如何用 Scala 中的地图替换产量？答案

【问题标题】：How to replace a yield with a map in Scala?如何用 Scala 中的地图替换产量？
【发布时间】：2012-02-05 14:52:36
【问题描述】：

我怎样才能删除这个产量？我想使用地图而不是：

val cols = for(x <- 0 to 6) yield for(y <- 0 to 5) yield apply(x, y)

这可能吗？

谢谢！

最好的问候，约翰

【问题讨论】：

一般来说：for (x <- seq) yield f(x)被翻译成seq.map(x => f(x))
不是你的问题的答案，但以防万一：val cols = Vector.tabulate(7,6)(apply)

标签： scala yield

【解决方案1】：

这很简单：

val cols = (0 to 6).map(x => (0 to 5).map( y => apply(x,y)))

更新

maps 可能存在差异。如果你想要没有嵌套结构的平面对象，最好使用flatMap 而不是map：

def apply(x: Int, y: Int) = (x,y)
scala> val cols = (0 to 6).map(x => (0 to 5).map( y => apply(x,y)))
//cols: scala.collection.immutable.IndexedSeq[scala.collection.immutable.IndexedSeq[(Int, Int)]] =  ...

scala> val cols = (0 to 6).flatMap(x => (0 to 5).map( y => apply(x,y)))
//cols: scala.collection.immutable.IndexedSeq[(Int, Int)] = ...

或者只是在最后展平结果：

scala> val cols = (0 to 6).map(x => (0 to 5).map( y => apply(x,y))).flatten
//cols: scala.collection.immutable.IndexedSeq[(Int, Int)] = ...

【讨论】：

我还有一个问题。当我使用此代码并打印此 cols println("cols: "+cols) 时，我得到 2 行。为什么？
@user1137701 你能指定你想要的输出吗？你得到了什么？我怀疑这是因为我使用了保存内部结构的map，而不是返回未嵌套对象的flatMap
嘿，是的，我得到了这个：code val array = Array.fill(7)(Array.fill(6)(None:Option[Coin])) def apply(x: Int , y: Int):Option[Coin] = if (0 (0 until 6).map( y => apply(x,y))) code 当我打印 cols 时，我得到了两次......

【解决方案2】：

当你有多个生成器时，最里面的一个转换为map，其余的转换为flatMap。（在您的情况下，这些不是嵌套生成器。）

以下示例可能会有所帮助：

scala> val xs, ys, zs = Vector(1, 4, 5)
xs: scala.collection.immutable.Vector[Int] = Vector(1, 4, 5)
ys: scala.collection.immutable.Vector[Int] = Vector(1, 4, 5)
zs: scala.collection.immutable.Vector[Int] = Vector(1, 4, 5)

scala> for {
     |   x <- xs
     |   y <- ys
     |   z <- zs
     | } yield (x, y, z)
res0: scala.collection.immutable.Vector[(Int, Int, Int)] = Vector((1,1,1), (1,1,4), (1,1,5), (1,4,1), (1,4,4), (1,4,5), (1,5,1), (1,5,4), (1,5,5), (4,1,1), (4,1,4), (4,1,5), (4,4,1), (4,4,4), (4,4,5), (4,5,1), (4,5,4), (4,5,5), (5,1,1), (5,1,4), (5,1,5), (5,4,1), (5,4,4), (5,4,5), (5,5,1), (5,5,4), (5,5,5))

scala> xs flatMap { x =>
     |   ys flatMap { y =>
     |     zs map { z =>
     |       (x, y, z)
     |     }
     |   }
     | }
res1: scala.collection.immutable.Vector[(Int, Int, Int)] = Vector((1,1,1), (1,1,4), (1,1,5), (1,4,1), (1,4,4), (1,4,5), (1,5,1), (1,5,4), (1,5,5), (4,1,1), (4,1,4), (4,1,5), (4,4,1), (4,4,4), (4,4,5), (4,5,1), (4,5,4), (4,5,5), (5,1,1), (5,1,4), (5,1,5), (5,4,1), (5,4,4), (5,4,5), (5,5,1), (5,5,4), (5,5,5))

scala> res0 == res1
res2: Boolean = true

scala> for {
     |   x <- xs
     |   y <- ys
     | } yield for {
     |   z <- zs
     | } yield (x, y, z)
res3: scala.collection.immutable.Vector[scala.collection.immutable.Vector[(Int, Int, Int)]] = Vector(Vector((1,1,1), (1,1,4), (1,1,5)), Vector((1,4,1), (1,4,4), (1,4,5)), Vector((1,5,1), (1,5,4), (1,5,5)), Vector((4,1,1), (4,1,4), (4,1,5)), Vector((4,4,1), (4,4,4), (4,4,5)), Vector((4,5,1), (4,5,4), (4,5,5)), Vector((5,1,1), (5,1,4), (5,1,5)), Vector((5,4,1), (5,4,4), (5,4,5)), Vector((5,5,1), (5,5,4), (5,5,5)))

scala> xs flatMap { x =>
     |   ys map { y =>
     |     zs map { z =>
     |       (x, y, z)
     |     }
     |   }
     | }
res4: scala.collection.immutable.Vector[scala.collection.immutable.Vector[(Int, Int, Int)]] = Vector(Vector((1,1,1), (1,1,4), (1,1,5)), Vector((1,4,1), (1,4,4), (1,4,5)), Vector((1,5,1), (1,5,4), (1,5,5)), Vector((4,1,1), (4,1,4), (4,1,5)), Vector((4,4,1), (4,4,4), (4,4,5)), Vector((4,5,1), (4,5,4), (4,5,5)), Vector((5,1,1), (5,1,4), (5,1,5)), Vector((5,4,1), (5,4,4), (5,4,5)), Vector((5,5,1), (5,5,4), (5,5,5)))

scala> res3 == res4
res5: Boolean = true

scala> for {
     |   x <- xs
     | } yield for {
     |   y <- ys
     |   z <- zs
     | } yield (x, y, z)
res6: scala.collection.immutable.Vector[scala.collection.immutable.Vector[(Int, Int, Int)]] = Vector(Vector((1,1,1), (1,1,4), (1,1,5), (1,4,1), (1,4,4), (1,4,5), (1,5,1), (1,5,4), (1,5,5)), Vector((4,1,1), (4,1,4), (4,1,5), (4,4,1), (4,4,4), (4,4,5), (4,5,1), (4,5,4), (4,5,5)), Vector((5,1,1), (5,1,4), (5,1,5), (5,4,1), (5,4,4), (5,4,5), (5,5,1), (5,5,4), (5,5,5)))

scala> xs map { x =>
     |   ys flatMap { y =>
     |     zs map { z =>
     |       (x, y, z)
     |     }
     |   }
     | }
res7: scala.collection.immutable.Vector[scala.collection.immutable.Vector[(Int, Int, Int)]] = Vector(Vector((1,1,1), (1,1,4), (1,1,5), (1,4,1), (1,4,4), (1,4,5), (1,5,1), (1,5,4), (1,5,5)), Vector((4,1,1), (4,1,4), (4,1,5), (4,4,1), (4,4,4), (4,4,5), (4,5,1), (4,5,4), (4,5,5)), Vector((5,1,1), (5,1,4), (5,1,5), (5,4,1), (5,4,4), (5,4,5), (5,5,1), (5,5,4), (5,5,5)))

scala> res6 == res7
res8: Boolean = true

【讨论】：