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< section class = "normal" id = "section-gitbook_65 3" >
< section class = "normal" id = "section-gitbook_7 3" >
< blockquote >
< p > 翻译： < / p >
< p > 校对： numbbbbb, coverxit < / p >
< p > 翻译：< a href = "https://github.com/coverxit" target = "_blank" > < / a >
校对： < a href = "https://github.com/numbbbbb" target = "_blank" > numbbbbb< / a > , < a href = "https://github.com/coverxit" target = "_blank" > coverxit< / a > , < a href = "https://github.com/stanzhai" target = "_blank" > stanzhai< / a > < / p >
< / blockquote >
< h1 id = "-" > 语句< / h1 >
< hr >
@ -606,162 +606,113 @@
< p > 循环语句用于重复执行代码块；分支语句用于执行满足特定条件的代码块；控制传递语句则用于修改代码的执行顺序。在稍后的叙述中，将会详细地介绍每一种类型的控制流语句。< / p >
< p > 是否将分号（< code > ;< / code > ）添加到语句的结尾处是可选的。但若要在同一行内写多条独立语句，请务必使用分号。< / p >
< blockquote >
< p > GRAMMAR OF A STATEMENT < / p >
< p > < em > statement< / em > → < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Expressions.html#//apple_ref/swift/grammar/expression" target = "_blank" > < em > expression< / em > < / a > < strong > ;< / strong > < em > opt< / em > < / p >
< p > < em > statement< / em > → < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Declarations.html#//apple_ref/swift/grammar/declaration" target = "_blank" > < em > declaration< / em > < / a > < strong > ;< / strong > < em > opt< / em > < / p >
< p > < em > statement< / em > → < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Statements.html#//apple_ref/swift/grammar/loop-statement" target = "_blank" > < em > loop-statement< / em > < / a > < strong > ;< / strong > < em > opt< / em > < / p >
< p > < em > statement< / em > → < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Statements.html#//apple_ref/swift/grammar/branch-statement" target = "_blank" > < em > branch-statement< / em > < / a > < strong > ;< / strong > < em > opt< / em > < / p >
< p > < em > statement< / em > → < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Statements.html#//apple_ref/swift/grammar/labeled-statement" target = "_blank" > < em > labeled-statement< / em > < / a > < / p >
< p > < em > statement< / em > → < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Statements.html#//apple_ref/swift/grammar/control-transfer-statement" target = "_blank" > < em > control-transfer-statement< / em > < / a > < strong > ;< / strong > < em > opt< / em > < / p >
< p > < em > statement< / em > → < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Statements.html#//apple_ref/swift/grammar/statement" target = "_blank" > < em > statment< / em > < / a > < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Statements.html#//apple_ref/swift/grammar/statements" target = "_blank" > < em > statements< / em > < / a > < strong > ;< / strong > < em > opt< / em > < / p >
< p > 语句语法< br > < em > 语句< / em > → < a href = "..\chapter3\04_Expressions.html#expression" > < em > 表达式< / em > < / a > < strong > ;< / strong > < em > 可选< / em > < br > < em > 语句< / em > → < a href = "..\chapter3\05_Declarations.html#declaration" > < em > 声明< / em > < / a > < strong > ;< / strong > < em > 可选< / em > < br > < em > 语句< / em > → < a href = "..\chapter3\10_Statements.html#loop_statement" > < em > 循环语句< / em > < / a > < strong > ;< / strong > < em > 可选< / em > < br > < em > 语句< / em > → < a href = "..\chapter3\10_Statements.html#branch_statement" > < em > 分支语句< / em > < / a > < strong > ;< / strong > < em > 可选< / em > < br > < em > 语句< / em > → < a href = "..\chapter3\10_Statements.html#labeled_statement" > < em > 标记语句(Labeled Statement)< / em > < / a > < br > < em > 语句< / em > → < a href = "..\chapter3\10_Statements.html#control_transfer_statement" > < em > 控制转移语句< / em > < / a > < strong > ;< / strong > < em > 可选< / em > < br > < em > 多条语句(Statements)< / em > → < a href = "..\chapter3\10_Statements.html#statement" > < em > 语句< / em > < / a > < a href = "..\chapter3\10_Statements.html#statements" > < em > 多条语句(Statements)< / em > < / a > < em > 可选< / em > < / p >
< / blockquote >
< p > < a name = "loop_statements" > < / a > < / p >
< h2 id = "-" > 循环语句< / h2 >
< p > 取决于特定的循环条件， < code > for< / code > 语句、< code > for-in< / code > 语句、< code > while< / code > 语句和< code > do-while< / code > 语句。< / p >
< p > 通过< code > break< / code > 语句和< code > continue< / code > 语句可以改变循环语句的控制流。有关这两条语句，详情参见 < a href = "#break_statement" > Break 语句< / a > 和 < a href = "#continue_statement" > Continue 语句< / a > 。< / p >
< blockquote >
< p > GRAMMAR OF A LOOP STATEMENT < / p >
< p > < em > loop-statement< / em > → < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Statements.html#//apple_ref/swift/grammar/for-statement" target = "_blank" > < em > for-statement< / em > < / a > < / p >
< p > < em > loop-statement< / em > → < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Statements.html#//apple_ref/swift/grammar/for-in-statement" target = "_blank" > < em > for-in-statement< / em > < / a > < / p >
< p > < em > loop-statement< / em > → < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Statements.html#//apple_ref/swift/grammar/while-statement" target = "_blank" > < em > while-statement< / em > < / a > < / p >
< p > < em > loop-statement< / em > → < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Statements.html#//apple_ref/swift/grammar/do-while-statement" target = "_blank" > < em > do-while-statement< / em > < / a > < / p >
< p > 循环语句语法< br > < em > 循环语句< / em > → < a href = "..\chapter3\10_Statements.html#for_statement" > < em > for语句< / em > < / a > < br > < em > 循环语句< / em > → < a href = "..\chapter3\10_Statements.html#for_in_statement" > < em > for-in语句< / em > < / a > < br > < em > 循环语句< / em > → < a href = "..\chapter3\10_Statements.html#wheetatype类型ile_statement" > < em > while语句< / em > < / a > < br > < em > 循环语句< / em > → < a href = "..\chapter3\10_Statements.html#do_while_statement" > < em > do-while语句< / em > < / a > < / p >
< / blockquote >
< h3 id = "for-" > For 语句< / h3 >
< p > < code > for< / code > 语句允许在重复执行代码块的同时，递增一个计数器。< / p >
< p > < code > for< / code > 语句的形式如下：< / p >
< pre > < code class = "lang-swift" > for `initialzation`; `condition`; `increment` {
`statements`
}
< / code > < / pre >
< blockquote >
< p > for < code > initialzation< / code > ; < code > condition< / code > ; < code > increment< / code > {< br > < code > statements< / code > < br > } < / p >
< / blockquote >
< p > < em > initialzation< / em > 、< em > condition< / em > 和 < em > increment< / em > 之间的分号，以及包围循环体 < em > statements< / em > 的大括号都是不可省略的。< / p >
< p > < code > for< / code > 语句的执行流程如下：< / p >
< ol >
< li > < p > < > initialzation< / em > 只会被执行一次，通常用于声明和初始化在接下来的循环中需要使用的变量。< / p >
< / li >
< li > < p > 计算 < em > condition< / em > 表达式：
如果为< code > true< / code > ， < em > stat ements < / em > 将会被执行 ，然后转到第3 步。如果为< code > false< / code > ， < em > statements< / em > 和 < em > increment< / em > 都不会被执行，< code > for< / code > 至此执行完毕。< / p >
< / li >
< li > < p > 计算 < em > increment< / em > 表达式， < / p >
< / li >
< li > < em > initialzation< / em > 只会被执行一次，通常用于声明和初始化在接下来的循环中需要使用的变量。< / li >
< li > 计算 < em > condition< / em > 表达式：
如果为< code > true< / code > ， < em > statements< / em > 将会被执行， < code > false< / code > ， < em > statements< / em > 和 < em > increment< / em > 都不会被执行，< code > for< / code > 至此执行完毕。< / li >
< li > 计算 < em > incr ement< / em > 表达式 ，然后转到第2 步。< / li >
< / ol >
< p > 定义在 < em > initialzation< / em > 中的变量仅在< code > for< / code > 语句的作用域以内有效。< em > condition< / em > 表达式的值的类型必须遵循< code > LogicValue< / code > 协议。< / p >
< blockquote >
< p > GRAMMAR OF A FOR STATEMENT < / p >
< p > < em > for-statement< / em > → < strong > for< / strong > < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Statements.html#//apple_ref/swift/grammar/for-init" target = "_blank" > < em > for-init< / em > < / a > < em > opt< / em > < strong > ;< / strong > < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Expressions.html#//apple_ref/swift/grammar/expression" target = "_blank" > < em > expression< / em > < / a > < em > opt< / em > < strong > ;< / strong > < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Expressions.html#//apple_ref/swift/grammar/expression" target = "_blank" > < em > expression< / em > < / a > < em > opt< / em > < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Declarations.html#//apple_ref/swift/grammar/code-block" target = "_blank" > < em > code-block< / em > < / a > < / p >
< p > < em > for-statement< / em > → < strong > for (< / strong > < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Statements.html#//apple_ref/swift/grammar/for-init" target = "_blank" > < em > for-init< / em > < / a > < em > opt< / em > < strong > ;< / strong > < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Expressions.html#//apple_ref/swift/grammar/expression" target = "_blank" > < em > expression< / em > < / a > < em > opt< / em > < strong > ;< / strong > < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Expressions.html#//apple_ref/swift/grammar/expression" target = "_blank" > < em > expression< / em > < / a > < em > opt< / em > < strong > )< / strong > < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Declarations.html#//apple_ref/swift/grammar/code-block" target = "_blank" > < em > code-block< / em > < / a > < / p >
< p > < em > for-statement< / em > → < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Declarations.html#//apple_ref/swift/grammar/variable-declaration" target = "_blank" > < em > variable-declaration< / em > < / a > | < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Expressions.html#//apple_ref/swift/grammar/expression-list" target = "_blank" > < em > expression-list< / em > < / a > < / p >
< p > For 循环语法< br > < em > for语句< / em > → < strong > for< / strong > < a href = "..\chapter3\10_Statements.html#for_init" > < em > for初始条件< / em > < / a > < em > 可选< / em > < strong > ;< / strong > < a href = "..\chapter3\04_Expressions.html#expression" > < em > 表达式< / em > < / a > < em > 可选< / em > < strong > ;< / strong > < a href = "..\chapter3\04_Expressions.html#expression" > < em > 表达式< / em > < / a > < em > 可选< / em > < a href = "..\chapter3\05_Declarations.html#code_block" > < em > 代码块< / em > < / a > < br > < em > for语句< / em > → < strong > for< / strong > < strong > (< / strong > < a href = "..\chapter3\10_Statements.html#for_init" > < em > for初始条件< / em > < / a > < em > 可选< / em > < strong > ;< / strong > < a href = "..\chapter3\04_Expressions.html#expression" > < em > 表达式< / em > < / a > < em > 可选< / em > < strong > ;< / strong > < a href = "..\chapter3\04_Expressions.html#expression" > < em > 表达式< / em > < / a > < em > 可选< / em > < strong > )< / strong > < a href = "..\chapter3\05_Declarations.html#code_block" > < em > 代码块< / em > < / a > < br > < em > for初始条件< / em > → < a href = "..\chapter3\05_Declarations.html#variable_declaration" > < em > 变量声明< / em > < / a > | < a href = "..\chapter3\04_Expressions.html#expression_list" > < em > 表达式列表< / em > < / a > < / p >
< / blockquote >
< h3 id = "for-in-" > For-In 语句< / h3 >
< p > < code > for-in< / code > 语句允许在重复执行代码块的同时，迭代集合（或遵循< code > Sequence< / code > 协议的任意类型）中的每一项。< / p >
< p > < code > for-in< / code > 语句的形式如下：< / p >
< pre > < code class = "lang-swift" > for `item` in `collection` {
`statements`
}
< / code > < / pre >
< blockquote >
< p > for < code > item< / code > in < code > collection< / code > {< br > < code > statements< / code > < br > } < / p >
< / blockquote >
< p > < code > for-in< / code > 语句在循环开始前会调用 < em > collection< / em > 表达式的< code > generate< / code > 方法来获取一个生成器类型（这是一个遵循< code > Generator< / code > 协议的类型）的值。接下来循环开始，调用 < em > collection< / em > 表达式的< code > next< / code > 方法。如果其返回值不是< code > None< / code > ，它将会被赋给 < em > item< / em > ，然后执行 < em > statements< / em > ，执行完毕后回到循环开始处；否则，将不会赋值给 < em > item< / em > 也不会执行 < em > statements< / em > ， < code > for-in< / code > 至此执行完毕。< / p >
< blockquote >
< p > GRAMMAR OF A FOR-IN STATEMENT < / p >
< p > < em > for-in-statement< / em > → < strong > for< / strong > < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Patterns.html#//apple_ref/swift/grammar/pattern" target = "_blank" > < em > pattern< / em > < / a > < strong > in< / strong > < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Expressions.html#//apple_ref/swift/grammar/expression" target = "_blank" > < em > expression< / em > < / a > < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Declarations.html#//apple_ref/swift/grammar/code-block" target = "_blank" > < em > code-block< / em > < / a > < / p >
< p > For-In 循环语法< br > < em > for-in语句< / em > → < strong > for< / strong > < a href = "..\chapter3\07_Patterns.html#pattern" > < em > 模式< / em > < / a > < strong > in< / strong > < a href = "..\chapter3\04_Expressions.html#expression" > < em > 表达式< / em > < / a > < a href = "..\chapter3\05_Declarations.html#code_block" > < em > 代码块< / em > < / a > < / p >
< / blockquote >
< h3 id = "while-" > While 语句< / h3 >
< p > < code > while< / code > 语句允许重复执行代码块。< / p >
< p > < code > while< / code > 语句的形式如下：< / p >
< pre > < code class = "lang-swift" > while `condition` {
`statements`
}
< / code > < / pre >
< blockquote >
< p > while < code > condition< / code > {< br > < code > statements< / code > < br > } < / p >
< / blockquote >
< p > < code > while< / code > 语句的执行流程如下：< / p >
< ol >
< li > < p > < em > condition< / em > 表达式：
如果为真< code > true< / code > ， < code > false< / code > ， < code > while< / code > 至此执行完毕。< / p >
< / li >
< li > < p > 执行 < em > statements< / em > ， < / p >
< / li >
< li > 计算 < em > condition< / em > 表达式：
如果为真< code > true< / code > ， < code > false< / code > ， < code > while< / code > 至此执行完毕。< / li >
< li > 执行 < em > statements< / em > ， < / li >
< / ol >
< p > 由于 < em > condition< / em > 的值在 < em > statements< / em > 执行前就已计算出，因此< code > while< / code > 语句中的 < em > statements< / em > 可能会被执行若干次，也可能不会被执行。< / p >
< p > < em > condition< / em > 表达式的值的类型必须遵循< code > LogicValue< / code > 协议。同时，< em > condition< / em > 表达式也可以使用可选绑定，详情参见< a href = "../chapter2/01_The_Basics.html#optional_binding" > 可选绑定< / a > 。< / p >
< blockquote >
< p > GRAMMAR OF A WHILE STATEMENT < / p >
< p > < em > while-statement< / em > → < strong > while< / strong > < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Statements.html#//apple_ref/swift/grammar/while-condition" target = "_blank" > < em > while-condition< / em > < / a > < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Declarations.html#//apple_ref/swift/grammar/code-block" target = "_blank" > < em > code-block< / em > < / a > < / p >
< p > < em > while-condition< / em > → < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Expressions.html#//apple_ref/swift/grammar/expression" target = "_blank" > < em > expression< / em > < / a > | < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Declarations.html#//apple_ref/swift/grammar/declaration" target = "_blank" > < em > declaration< / em > < / a > < / p >
< p > While 循环语法< br > < em > while语句< / em > → < strong > while< / strong > < a href = "..\chapter3\10_Statements.html#while_condition" > < em > while条件< / em > < / a > < a href = "..\chapter3\05_Declarations.html#code_block" > < em > 代码块< / em > < / a > < br > < em > while条件< / em > → < a href = "..\chapter3\04_Expressions.html#expression" > < em > 表达式< / em > < / a > | < a href = "..\chapter3\05_Declarations.html#declaration" > < em > 声明< / em > < / a > < / p >
< / blockquote >
< h3 id = "do-while-" > Do-While 语句< / h3 >
< p > < code > do-while< / code > 语句允许代码块被执行一次或多次。< / p >
< p > < code > do-while< / code > 语句的形式如下：< / p >
< pre > < code class = "lang-swift" > do {
`statements`
} while `condition`
< / code > < / pre >
< blockquote >
< p > do {< br > < code > statements< / code > < br > } while < code > condition< / code > < / p >
< / blockquote >
< p > < code > do-while< / code > 语句的执行流程如下：< / p >
< ol >
< li > < p > < em > statements< / em > ， < / p >
< / li >
< li > < p > 计算 < em > condition< / em > 表达式：
如果为< code > true< / code > ， < code > false< / code > ， < code > do-while< / code > 至此执行完毕。< / p >
< / li >
< li > 执行 < em > statements< / em > ， < / li >
< li > 计算 < em > condition< / em > 表达式：
如果为< code > true< / code > ， < code > false< / code > ， < code > do-while< / code > 至此执行完毕。< / li >
< / ol >
< p > 由于 < em > condition< / em > 表达式的值是在 < em > statements< / em > 执行后才计算出，因此< code > do-while< / code > 语句中的 < em > statements< / em > 至少会被执行一次。< / p >
< p > < em > condition< / em > 表达式的值的类型必须遵循< code > LogicValue< / code > 协议。同时，< em > condition< / em > 表达式也可以使用可选绑定，详情参见< a href = "../chapter2/01_The_Basics.html#optional_binding" > 可选绑定< / a > 。< / p >
< blockquote >
< p > GRAMMAR OF A DO-WHILE STATEMENT < / p >
< p > < em > do-while-statement< / em > → < strong > do< / strong > < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Declarations.html#//apple_ref/swift/grammar/code-block" target = "_blank" > < em > code-block< / em > < / a > < strong > while< / strong > < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Statements.html#//apple_ref/swift/grammar/while-condition" target = "_blank" > < em > while-condition< / em > < / a > < / p >
< p > Do-While 循环语法< br > < em > do-while语句< / em > → < strong > do< / strong > < a href = "..\chapter3\05_Declarations.html#code_block" > < em > 代码块< / em > < / a > < strong > while< / strong > < a href = "..\chapter3\10_Statements.html#while_condition" > < em > while条件< / em > < / a > < / p >
< / blockquote >
< p > < a name = "branch_statements" > < / a > < / p >
< h2 id = "-" > 分支语句< / h2 >
< p > 取决于一个或者多个条件的值， ， < code > if< / code > 语句和< code > switch< / code > 语句。< / p >
< p > < code > switch< / code > 语句中的控制流可以用< code > break< / code > 语句修改，详情请见< a href = "#break_statement" > Break 语句< / a > 。< / p >
< blockquote >
< p > GRAMMAR OF A BRANCH STATEMENT < / p >
< p > < em > branch-statement< / em > → < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Statements.html#//apple_ref/swift/grammar/if-statement" target = "_blank" > < em > if-statement< / em > < / a > < / p >
< p > < em > branch-statement< / em > → < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Statements.html#//apple_ref/swift/grammar/switch-statement" target = "_blank" > < em > switch-statement< / em > < / a > < / p >
< p > 分支语句语法< br > < em > 分支语句< / em > → < a href = "..\chapter3\10_Statements.html#if_statement" > < em > if语句< / em > < / a > < br > < em > 分支语句< / em > → < a href = "..\chapter3\10_Statements.html#switch_statement" > < em > switch语句< / em > < / a > < / p >
< / blockquote >
< h3 id = "if-" > If 语句< / h3 >
< p > 取决于一个或多个条件的值，< code > if< / code > 语句将决定执行哪一块代码。< / p >
< p > < code > if< / code > 语句有两种标准形式，在这两种形式里都必须有大括号。< / p >
< p > 第一种形式是当且仅当条件为真时执行代码，像下面这样：< / p >
< pre > < code class = "lang-swift" > if `condition` {
`statements`
}
< / code > < / pre >
< blockquote >
< p > if < code > condition< / code > {< br > < code > statements< / code > < br > } < / p >
< / blockquote >
< p > 第二种形式是在第一种形式的基础上添加 < em > else 语句< / em > ，当只有一个 else 语句时，像下面这样：< / p >
< pre > < code class = "lang-swift" > if `condition` {
`statements to execute if condition is true`
< blockquote >
< p > if < code > condition< / code > {
< code > statements to execute if condition is true< / code >
} else {
` statements to execute if condition is false`
}
< / code > < / pr e>
< code > < / code >
}< / p >
< / blockquot e>
< p > 同时， < code > if< / code > 语句，从而形成一条链来测试更多的条件，像下面这样：< / p >
< pre > < code class = "lang-swift" > if `condition 1` {
`statements to execute if condition 1 is true`
} else if `condition 2` {
`statements to execute if condition 2 is true`
}
else {
`statements to execute if both conditions are false`
}
< / code > < / pre >
< blockquote >
< p > if < code > condition 1< / code > {< br > < code > statements to execute if condition 1 is true< / code > < br > } else if < code > condition 2< / code > {< br > < code > statements to execute if condition 2 is true< / code > < br > }< br > else {< br > < code > statements to execute if both conditions are false< / code > < br > } < / p >
< / blockquote >
< p > < code > if< / code > 语句中条件的值的类型必须遵循< code > LogicValue< / code > 协议。同时，条件也可以使用可选绑定，详情参见< a href = "../chapter2/01_The_Basics.html#optional_binding" > 可选绑定< / a > 。< / p >
< blockquote >
< p > GRAMMAR OF AN IF STATEMENT < / p >
< p > < em > if-statement< / em > → < strong > if< / strong > < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Statements.html#//apple_ref/swift/grammar/if-condition" target = "_blank" > < em > if-condition< / em > < / a > < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Declarations.html#//apple_ref/swift/grammar/code-block" target = "_blank" > < em > code-block< / em > < / a > < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Statements.html#//apple_ref/swift/grammar/else-clause" target = "_blank" > < em > else-clause< / em > < / a > < em > opt< / em > < / p >
< p > < em > if-condition< / em > → < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Expressions.html#//apple_ref/swift/grammar/expression" target = "_blank" > < em > expression< / em > < / a > | < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Declarations.html#//apple_ref/swift/grammar/declaration" target = "_blank" > < em > declaration< / em > < / a > < / p >
< p > < em > else-clause< / em > → < strong > else< / strong > < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Declarations.html#//apple_ref/swift/grammar/code-block" target = "_blank" > < em > code-block< / em > < / a > | < strong > else< / strong > < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Statements.html#//apple_ref/swift/grammar/if-statement" target = "_blank" > < em > if-statement< / em > < / a > < em > opt< / em > < / p >
< p > If语句语法< br > < em > if语句< / em > → < strong > if< / strong > < a href = "..\chapter3\10_Statements.html#if_condition" > < em > if条件< / em > < / a > < a href = "..\chapter3\05_Declarations.html#code_block" > < em > 代码块< / em > < / a > < a href = "..\chapter3\10_Statements.html#else_clause" > < em > else子句(Clause)< / em > < / a > < em > 可选< / em > < br > < em > if条件< / em > → < a href = "..\chapter3\04_Expressions.html#expression" > < em > 表达式< / em > < / a > | < a href = "..\chapter3\05_Declarations.html#declaration" > < em > 声明< / em > < / a > < br > < em > else子句(Clause)< / em > → < strong > else< / strong > < a href = "..\chapter3\05_Declarations.html#code_block" > < em > 代码块< / em > < / a > | < strong > else< / strong > < a href = "..\chapter3\10_Statements.html#if_statement" > < em > if语句< / em > < / a > < / p >
< / blockquote >
< h3 id = "switch-" > Switch 语句< / h3 >
< p > 取决于< code > switch< / code > 语句的< em > 控制表达式（ ） < / em > ， < code > switch< / code > 语句将决定执行哪一块代码。< / p >
< p > < code > switch< / code > 语句的形式如下：< / p >
< pre > < code class = "lang-swift" > switch `control expression` {
case `pattern 1`:
`statements`
case `pattern 2` where `condition`:
`statements`
case `pattern 3` where `condition`,
`pattern 4` where `condition`:
`statements`
default:
`statements`
}
< / code > < / pre >
< blockquote >
< p > switch < code > control expression< / code > {< br > case < code > pattern 1< / code > :< br > < code > statements< / code > < br > case < code > pattern 2< / code > where < code > condition< / code > :< br > < code > statements< / code > < br > case < code > pattern 3< / code > where < code > condition< / code > ,< br > < code > pattern 4< / code > where < code > condition< / code > :< br > < code > statements< / code > < br > default:< br > < code > statements< / code > < br > } < / p >
< / blockquote >
< p > < code > switch< / code > 语句的< em > 控制表达式（ ） < / em > 会首先被计算，然后与每一个 case 的模式（ ） ， < em > statements< / em > 。另外，每一个 case 分支都不能为空，也就是说在每一个 case 分支中至少有一条语句。如果你不想在匹配到的 case 分支中执行代码，只需在该分支里写一条< code > break< / code > 语句即可。< / p >
< p > 可以用作控制表达式的值是十分灵活的，除了标量类型(scalar types， < code > Int< / code > 、< code > Character< / code > )外， ， （ ） ， < code > switch< / code > 语句中使用这些类型，详情参见< a href = "../chapter2/05_Control_Flow.html" > 控制流< / a > 一章的 < a href = "../chapter2/05_Control_Flow.html#switch" > Switch< / a > 。< / p >
< p > 你可以在模式后面添加一个起保护作用的表达式(guard expression)。< em > 起保护作用的表达式< / em > 是这样构成的：关键字< code > where< / code > 后面跟着一个作为额外测试条件的表达式。因此，当且仅当< em > 控制表达式< / em > 匹配一个< em > case< / em > 的某个模式且起保护作用的表达式为真时，对应 case 分支中的 < em > statements< / em > 才会被执行。在下面的例子中，< em > 控制表达式< / em > 只会匹配含两个相等元素的元组，如< code > (1, 1)< / code > ： < / p >
@ -775,16 +726,7 @@ else {
< h4 id = "-fall-through-" > 不存在隐式的贯穿(fall through)< / h4 >
< p > 当匹配的 case 分支中的代码执行完毕后，程序会终止< code > switch< / code > 语句，而不会继续执行下一个 case 分支。这就意味着，如果你想执行下一个 case 分支，需要显式地在你需要的 case 分支里使用< code > fallthrough< / code > 语句。关于< code > fallthrough< / code > 语句的更多信息，详情参见 < a href = "#fallthrough_statement" > Fallthrough 语句< / a > 。< / p >
< blockquote >
< p > GRAMMAR OF A SWITCH STATEMENT < / p >
< p > < em > switch-statement< / em > → < strong > switch< / strong > < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Expressions.html#//apple_ref/swift/grammar/expression" target = "_blank" > < em > expression< / em > < / a > < strong > {< / strong > < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Statements.html#//apple_ref/swift/grammar/switch-cases" target = "_blank" > < em > switch-cases< / em > < / a > < em > opt< / em > < strong > }< / strong > < / p >
< p > < em > switch-cases< / em > → < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Statements.html#//apple_ref/swift/grammar/switch-case" target = "_blank" > < em > switch-case< / em > < / a > < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Statements.html#//apple_ref/swift/grammar/switch-cases" target = "_blank" > < em > switch-cases< / em > < / a > < em > opt< / em > < / p >
< p > < em > switch-case< / em > → < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Statements.html#//apple_ref/swift/grammar/case-label" target = "_blank" > < em > case-label< / em > < / a > < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Statements.html#//apple_ref/swift/grammar/statements" target = "_blank" > < em > statement< / em > < / a > | < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Statements.html#//apple_ref/swift/grammar/default-label" target = "_blank" > < em > default-label< / em > < / a > < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Statements.html#//apple_ref/swift/grammar/statements" target = "_blank" > < em > statements< / em > < / a > < / p >
< p > < em > switch-case< / em > → < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Statements.html#//apple_ref/swift/grammar/case-label" target = "_blank" > < em > case-label< / em > < / a > < strong > ;< / strong > | < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Statements.html#//apple_ref/swift/grammar/default-label" target = "_blank" > < em > default-label< / em > < / a > < strong > ;< / strong > < / p >
< p > < em > case-label< / em > → < strong > case< / strong > < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Statements.html#//apple_ref/swift/grammar/case-item-list" target = "_blank" > < em > case-item-list< / em > < / a > < strong > :< / strong > < / p >
< p > < em > case-item-list< / em > → < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Patterns.html#//apple_ref/swift/grammar/pattern" target = "_blank" > < em > pattern< / em > < / a > < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Statements.html#//apple_ref/swift/grammar/guard-clause" target = "_blank" > < em > guard-clause< / em > < / a > < em > opt< / em > | < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Patterns.html#//apple_ref/swift/grammar/pattern" target = "_blank" > < em > pattern< / em > < / a > < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Statements.html#//apple_ref/swift/grammar/guard-clause" target = "_blank" > < em > guard-clause< / em > < / a > < em > opt< / em > , < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Statements.html#//apple_ref/swift/grammar/case-item-list" target = "_blank" > < em > case-item-list< / em > < / a > < / p >
< p > < em > default-label< / em > → < strong > default :< / strong > < / p >
< p > < em > guard-clause< / em > → < strong > where< / strong > < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Statements.html#//apple_ref/swift/grammar/guard-expression" target = "_blank" > < em > guard-expression< / em > < / a > < / p >
< p > < em > guard-expression< / em > → < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Expressions.html#//apple_ref/swift/grammar/expression" target = "_blank" > < em > expression< / em > < / a > < / p >
< p > Switch语句语法< br > < em > switch语句< / em > → < strong > switch< / strong > < a href = "..\chapter3\04_Expressions.html#expression" > < em > 表达式< / em > < / a > < strong > {< / strong > < a href = "..\chapter3\10_Statements.html#switch_cases" > < em > SwitchCase列表< / em > < / a > < em > 可选< / em > < strong > }< / strong > < br > < em > SwitchCase列表< / em > → < a href = "..\chapter3\10_Statements.html#switch_case" > < em > SwitchCase< / em > < / a > < a href = "..\chapter3\10_Statements.html#switch_cases" > < em > SwitchCase列表< / em > < / a > < em > 可选< / em > < br > < em > SwitchCase< / em > → < a href = "..\chapter3\10_Statements.html#case_label" > < em > case标签< / em > < / a > < a href = "..\chapter3\10_Statements.html#statements" > < em > 多条语句(Statements)< / em > < / a > | < a href = "..\chapter3\10_Statements.html#default_label" > < em > default标签< / em > < / a > < a href = "..\chapter3\10_Statements.html#statements" > < em > 多条语句(Statements)< / em > < / a > < br > < em > SwitchCase< / em > → < a href = "..\chapter3\10_Statements.html#case_label" > < em > case标签< / em > < / a > < strong > ;< / strong > | < a href = "..\chapter3\10_Statements.html#default_label" > < em > default标签< / em > < / a > < strong > ;< / strong > < br > < em > case标签< / em > → < strong > case< / strong > < a href = "..\chapter3\10_Statements.html#case_item_list" > < em > case项列表< / em > < / a > < strong > :< / strong > < br > < em > case项列表< / em > → < a href = "..\chapter3\07_Patterns.html#pattern" > < em > 模式< / em > < / a > < a href = "..\chapter3\10_Statements.html#guard_clause" > < em > guard-clause< / em > < / a > < em > 可选< / em > | < a href = "..\chapter3\07_Patterns.html#pattern" > < em > 模式< / em > < / a > < a href = "..\chapter3\10_Statements.html#guard_clause" > < em > guard-clause< / em > < / a > < em > 可选< / em > < strong > ,< / strong > < a href = "..\chapter3\10_Statements.html#case_item_list" > < em > case项列表< / em > < / a > < br > < em > default标签< / em > → < strong > default< / strong > < strong > :< / strong > < br > < em > guard-clause< / em > → < strong > where< / strong > < a href = "..\chapter3\10_Statements.html#guard_expression" > < em > guard-expression< / em > < / a > < br > < em > guard-expression< / em > → < a href = "..\chapter3\04_Expressions.html#expression" > < em > 表达式< / em > < / a > < / p >
< / blockquote >
< p > < a name = "labeled_statement" > < / a >
< a name = "control_transfer_statements" > < / a > 带标签的语句< / p >
@ -792,48 +734,39 @@ else {
< p > 标签的作用域是该标签所标记的语句之后的所有语句。你可以不使用带标签的语句，但只要使用它，标签名就必唯一。< / p >
< p > 关于使用带标签的语句的例子，详情参见< a href = "../chapter2/05_Control_Flow.html" > 控制流< / a > 一章的< a href = "../chapter2/05_Control_Flow.html#labeled_statements" > 带标签的语句< / a > 。< / p >
< blockquote >
< p > GRAMMAR OF A LABELED STATEMENT < / p >
< p > < em > labeled-statement< / em > → < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Statements.html#//apple_ref/swift/grammar/statement-label" target = "_blank" > < em > statement-label< / em > < / a > < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Statements.html#//apple_ref/swift/grammar/loop-statement" target = "_blank" > < em > loop-statement< / em > < / a > | < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Statements.html#//apple_ref/swift/grammar/statement-label" target = "_blank" > < em > statement-label< / em > < / a > < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Statements.html#//apple_ref/swift/grammar/switch-statement" target = "_blank" > < em > switch-statement< / em > < / a > < / p >
< p > < em > statement-label< / em > → < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Statements.html#//apple_ref/swift/grammar/label-name" target = "_blank" > < em > label-name< / em > < / a > < strong > :< / strong > < / p >
< p > < em > label-name< / em > → < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/LexicalStructure.html#//apple_ref/swift/grammar/identifier" target = "_blank" > < em > identifier< / em > < / a > < / p >
< p > 标记语句语法< br > < em > 标记语句(Labeled Statement)< / em > → < a href = "..\chapter3\10_Statements.html#statement_label" > < em > 语句标签< / em > < / a > < a href = "..\chapter3\10_Statements.html#loop_statement" > < em > 循环语句< / em > < / a > | < a href = "..\chapter3\10_Statements.html#statement_label" > < em > 语句标签< / em > < / a > < a href = "..\chapter3\10_Statements.html#switch_statement" > < em > switch语句< / em > < / a > < br > < em > 语句标签< / em > → < a href = "..\chapter3\10_Statements.html#label_name" > < em > 标签名称< / em > < / a > < strong > :< / strong > < br > < em > 标签名称< / em > → < a href = "..\chapter3\02_Lexical_Structure.html#identifier" > < em > 标识符< / em > < / a > < / p >
< / blockquote >
< h2 id = "-" > 控制传递语句< / h2 >
< p > 通过无条件地把控制权从一片代码传递到另一片代码， < code > break< / code > 语句、< code > continue< / code > 语句、< code > fallthrough< / code > 语句和< code > return< / code > 语句。< / p >
< blockquote >
< p > GRAMMAR OF A CONTROL TRANSER STATEMENT < / p >
< p > < em > control-transfer-statement< / em > → < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Statements.html#//apple_ref/swift/grammar/break-statement" target = "_blank" > < em > break-statement< / em > < / a > < / p >
< p > < em > control-transfer-statement< / em > → < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Statements.html#//apple_ref/swift/grammar/continue-statement" target = "_blank" > < em > continue-statement< / em > < / a > < / p >
< p > < em > control-transfer-statement< / em > → < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Statements.html#//apple_ref/swift/grammar/fallthrough-statement" target = "_blank" > < em > fallthrough-statement< / em > < / a > < / p >
< p > < em > control-transfer-statement< / em > → < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Statements.html#//apple_ref/swift/grammar/return-statement" target = "_blank" > < em > return-statement< / em > < / a > < / p >
< p > 控制传递语句(Control Transfer Statement) 语法< br > < em > 控制传递语句< / em > → < a href = "..\chapter3\10_Statements.html#break_statement" > < em > break语句< / em > < / a > < br > < em > 控制传递语句< / em > → < a href = "..\chapter3\10_Statements.html#continue_statement" > < em > continue语句< / em > < / a > < br > < em > 控制传递语句< / em > → < a href = "..\chapter3\10_Statements.html#fallthrough_statement" > < em > fallthrough语句< / em > < / a > < br > < em > 控制传递语句< / em > → < a href = "..\chapter3\10_Statements.html#return_statement" > < em > return语句< / em > < / a > < / p >
< / blockquote >
< p > < a name = "break_statement" > < / a > < / p >
< h3 id = "break-" > Break 语句< / h3 >
< p > < code > break< / code > 语句用于终止循环或< code > switch< / code > 语句的执行。使用< code > break< / code > 语句时，可以只写< code > break< / code > 这个关键词，也可以在< code > break< / code > 后面跟上标签名（ ） ， ： < / p >
< pre > < code class = "lang-swift" > break
break `label name`
< / code > < / pr e>
< blockquote >
< p > break< br > break < code > label name< / code > < / p >
< / blockquot e>
< p > 当< code > break< / code > 语句后面带标签名时，可用于终止由这个标签标记的循环或< code > switch< / code > 语句的执行。< / p >
< p > 而当只写< code > break< / code > 时，则会终止< code > switch< / code > 语句或上下文中包含< code > break< / code > 语句的最内层循环的执行。< / p >
< p > 在这两种情况下，控制权都会被传递给循环或< code > switch< / code > 语句外面的第一行语句。< / p >
< p > 关于使用< code > break< / code > 语句的例子，详情参见< a href = "../chapter2/05_Control_Flow.html" > 控制流< / a > 一章的 < a href = "../chapter2/05_Control_Flow.html#break" > Break< / a > 和< a href = "../chapter2/05_Control_Flow.html#labeled_statements" > 带标签的语句< / a > 。< / p >
< blockquote >
< p > GRAMMAR OF A BREAK STATEMENT < / p >
< p > < em > break-statement< / em > → < strong > break< / strong > < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Statements.html#//apple_ref/swift/grammar/label-name" target = "_blank" > < em > label-name< / em > < / a > < em > opt< / em > < / p >
< p > Break 语句语法< br > < em > break语句< / em > → < strong > break< / strong > < a href = "..\chapter3\10_Statements.html#label_name" > < em > 标签名称< / em > < / a > < em > 可选< / em > < / p >
< / blockquote >
< p > < a name = "continue_statement" > < / a > < / p >
< h3 id = "continue-" > Continue 语句< / h3 >
< p > < code > continue< / code > 语句用于终止循环中当前迭代的执行，但不会终止该循环的执行。使用< code > continue< / code > 语句时，可以只写< code > continue< / code > 这个关键词，也可以在< code > continue< / code > 后面跟上标签名（ ） ， ： < / p >
< pre > < code class = "lang-swift" > continue
continue `label name`
< / code > < / pr e>
< blockquote >
< p > continue< br > continue < code > label name< / code > < / p >
< / blockquot e>
< p > 当< code > continue< / code > 语句后面带标签名时，可用于终止由这个标签标记的循环中当前迭代的执行。< / p >
< p > 而当只写< code > break< / code > 时，可用于终止上下文中包含< code > continue< / code > 语句的最内层循环中当前迭代的执行。< / p >
< p > 在这两种情况下，控制权都会被传递给循环外面的第一行语句。< / p >
< p > 在< code > for< / code > 语句中，< code > continue< / code > 语句执行后，< em > increment< / em > 表达式还是会被计算，这是因为每次循环体执行完毕后 < em > increment< / em > 表达式都会被计算。< / p >
< p > 关于使用< code > continue< / code > 语句的例子，详情参见< a href = "../chapter2/05_Control_Flow.html" > 控制流< / a > 一章的 < a href = "../chapter2/05_Control_Flow.html#continue" > Continue< / a > 和< a href = "../chapter2/05_Control_Flow.html#labeled_statements" > 带标签的语句< / a > 。< / p >
< blockquote >
< p > GRAMMAR OF A CONTINUE STATEMENT < / p >
< p > < em > continue-statement< / em > → < strong > continue< / strong > < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Statements.html#//apple_ref/swift/grammar/label-name" target = "_blank" > < em > label-name< / em > < / a > < em > opt< / em > < / p >
< p > Continue 语句语法< br > < em > continue语句< / em > → < strong > continue< / strong > < a href = "..\chapter3\10_Statements.html#label_name" > < em > 标签名称< / em > < / a > < em > 可选< / em > < / p >
< / blockquote >
< p > < a name = "fallthrough_statement" > < / a > < / p >
< h3 id = "fallthrough-" > Fallthrough 语句< / h3 >
@ -841,20 +774,18 @@ continue `label name`
< p > < code > fallthrough< / code > 语句可出现在< code > switch< / code > 语句中的任意 case 里，但不能出现在最后一个 case 分支中。同时，< code > fallthrough< / code > 语句也不能把控制权传递给使用了可选绑定的 case 分支。< / p >
< p > 关于在< code > switch< / code > 语句中使用< code > fallthrough< / code > 语句的例子，详情参见< a href = "../chapter2/05_Control_Flow.html" > 控制流< / a > 一章的< a href = "../chapter2/05_Control_Flow.html#control_transfer_statements" > 控制传递语句< / a > 。< / p >
< blockquote >
< p > GRAMMAR OF A FALLTHROUGH STATEMENT < / p >
< p > < em > continue-statement< / em > → < strong > fallthrough< / strong > < / p >
< p > Fallthrough 语句语法< br > < em > fallthrough语句< / em > → < strong > fallthrough< / strong > < / p >
< / blockquote >
< h3 id = "return-" > Return 语句< / h3 >
< p > < code > return< / code > 语句用于在函数或方法的实现中将控制权传递给调用者，接着程序将会从调用者的位置继续向下执行。< / p >
< p > 使用< code > return< / code > 语句时，可以只写< code > return< / code > 这个关键词，也可以在< code > return< / code > 后面跟上表达式，像下面这样：< / p >
< pre > < code class = "lang-swift" > return
return `expression`
< / code > < / pr e>
< blockquote >
< p > return< br > return < code > expression< / code > < / p >
< / blockquot e>
< p > 当< code > return< / code > 语句后面带表达式时， ， < / p >
< p > 而当只写< code > return< / code > 时，仅仅是将控制权从该函数或方法传递给调用者，而不返回一个值。（这就是说，该函数或方法的返回类型为< code > Void< / code > 或< code > ()< / code > ） < / p >
< blockquote >
< p > GRAMMAR OF A RETURN STATEMENT < / p >
< p > < em > return-statement< / em > → < strong > return< / strong > < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Expressions.html#//apple_ref/swift/grammar/expression" target = "_blank" > < em > expression< / em > < / a > < em > opt< / em > < / p >
< p > Return 语句语法< br > < em > return语句< / em > → < strong > return< / strong > < a href = "..\chapter3\04_Expressions.html#expression" > < em > 表达式< / em > < / a > < em > 可选< / em > < / p >
< / blockquote >
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