如何解决提取集合
| 最近，我为Anys的笛卡尔乘积编写了一个迭代器，并从List列表开始，但认识到我可以轻松切换到更抽象的特征Seq。 我知道，您喜欢看代码。 :)class Cartesian (val ll: Seq[Seq[_]]) extends Iterator [Seq[_]] {
def combicount: Int = (1 /: ll) (_ * _.length)
val last = combicount
var iter = 0
override def hasNext (): Boolean = iter < last
override def next (): Seq[_] = {
val res = combination (ll,iter)
iter += 1
res
}
def combination (xx: Seq [Seq[_]],i: Int): List[_] = xx match {
case Nil => Nil
case x :: xs => x (i % x.length) :: combination (xs,i / x.length)
}
}
object Main extends Application {
val illi = new Cartesian (List (\"abc\".toList,\"xy\".toList,\"AB\".toList))
// val ivvi = new Cartesian (Vector (Vector (1,2,3),Vector (10,20)))
val issi = new Cartesian (Seq (Seq (1,Seq (10,20)))
// val iaai = new Cartesian (Array (Array (1,Array (10,20)))
(0 to 5).foreach (dummy => println (illi.next ()))
// (0 to 5).foreach (dummy => println (issi.next ()))
}
/*
List(a,x,A)
List(b,A)
List(c,A)
List(a,y,A)
*/
解决方法
有两种解决方案。第一个是不要求容器是某个泛型超类的子类，而是可以转换为一个（通过使用隐式函数参数）。如果容器已经是所需类型的子类，则有一个预定义的标识转换，仅将其返回。
import collection.mutable.Builder
import collection.TraversableLike
import collection.generic.CanBuildFrom
import collection.mutable.SeqLike
class Cartesian[T,ST[T],TT[S]](val ll: TT[ST[T]])(implicit cbf: CanBuildFrom[Nothing,T,ST[T]],seqLike: ST[T] => SeqLike[T,traversableLike: TT[ST[T]] => TraversableLike[ST[T],TT[ST[T]]] ) extends Iterator[ST[T]] {
def combicount (): Int = (1 /: ll) (_ * _.length)
val last = combicount - 1
var iter = 0
override def hasNext (): Boolean = iter < last
override def next (): ST[T] = {
val res = combination (ll,iter,cbf())
iter += 1
res
}
def combination (xx: TT[ST[T]],i: Int,builder: Builder[T,ST[T]]): ST[T] =
if (xx.isEmpty) builder.result
else combination (xx.tail,i / xx.head.length,builder += xx.head (i % xx.head.length) )
}
scala> new Cartesian[String,Vector,Vector](Vector(Vector(\"a\"),Vector(\"xy\"),Vector(\"AB\")))
res0: Cartesian[String,Vector] = empty iterator
scala> new Cartesian[String,Array,Array](Array(Array(\"a\"),Array(\"xy\"),Array(\"AB\")))
res1: Cartesian[String,Array] = empty iterator
foldLeft
get(i: Int)
length
get(i: Int)
trait HasGet[T,CC[_]] {
def get(cc: CC[T],i: Int): T
}
object HasGet {
implicit def seqLikeHasGet[T,CC[X] <: SeqLike[X,_]] = new HasGet[T,CC] {
def get(cc: CC[T],i: Int): T = cc(i)
}
implicit def arrayHasGet[T] = new HasGet[T,Array] {
def get(cc: Array[T],i: Int): T = cc(i)
}
}
trait HasLength[CC] {
def length(cc: CC): Int
}
object HasLength {
implicit def seqLikeHasLength[CC <: SeqLike[_,_]] = new HasLength[CC] {
def length(cc: CC) = cc.length
}
implicit def arrayHasLength[T] = new HasLength[Array[T]] {
def length(cc: Array[T]) = cc.length
}
}
trait HasFold[T,CC[_]] {
def foldLeft[A](cc: CC[T],zero: A)(op: (A,T) => A): A
}
object HasFold {
implicit def seqLikeHasFold[T,_]] = new HasFold[T,CC] {
def foldLeft[A](cc: CC[T],T) => A): A = cc.foldLeft(zero)(op)
}
implicit def arrayHasFold[T] = new HasFold[T,Array] {
def foldLeft[A](cc: Array[T],T) => A): A = {
var i = 0
var result = zero
while (i < cc.length) {
result = op(result,cc(i))
i += 1
}
result
}
}
}
class Cartesian[T,ST[_],TT[Y]](val ll: TT[ST[T]])(implicit cbf: CanBuildFrom[Nothing,stHasLength: HasLength[ST[T]],stHasGet: HasGet[T,ST],ttHasFold: HasFold[ST[T],TT],ttHasGet: HasGet[ST[T],ttHasLength: HasLength[TT[ST[T]]]) extends Iterator[ST[T]] {
def combicount (): Int = ttHasFold.foldLeft(ll,1)((a,l) => a * stHasLength.length(l))
val last = combicount - 1
var iter = 0
override def hasNext (): Boolean = iter < last
override def next (): ST[T] = {
val res = combination (ll,j: Int,ST[T]]): ST[T] =
if (ttHasLength.length(xx) == j) builder.result
else {
val head = ttHasGet.get(xx,j)
val headLength = stHasLength.length(head)
combination (xx,j + 1,i / headLength,builder += stHasGet.get(head,(i % headLength) ))
}
}
scala> new Cartesian[String,List](List(Vector(\"a\"),Vector(\"AB\")))
res6: Cartesian[String,List] = empty iterator
scala> new Cartesian[String,Array(\"AB\")))
res7: Cartesian[String,Array] = empty iterator
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