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Zhaoyu ZhangZhaoyu Zhang
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Simplified Chinese Translation of the cheatsheet
translated the cheatsheet into Chinese, using the English and Japanese version as references.
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‎zh-cn/cheatsheets/index.md

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---
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layout: cheatsheet
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istranslation: true
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title: Scalacheat
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by: Brendan O'Connor
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about: 感谢 <a href="http://brenocon.com/">Brendan O'Connor</a>, 本速查表可以用于快速地查找Scala语法结构。Licensed by Brendan O'Connor under a CC-BY-SA 3.0 license.
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languages: [zh-cn]
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---
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###### Contributed by {{ page.by }}
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| | |
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| ------ | ------ |
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| <span id="variables" class="h2">变量</span> | |
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| `var x = 5` | 变量 |
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| <span class="label success">Good</span> `val x = 5`<br> <span class="label important">Bad</span> `x=6` | 常量 |
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| `var x: Double = 5` | 指定数据类型 |
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| <span id="functions" class="h2">函数</span> | |
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| <span class="label success">Good</span> `def f(x: Int) = { x*x }`<br> <span class="label important">Bad</span> `def f(x: Int) { x*x }` | 定义函数 <br> 隐藏错误: 没有“=”程序会返回Unit类型，这将会引起重大灾难。 |
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| <span class="label success">Good</span> `def f(x: Any) = println(x)`<br> <span class="label important">Bad</span> `def f(x) = println(x)` | 定义函数 <br> 语法错误: 每个参数都需要指定类型。 |
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| `type R = Double` | 类型别名 |
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| `def f(x: R)` vs.<br> `def f(x: => R)` | 按值调用 <br> 按名称调用 （惰性参数） |
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| `(x:R) => x*x` | 匿名函数 |
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| `(1 to 5).map(_*2)` vs.<br> `(1 to 5).reduceLeft( _+_ )` | 匿名函数: 下划线是arg参数的占位符。 |
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| `(1 to 5).map( x => x*x )` | 匿名函数: 必须命名以后才可以使用一个arg参数两次。 |
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| <span class="label success">Good</span> `(1 to 5).map(2*)`<br> <span class="label important">Bad</span> `(1 to 5).map(*2)` | 匿名函数: 边界前缀方法，理智的人都用`2*_`。 |
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| `(1 to 5).map { x => val y=x*2; println(y); y }` | 匿名函数: 上个方法的程序块风格。 |
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| `(1 to 5) filter {_%2 == 0} map {_*2}` | 匿名函数: 管道风格（或者叫括号风格）。 |
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| `def compose(g:R=>R, h:R=>R) = (x:R) => g(h(x))` <br> `val f = compose({_*2}, {_-1})` | 匿名函数: 要传入多个程序块的话，需要外部括号。 |
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| `val zscore = (mean:R, sd:R) => (x:R) => (x-mean)/sd` | 柯里化, 很显然的语法。 |
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| `def zscore(mean:R, sd:R) = (x:R) => (x-mean)/sd` | 柯里化, 很显然的语法。 |
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| `def zscore(mean:R, sd:R)(x:R) = (x-mean)/sd` | 柯里化，语法糖，然并卵。 |
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| `val normer = zscore(7, 0.4) _` | 需要接上下划线才能得到偏微商（只适用于语法糖版本）。 |
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| `def mapmake[T](g:T=>T)(seq: List[T]) = seq.map(g)` | 泛型 |
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| `5.+(3); 5 + 3` <br> `(1 to 5) map (_*2)` | 前缀语法糖 |
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| `def sum(args: Int*) = args.reduceLeft(_+_)` | 可变参数 |
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| <span id="packages" class="h2">包</span> | |
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| `import scala.collection._` | 通配符导入 |
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| `import scala.collection.Vector` <br> `import scala.collection.{Vector, Sequence}` | 选择性导入 |
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| `import scala.collection.{Vector => Vec28}` | 重命名导入 |
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| `import java.util.{Date => _, _}` | 导入java.util包里除Date之外的所有文件. |
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| `package pkg` _at start of file_ <br> `package pkg { ... }` | 声明一个包 |
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| <span id="data_structures" class="h2">数据结构</span> | |
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| `(1,2,3)` | 元组 (`Tuple3`) |
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| `var (x,y,z) = (1,2,3)` | 解构绑定：通过模式匹配来解构元组。 |
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| <span class="label important">Bad</span>`var x,y,z = (1,2,3)` | 隐藏错误：每一个变量都被赋值了整个元组。 |
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| `var xs = List(1,2,3)` | 列表 (不可变). |
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| `xs(2)` | 用括号索引 ([slides](http://www.slideshare.net/Odersky/fosdem-2009-1013261/27)) |
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| `1 :: List(2,3)` | 在列表开头添加元素 |
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| `1 to 5` _same as_ `1 until 6` <br> `1 to 10 by 2` | 范围（语法糖） |
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| `()` _(empty parens)_ | Unit类型的专有成员 (相当于 C/Java 里的void). |
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| <span id="control_constructs" class="h2">控制结构</span> | |
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| `if (check) happy else sad` | 条件 |
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| `if (check) happy` _same as_ <br> `if (check) happy else ()` | 条件（语法糖） |
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| `while (x < 5) { println(x); x += 1}` | while循环 |
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| `do { println(x); x += 1} while (x < 5)` | do while循环 |
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| `import scala.util.control.Breaks._`<br>`breakable {`<br>` for (x <- xs) {`<br>` if (Math.random < 0.1) break`<br>` }`<br>`}`| break. ([slides](http://www.slideshare.net/Odersky/fosdem-2009-1013261/21)) |
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| `for (x <- xs if x%2 == 0) yield x*10` _same as_ <br>`xs.filter(_%2 == 0).map(_*10)` | for循环: filter/map |
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| `for ((x,y) <- xs zip ys) yield x*y` _same as_ <br>`(xs zip ys) map { case (x,y) => x*y }` | for循环: 解构绑定 |
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| `for (x <- xs; y <- ys) yield x*y` _same as_ <br>`xs flatMap {x => ys map {y => x*y}}` | for循环: 叉乘 |
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| `for (x <- xs; y <- ys) {`<br> `println("%d/%d = %.1f".format(x, y, x/y.toFloat))`<br>`}` | for循环: 势在必行的格式<br>[sprintf-style](http://java.sun.com/javase/6/docs/api/java/util/Formatter.html#syntax) |
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| `for (i <- 1 to 5) {`<br> `println(i)`<br>`}` | for循环: 包括上边界的遍历 |
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| `for (i <- 1 until 5) {`<br> `println(i)`<br>`}` | for循环: 忽略上边界的遍历 |
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| <span id="pattern_matching" class="h2">模式匹配</span> | |
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| <span class="label success">Good</span> `(xs zip ys) map { case (x,y) => x*y }`<br> <span class="label important">Bad</span> `(xs zip ys) map( (x,y) => x*y )` | 模式匹配中，函数的参数需要使用case。 |
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| <span class="label important">Bad</span><br>`val v42 = 42`<br>`Some(3) match {`<br>` case Some(v42) => println("42")`<br>` case _ => println("Not 42")`<br>`}` | "v42" 被解释为可以匹配任何Int类型值的名称，打印输出"42"。 |
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| <span class="label success">Good</span><br>`val v42 = 42`<br>`Some(3) match {`<br>`` case Some(`v42`) => println("42")``<br>`case _ => println("Not 42")`<br>`}` | 有倒引号的 "\`v42\`" 被解释为已经存在的val `v42`，所以输出的是 "Not 42". |
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| <span class="label success">Good</span><br>`val UppercaseVal = 42`<br>`Some(3) match {`<br>` case Some(UppercaseVal) => println("42")`<br>` case _ => println("Not 42")`<br>`}` | `UppercaseVal` 被视作已经存在的 val，而不是一个新的模式变量，因为它是以大写字母开头的，所以`UppercaseVal` 包含的值是 `3`, 结果输出的是"Not 42"。|
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| <span id="object_orientation" class="h2">面向对象</span> | |
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| `class C(x: R)` _same as_ <br>`class C(private val x: R)`<br>`var c = new C(4)` | 构造器参数 - 私有 |
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| `class C(val x: R)`<br>`var c = new C(4)`<br>`c.x` | 构造器参数 - 公有 |
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| `class C(var x: R) {`<br>`assert(x > 0, "positive please")`<br>`var y = x`<br>`val readonly = 5`<br>`private var secret = 1`<br>`def this = this(42)`<br>`}`|<br>类本身就是构造器<br>声明一个公有成员变量<br>声明一个可获得但不可设置的成员变量<br>声明一个私有变量r<br>可选构造器|
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| `new{ ... }` | 匿名类 |
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| `abstract class D { ... }` | 定义一个抽象类。（不可创建） |
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| `class C extends D { ... }` | 定义一个继承子类。 |
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| `class D(var x: R)`<br>`class C(x: R) extends D(x)` | 继承与构造器参数（愿望清单: 默认自动传参）
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| `object O extends D { ... }` | 定义一个单例模式（模块化的） |
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| `trait T { ... }`<br>`class C extends T { ... }`<br>`class C extends D with T { ... }` | 特性<br>带有实现的接口，没有构造参数。 [mixin-able]({{ site.baseurl }}/tutorials/tour/mixin-class-composition.html).
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| `trait T1; trait T2`<br>`class C extends T1 with T2`<br>`class C extends D with T1 with T2` | 多重特性 |
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| `class C extends D { override def f = ...}` | 必须声明函数的重载。 |
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| `new java.io.File("f")` | 创建类。 |
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| <span class="label important">Bad</span> `new List[Int]`<br> <span class="label success">Good</span> `List(1,2,3)` | 类型错误： 抽象类型<br>相反，习惯上：调用的“工厂“会推测类型 |
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| `classOf[String]` | 类字面常量 |
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| `x.isInstanceOf[String]` | 类型检查 (运行时) |
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| `x.asInstanceOf[String]` | 类型强制转换 (运行时) |
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| `x: String` | 归属 (编译时) |
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