyWelcome is Bonjour!
|
||
</code></pre><blockquote>
|
||
<p>注意：字符串插值所有可用的选项在 字符串插值 这章中讲述。</p>
|
||
</blockquote>
|
||
|
||
|
||
</section>
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