Scala：流上的 filter 和 takeWhile 有什么区别？
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Scala: What is the difference between filter and takeWhile on a stream?
提问by AbrahamDaniel
I just started to learn Scala, and I'm confused between the filter and takeWhile while working on streams.
我刚开始学习 Scala，在处理流时，我对 filter 和 takeWhile 感到困惑。
I came across this program to generate prime numbers, which uses both takeWhile and filter on a stream.
我遇到了这个程序来生成素数，它在流上同时使用 takeWhile 和 filter。
lazy val ps: Stream[Int] = 2 #:: Stream.from(3).filter(i =>
ps.takeWhile{j => j * j <= i}.forall{ k => i % k > 0});
While experimenting i found
在试验时我发现
Stream.from(1).takeWhile(_ < 10).toList
returns me
还我
List[Int] = List(1, 2, 3, 4, 5, 6, 7, 8, 9)
while
尽管
Stream.from(1).filter(_ < 10).toList
runs infinitely.
无限运行。
回答by PawelN
takeWhile()stop evaluation immediately after condition is not met.
takeWhile()不满足条件后立即停止评估。
filterhave to evaluate the whole Stream.
filter必须评估整个流。
I suggest You to analyze this two function in below code:
我建议你在下面的代码中分析这两个函数：
回答by Alexey Romanov
The difference isn't stream-specific, but the same for all collections extending GenTraversableLike:
差异不是特定于流的，但对于所有扩展的集合都是相同的GenTraversableLike：
filter
filter
Returns all the elementsthat satisfy the predicate
p
返回满足谓词的所有元素
p
takeWhile
takeWhile
Returns the longest prefixwhose elements satisfy the predicate
p.
返回其元素满足谓词的最长前缀
p。
E.g.
例如
> List(1, 2, 3, 4).filter(_ % 2 == 1)
List(1, 3)
> List(1, 2, 3, 4).takeWhile(_ % 2 == 1)
List(1)
In particular, Stream.from(1).filter(_ < 10).toListdoesn't finish because it has to check every element of the stream: it doesn't "know" there won't be any elements which satisfy _ < 10after 9 (and actually, there are, thanks to wraparound).
特别是，Stream.from(1).filter(_ < 10).toList没有完成，因为它必须检查流的每个元素：它不“知道”_ < 10在 9 之后不会有任何满足的元素（实际上，由于环绕，有）。
