scala：产生两个循环的成对组合
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scala: yield pairwise combinations of two loops
提问by Jakub M.
I would like to create a collection with tuples containing all pairwise combinations of two lists. Something like:
我想创建一个包含两个列表的所有成对组合的元组的集合。就像是：
for ( x <- xs )
for ( y <- ys )
yield (x,y)
In Python this would work, in Scala apparently foryields only for the last loop (so this evaluates to Unit)
在 Python 中这是可行的，在 Scala 中显然for只在最后一个循环中产生（因此计算结果为Unit）
What is the cleanest way to implement it in Scala?
在 Scala 中实现它的最简洁的方法是什么？
回答by Nicolas
You were almost there:
你快到了：
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))
回答by tgr
A little bit more explicit according to Nicolas:
In Scala you can use multiple generators in a single for-comprehension.
根据 Nicolas 的说法更明确一点：
在 Scala 中，您可以在单个 for-comprehension 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))
You can even evaluate in the comprehension.
您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))
Or make an assignment.
或者做一个任务。
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))
