查找索引*在哪里*

Question

Vector 中有一个 indexWhere 函数可以查找匹配项的索引。

def indexWhere(p: (A) ⇒ Boolean, from: Int): Int
> Finds index of the first element satisfying some predicate after or 
> at some start index.

http://www.scala-lang.org/api/current/index.html#scala.collection.immutable.Vector

我编写了这个函数来查找发生这种匹配的所有索引。

  def getAllIndexesWhere[A,B](as: List[A])(f: (B => Boolean))(g: A => B): Vector[B] = {
    def go(y: List[A], acc: List[Option[B]]): Vector[B] = as match {
      case x :: xs => val result = if (f(g(x))) Some(g(x)) else None
                      go(xs, acc :+ result)
      case Nil => acc.flatten.toVector
    }
    go(as, Nil)
  }

但是，是否已经有集合的内置功能？

Answer 1

zipWithIndex、filter 和 map 是内置函数，可以组合起来获取某个谓词的所有索引。

获取列表中偶数值的索引。

scala> List(1,2,3,4,5,6,7,8,9,10).zipWithIndex.filter(_._1 % 2 == 0).map(_._2)
res0: List[Int] = List(1, 3, 5, 7, 9)

您也可以将collect 用作@0__ 注释。

scala> List(1,2,3,4,5,6,7,8,9,10).zipWithIndex.collect{ case(a,b) if a % 2 == 0 => b}
res1: List[Int] = List(1, 3, 5, 7, 9)