前言

有一段时间没有使用Scala, 我们今天回顾下Scala的常用的函数算子, 并使用一些练习加深理解和印象. 这些函数对于理解Spark的相关算子非常有用. 建议自己练习.

所有的练习代码, 都可以在 https://github.com/SeanYanxml/arsenal/tree/master/arsenal-scala/quick-scala/quick-scala 上找到.

常见集合函数

  • foreach()

override def foreach[U](f : scala.Function1[A, U]) : scala.Unit = { /* compiled code */ }

遍历数组&集合.

scala> val lst0 = List(1,2,3,4,5,6)
lst0: List[Int] = List(1, 2, 3, 4, 5, 6)

scala> lst0.foreach(print(_))
123456
  • map()

final override def map[B, That](f : scala.Function1[A, B])(implicit bf : scala.collection.generic.CanBuildFrom[scala.collection.immutable.List[A], B, That]) : That = { /* compiled code */ }

用于遍历数组、Map集合、List集合、Tuple元祖内的每一个元素. 传入的是一个

scala>     val lst0 = List(1,7,9,8,0,3,5,4,6,2)
lst0: List[Int] = List(1, 7, 9, 8, 0, 3, 5, 4, 6, 2)

scala>     val lst1 = lst0.map(_*10)
lst1: List[Int] = List(10, 70, 90, 80, 0, 30, 50, 40, 60, 20)
  • Tips - foreach()&map()
    foreach()与map()方法的区别在于, foreach()无返回值, 而map()有返回值.
scala> val lst0 = List(1,2,3,4,5)
lst0: List[Int] = List(1, 2, 3, 4, 5)

scala> val lst1 = lst0.foreach(_*10)
lst1: Unit = ()
# 无输出
scala> lst1

scala> val lst2 = lst0.map(_*10)
lst2: List[Int] = List(10, 20, 30, 40, 50)
# 含有输出
scala> lst2
res2: List[Int] = List(10, 20, 30, 40, 50)
  • map().flatten/flatMap()

  • def flatten[B](implicit asTraversable : scala.Function1[A, scala.collection.GenTraversableOnce[B]]) : CC[B] = { /* compiled code */ }

  • final override def flatMap[B, That](f : scala.Function1[A, scala.collection.GenTraversableOnce[B]])(implicit bf : scala.collection.generic.CanBuildFrom[scala.collection.immutable.List[A], B, That]) : That = { /* compiled code */ }

先进行map()函数操作, 再将其进行压平.

scala> val lines = List("hello tom hello jerry", "hello jerry", "hello kitty")
lines: List[String] = List(hello tom hello jerry, hello jerry, hello kitty)

# 获得里面是字符串数组的List对象
scala> val linesSplit = lines.map(_.split(" "))
linesSplit: List[Array[String]] = List(Array(hello, tom, hello, jerry), Array(hello, jerry), Array(hello, kitty))

# 将所有的数组都压平,压在一个List内部
scala> val linesSplitFlat = lines.map(_.split(" ")).flatten
linesSplitFlat: List[String] = List(hello, tom, hello, jerry, hello, jerry, hello, kitty)

# 使用flatMap
scala> val linesSplitFlat2 = lines.flatMap(_.split(" "))
linesSplitFlat2: List[String] = List(hello, tom, hello, jerry, hello, jerry, hello, kitty)```
  • filter()

def filter(p : scala.Function1[A, scala.Boolean]) : Repr = { /* compiled code */ }

过滤, 过滤出数组或集合内满足筛选条件的数据.

scala> val lst0 = List(1,7,9,8,0,3,5,4,6,2)
lst0: List[Int] = List(1, 7, 9, 8, 0, 3, 5, 4, 6, 2)

scala> val list3  = lst0.filter(_>5)
list3: List[Int] = List(7, 9, 8, 6)
  • sorted/sortedBy()/sortedWith()
  • def sorted[B >: A](implicit ord : scala.math.Ordering[B]) : Repr = { /* compiled code */ }
  • def sortBy[B](f : scala.Function1[A, B])(implicit ord : scala.math.Ordering[B]) : Repr = { /* compiled code */ }
  • def sortWith(lt : scala.Function2[A, A, scala.Boolean]) : Repr = { /* compiled code */ }

三个函数都可以用于排序. 其中sorted是简单排序, sortedBy可以指定已某一个属性进行排序, sortedWith可以指定排序的比较函数.

scala>     val lst0 = List(1,7,9,8,0,3,5,4,6,2)
lst0: List[Int] = List(1, 7, 9, 8, 0, 3, 5, 4, 6, 2)

scala>     val lst3_1 = lst0.sorted
lst3_1: List[Int] = List(0, 1, 2, 3, 4, 5, 6, 7, 8, 9)

scala>     val lst3_2 = lst0.sortBy(x =>x)
lst3_2: List[Int] = List(0, 1, 2, 3, 4, 5, 6, 7, 8, 9)

scala>     val lst3_2 = lst0.sortBy(x => (-x))
lst3_2: List[Int] = List(9, 8, 7, 6, 5, 4, 3, 2, 1, 0)

# 传入一个compare()函数
scala>     val lst3_3 = lst0.sortWith((x,y) => (x>y))
lst3_3: List[Int] = List(9, 8, 7, 6, 5, 4, 3, 2, 1, 0)
  • reverse

override def reverse : scala.collection.immutable.List[A] = { /* compiled code */ }

反转.

scala>     val lst0 = List(1,7,9,8,0,3,5,4,6,2)
lst0: List[Int] = List(1, 7, 9, 8, 0, 3, 5, 4, 6, 2)

scala>     val lst4 = lst0.reverse
lst4: List[Int] = List(2, 6, 4, 5, 3, 0, 8, 9, 7, 1)
  • grouped()/groupedBy()
  • def grouped(size : scala.Int) : scala.collection.Iterator[Repr] = { /* compiled code */ }
  • def groupBy[K](f : scala.Function1[A, K]) : scala.collection.immutable.Map[K, Repr] = { /* compiled code */ }

grouped()是将几个元素进行组合, 返回的是一个List<Iterator>的List;
groupedBy()指定分类的函数, 返回的是一个Map<K,List[Value]>的Map.

scala>     val lst0 = List(1,7,9,8,0,3,5,4,6,2)
lst0: List[Int] = List(1, 7, 9, 8, 0, 3, 5, 4, 6, 2)

#grouped()
scala>     val lst5 = lst0.grouped(4)
lst5: Iterator[List[Int]] = non-empty iterator

scala> lst5.toList
res1: List[List[Int]] = List(List(1, 7, 9, 8), List(0, 3, 5, 4), List(6, 2))

# groupedBy()
scala> val lst0 = List(1,7,9,8,0,3,5,4,6,2,3,2,3)
lst0: List[Int] = List(1, 7, 9, 8, 0, 3, 5, 4, 6, 2, 3, 2, 3)

scala> val lst8 = lst0.groupBy(x => (x))
lst8: scala.collection.immutable.Map[Int,List[Int]] = Map(0 -> List(0), 5 -> List(5), 1 -> List(1), 6 -> List(6), 9 -> List(9), 2 -> List(2, 2), 7 -> List(7), 3 -> List(3, 3, 3), 8 -> List(8), 4 -> List(4))

scala> val lst8 = lst0.groupBy(x => (x)).toList
lst8: List[(Int, List[Int])] = List((0,List(0)), (5,List(5)), (1,List(1)), (6,List(6)), (9,List(9)), (2,List(2, 2)), (7,List(7)), (3,List(3, 3, 3)), (8,List(8)), (4,List(4)))

scala> val lst8 = lst0.groupBy(x => (x%2==1)).toList
lst8: List[(Boolean, List[Int])] = List((false,List(8, 0, 4, 6, 2, 2)), (true,List(1, 7, 9, 3, 5, 3, 3)))
  • reduce()

def reduce[A1 >: A](op : scala.Function2[A1, A1, A1]) : A1 = { /* compiled code */ }

并行化计算.

scala>     val array = Array(1,2,3,4,5,6,7)
array: Array[Int] = Array(1, 2, 3, 4, 5, 6, 7)

scala>     val sum = array.reduce(_+_)
sum: Int = 28

scala>     array.par.reduce(_+_)
res3: Int = 28
  • flod()

def fold[U >: T](z : U)(op : scala.Function2[U, U, U]) : U = { /* compiled code */ }

scala>     val array = Array(1,2,3,4,5,6,7)
array: Array[Int] = Array(1, 2, 3, 4, 5, 6, 7)

scala>     array.fold(10)(_+_)
res4: Int = 38

scala>     array.par.fold(10)(_+_)
res5: Int = 98

scala>     array.par.fold(0)(_+_)
res6: Int = 28
  • aggregate()

def aggregate[B](z : => B)(seqop : scala.Function2[B, A, B], combop : scala.Function2[B, B, B]) : B = { /* compiled code */ }

聚合函数.

scala>     val arr = List(List(1, 2, 3), List(3, 4, 5), List(2), List(0))
arr: List[List[Int]] = List(List(1, 2, 3), List(3, 4, 5), List(2), List(0))

scala>     arr.aggregate(0)(_+_.sum, _+_)
res7: Int = 20
  • 交集、并集、差集 (intersect/union/diff)
  • def intersect[B >: A](that : scala.collection.GenSeq[B]) : Repr = { /* compiled code */ }
  • override def union[B >: A, That](that : scala.collection.GenSeq[B])(implicit bf : scala.collection.generic.CanBuildFrom[Repr, B, That]) : That = { /* compiled code */ }
  • def diff[B >: A](that : scala.collection.GenSeq[B]) : Repr = { /* compiled code */ }
scala>     val l1 = List(5,6,4,7)
l1: List[Int] = List(5, 6, 4, 7)

scala>     val l2 = List(1,2,3,4)
l2: List[Int] = List(1, 2, 3, 4)

# 并集
scala>     val unionL1 = l1.union(l2)
unionL1: List[Int] = List(5, 6, 4, 7, 1, 2, 3, 4)

# 交集
scala>     val insercetionL1 = l1.intersect(l2)
insercetionL1: List[Int] = List(4)

# 差集
scala>     val differenceL1 = l1.diff(l2)
differenceL1: List[Int] = List(5, 6, 7)

其他函数

  • split()

public String[] split(String regex) {
return split(regex, 0);
}

分割字符串, 返回一个字符串数组.

scala> val str1 = "a b c d e"
str1: String = a b c d e

scala> val strArray1 = str1.split(" ")
strArray1: Array[String] = Array(a, b, c, d, e)

scala> strArray1
res4: Array[String] = Array(a, b, c, d, e)

练习

Test1
  • 创建一个List
    val lst0 = List(1,7,9,8,0,3,5,4,6,2)
  • 将lst0中每个元素乘以10后生成一个新的集合
  • 将lst0中的偶数取出来生成一个新的集合
  • 将lst0排序后生成一个新的集合
  • 反转顺序
  • 将lst0中的元素4个一组,类型为Iterator[List[Int]]
  • 将Iterator转换成List
  • 将多个list压扁成一个List
Test2
  	val lines = List("hello tom hello jerry", "hello jerry", "hello kitty")
  • 先按空格切分，再压平
  • 计算WordCount
Test3
    val array = Array(1,2,3,4,5,6,7)
  • 并行计算求和(reduce / fold)
Test4
    val arr = List(List(1, 2, 3), List(3, 4, 5), List(2), List(0))
  • 聚合计算
Test5
 val l1 = List(5,6,4,7)
 val l2 = List(1,2,3,4)
  • 求差集、交集、并集

相关练习与结果

package com.yanxml.quick_scala.basic.train

import org.junit.Test

/**
 * 快速训练Scala的基础语法.
 * @Date 2019-04-24
 * */
class QuickBasicTrain {
  
  @Test
  def train(){
      //创建一个List
    val lst0 = List(1,7,9,8,0,3,5,4,6,2)
    //将lst0中每个元素乘以10后生成一个新的集合
    val lst1 = lst0.map(_*10)

    //将lst0中的偶数取出来生成一个新的集合
    val lst2 = lst0.filter(_%2==0)
    //将lst0排序后生成一个新的集合
    val lst3_1 = lst0.sorted
    val lst3_2 = lst0.sortBy(x =>x)
    val lst3_3 = lst0.sortWith((x,y) => (x>y))
    //反转顺序
    val lst4 = lst0.reverse
    //将lst0中的元素4个一组,类型为Iterator[List[Int]]
    val lst5 = lst0.grouped(4)
    //将Iterator转换成List
    val lst6 = lst0.grouped(4).toList
    //将多个list压扁成一个List
    val lst7 = lst0.grouped(4).toList.flatten

    val lst8 = lst0.groupBy(x => (x%2==1))

  	val lines = List("hello tom hello jerry", "hello jerry", "hello kitty")
    //先按空格切分，在压平
    val linesSplitFlat = lines.map(_.split(" ")).flatten
    val linesSplitFlat2 = lines.flatMap(_.split(" "))
    val linesCountOfOne = linesSplitFlat.map((_,1))
    // (<hello>(<Hello,1><hello,2>))(...)
    val linesGroupBy = linesCountOfOne.groupBy(_._1)
    // 错误示范: linesGroupBy.map(_._1,_._2.size)
    val linesCount = linesGroupBy.map(t => (t._1, t._2.size))
    // 默认Map是不支持排序的 需要
    val linesCountSorted = linesCount.toList.sortBy(_._2)
    
    val linesCountAllOne = lines.map(_.split(" ")).flatten.map((_,1)).groupBy(_._1).map(t => (t._1,t._2.size))
    val linesCountAllOne2 = lines.map(_.split(" ")).flatten.map((_,1)).groupBy(_._1).mapValues(_.foldLeft(0)(_+_._2))
    
    //并行计算求和
 
    //化简：reduce
    val array = Array(1,2,3,4,5,6,7)
    array.map(println)
    array.map(x=> println(x+1))
    array.foreach(println)
    // 默认使用reduceLeft (((1+2)+3)+4)+5)+6)+7
    val sum = array.reduce(_+_)
    // 转换为并行化的接口
    array.par.reduce(_+_)
    //将非特定顺序的二元操作应用到所有元素

    //安装特点的顺序
    
    //折叠：有初始值（无特定顺序）
    // 使用柯里化 默认设置
    array.fold(10)(_+_)
    
    array.par.fold(10)(_+_)
    array.par.fold(0)(_+_)
    //折叠：有初始值（有特定顺序）


    //聚合
    val arr = List(List(1, 2, 3), List(3, 4, 5), List(2), List(0))
    // aggregate(<初始值>)((函数1),(函数2))
    // 聚合函数求和
    arr.aggregate(0)(_+_.sum, _+_)

    val l1 = List(5,6,4,7)
    val l2 = List(1,2,3,4)
    //求并集
    val unionL1 = l1.union(l2)
    
    //求交集
    val insercetionL1 = l1.intersect(l2) 
    //求差集
    val differenceL1 = l1.diff(l2)
      
//    println(r3)
     
  }
  
}

后记

虽然, 有些函数的定义还不能像Java一样完全看懂, 但是记录于此. 多看几次.
此外, 有许多的函数暂没有列举出来. 后续再进行补充.

Reference

[1]. Scala 中的foreach和map方法比较

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