如何拆分一个迭代器? [英] How to split up an Iterator?

问题描述

如何将迭代器拆分为具有重复项和其余部分的前缀?例如，

How to split an iterator into a prefix with duplicates and the rest ? For instance,

def splitDupes(it: Iterator[Int]): (Iterator[Int], Iterator[Int]) = ???

val (xs, ys) = splitDupes(List(1, 1, 1, 2, 3, 4, 5).iterator)
xs.toList // List(1, 1, 1)
ys.toList // List(2, 3, 4, 5)

val (xs, ys) = splitDupes(List(1, 2, 3, 4, 5).iterator)
xs.toList // List(1)
ys.toList // List(2, 3, 4, 5)

val (xs, ys) = splitDupes(List(1, 1, 1, 1, 1).iterator)
xs.toList // List(1, 1, 1, 1, 1)
ys.toList // List()

val (xs, ys) = splitDupes(List[Int]().iterator)
xs.toList // List()
ys.toList // List()

我可以用它按块读取文本文件吗?

Can I use it to read a text file by chunks ?

推荐答案

您可以使用span方法将Iterable拆分为满足谓词的前缀和不满足条件的后缀.对于Iterator s span做正确的事情，并且在前缀用完之前迭代后缀的情况下，将元素懒惰地存储在前缀Iterator中.

You can use the span method to split an Iterable into a prefix that satisfies a predicate and a suffix that doesn't. For Iterators span does the correct thing, and lazily stores elements in the prefix Iterator, in case the suffix was iterated before the prefix has run out.

def splitDupes[T](it: Iterator[T]): (Iterator[T], Iterator[T]) = {
  if (it.isEmpty) (Iterator.empty, Iterator.empty)
  else {
    val head = it.next()
    val (dupes, rest) = it.span(_ == head)
    (Iterator(head) ++ dupes, rest)
  }
}

示例:

scala> val (dupes, rest) = splitDupes(Iterator(1,1,1,2,3,2,1))
dupes: Iterator[Int] = <iterator>
rest: Iterator[Int] = <iterator>

scala> (dupes.toList, rest.toList)
res1: (List[Int], List[Int]) = (List(1, 1, 1),List(2, 3, 2, 1))

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