概述

scala語言的for語法很靈活. 除了普通的直接對集合的迴圈, 以及迴圈中的判斷和值返回. 非常靈活.

for 可以通過yield(生產)返回值, 最終組成for迴圈的物件型別.for 迴圈中的 yield 會把當前的元素記下來，儲存在集合中，迴圈結束後將返回該集合。如果被迴圈的是 Map，返回的就是Map，被迴圈的是 List，返回的就是List，以此類推。

守衛( guards) (for loop ‘if’ conditions)

可以在 for 迴圈結構中加上 ‘if’ 表示式, 和yield聯合起來用.

普通對集合或迭代迴圈

scala> for(i<- 1
 
 to 5) println(i)
1
2
3
4
5
scala> for(i<- 1 until 5) println(i)
1
2
3
4

yield返回值

scala> for (i <- 1 to 5) yield i
res0: scala.collection.immutable.IndexedSeq[Int] = Vector(1, 2, 3, 4, 5)

scala> val a= for (i <- 1 to 5) yield i
a: scala.collection.immutable.IndexedSeq[Int] = Vector
 
(1, 2, 3, 4, 5)

scala> a
res1: scala.collection.immutable.IndexedSeq[Int] = Vector(1, 2, 3, 4, 5)

scala> val a= for (i <- 1 until 5) yield i
a: scala.collection.immutable.IndexedSeq[Int] = Vector(1, 2, 3, 4)

scala> a
res2: scala.collection.immutable.IndexedSeq[Int] = Vector(1, 2, 3, 4
 
)

scala> val a= for (i <- 1 until 5) yield i*2
a: scala.collection.immutable.IndexedSeq[Int] = Vector(2, 4, 6, 8)

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

scala> for ( e <- a) yield e
res3: Array[Int] = Array(1, 2, 3, 4, 5)

迴圈過濾 if 判斷, 並返回值

scala> for ( e <- a if e%2 == 0) yield e
res4: Array[Int] = Array(2, 4)
scala> a
res10: Array[Int] = Array(1, 2, 3, 4, 5)

scala> val b = 6 to 7
b: scala.collection.immutable.Range.Inclusive = Range 6 to 7

scala> for {
     | x <-a
     | y <-b
     | } yield (x,y)
res11: Array[(Int, Int)] = Array((1,6), (1,7), (2,6), (2,7), (3,6), (3,7), (4,6), (4,7), (5,6), (5,7))

scala> for {
     | y <- b
     | x <- a
     | } yield (x,y)
res12: scala.collection.immutable.IndexedSeq[(Int, Int)] = Vector((1,6), (2,6), (3,6), (4,6), (5,6), (1,7), (2,7), (3,7), (4,7), (5,7))

for 複雜例項

找出.txt字尾檔案

scala> def getTextFile(path:String) : Array[java.io.File] =
     | for {
     | file <- new File(path).listFiles
     | if file.isFile
     | if file.getName.endsWith(".txt")
     | } yield file
getTextFile: (path: String)Array[java.io.File]

scala> getTextFile(".")
res9: Array[java.io.File] = Array(./a.txt, ./test.txt)

參考

https://unmi.cc/scala-yield-samples-for-loop/

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