scala:产生两个循环的成对组合

Question

我想创建一个包含元组的集合,其中包含两个列表的所有成对组合.就像是:

for ( x <- xs )
  for ( y <- ys ) 
    yield (x,y)

在Python中这可行,在Scala中显然for仅为最后一个循环产生(所以这个计算结果Unit)

在Scala中实现它的最简洁方法是什么？

Answer 1

你几乎在那里:

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

scala> val ys = List (4,5,6)
ys: List[Int] = List(4, 5, 6)

scala> for (x <- xs; y <- ys) yield (x,y)
res3: List[(Int, Int)] = List((1,4), (1,5), (1,6), (2,4), (2,5), (2,6), (3,4), (3,5), (3,6))

Answer 2

根据Nicolas更明确一点:
在Scala中,你可以在一个单一的理解中使用多个生成器.

val xs = List(1,2,3)
val ys = List(4,5)

for {
  x <- xs
  y <- ys
} yield (x,y)

res0: List[(Int, Int)] = List((1,4), (1,5), (2,4), (2,5), (3,4), (3,5))

你甚至可以在理解中进行评估.

for {
  x <- xs
  y <- ys
  if (x + y == 6)
} yield (x,y)

res1: List[(Int, Int)] = List((1,5), (2,4))

或者做作业.

for {
  x <- xs
  y <- ys
  val z = x + y
} yield (x,y,z)

res2: List[(Int,Int,Int)] = List((1,4,5), (1,5,6), (2,4,6), (2,5,7), (3,4,7), (3,5,8))