F#如何根据谓词而不是固定长度来对序列进行窗口化 [英] F# how to Window a sequence based on predicate rather than fixed length

问题描述

鉴于以下输入序列，我想生成所需的输出. 我知道，如果所有窗口的长度都是固定的，则可以使用Seq.window几乎获得所需的结果.但是，在这种情况下它们不是固定长度的，我想在遇到"a"时开始一个新序列. 标准馆藏图书馆有可能吗?

Given the following input sequence, I would like to generate the desired output. I know that Seq.window can be used to almost get the desired result if all the windows are a fixed length. However in this case they are not fixed legnth, I would like to start a new sequence whenever "a" is encountered. Is this possible with the standard collections library?

let inputSequence = 
      ["a"; "b"; "c";
       "a"; "b"; "c"; "d";
       "a"; "b"; 
       "a"; "d"; "f";
       "a"; "x"; "y"; "z"]

let desiredResult = 
   [["a"; "b"; "c";]
    ["a"; "b"; "c"; "d";]
    ["a"; "b"; ]
    ["a"; "d"; "f";]
    ["a"; "x"; "y"; "z"]]

推荐答案

这是一种使用可变状态但非常简洁的方法:

Here's a way that uses mutable state but is pretty concise:

let mutable i = 0
[ for x in inputSequence do
    if x = "a" then i <- i + 1
    yield i, x ]
|> List.groupBy fst
|> List.map snd
|> List.map (List.map snd)

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