用于组合序列的惯用scala解决方案

时间:2013-09-19 22:51:02

标签: scala functional-programming sequence seq

想象一个函数combineSequences: (seqs: Set[Seq[Int]])Set[Seq[Int]],它在第一个序列的最后一项与第二个序列的第一项匹配时组合序列。例如,如果您有以下序列:

(1, 2)
(2, 3)
(5, 6, 7, 8)
(8, 9, 10)
(3, 4, 10)

combineSequences的结果是:

(5, 6, 7, 8, 8, 9, 10)
(1, 2, 2, 3, 3, 4, 10)

因为序列1,2和5组合在一起。如果多个序列可以组合以创建不同的结果,则决策是任意的。例如,如果我们有序列:

(1, 2)
(2, 3)
(2, 4)

有两个正确的答案。之一:

(1, 2, 2, 3)
(2, 4)

或者:

(1, 2, 2, 4)
(2, 3)

我只能想到一个非常必要且相当不透明的实现。我想知道是否有人有一个更惯用的scala解决方案。我现在已经遇到过几次相关的问题了。

1 个答案:

答案 0 :(得分:3)

当然不是最优化的解决方案,但我已经考虑了可读性。

def combineSequences[T]( seqs: Set[Seq[T]] ): Set[Seq[T]] = {
  if ( seqs.isEmpty ) seqs
  else {
    val (seq1, otherSeqs) = (seqs.head, seqs.tail)
    otherSeqs.find(_.headOption == seq1.lastOption) match {
      case Some( seq2 ) => combineSequences( otherSeqs - seq2 + (seq1 ++ seq2) )
      case None =>
        otherSeqs.find(_.lastOption == seq1.headOption) match {
          case Some( seq2 ) => combineSequences( otherSeqs - seq2 + (seq2 ++ seq1) )
          case None => combineSequences( otherSeqs ) + seq1
        }
    }
  }
}

REPL测试:

scala> val seqs = Set(Seq(1, 2), Seq(2, 3), Seq(5, 6, 7, 8), Seq(8, 9, 10), Seq(3, 4, 10))
seqs: scala.collection.immutable.Set[Seq[Int]] = Set(List(1, 2), List(2, 3), List(8, 9, 10), List(5, 6, 7, 8), List(3, 4, 10))
scala> combineSequences( seqs )
res10: Set[Seq[Int]] = Set(List(1, 2, 2, 3, 3, 4, 10), List(5, 6, 7, 8, 8, 9, 10))
scala> val seqs = Set(Seq(1, 2), Seq(2, 3, 100), Seq(5, 6, 7, 8), Seq(8, 9, 10), Seq(100, 4, 10))
seqs: scala.collection.immutable.Set[Seq[Int]] = Set(List(100, 4, 10), List(1, 2), List(8, 9, 10), List(2, 3, 100), List(5, 6, 7, 8))
scala> combineSequences( seqs )
res11: Set[Seq[Int]] = Set(List(5, 6, 7, 8, 8, 9, 10), List(1, 2, 2, 3, 100, 100, 4, 10))