F#:如何将一个序列分成一个序列 [英] F#: How do i split up a sequence into a sequence of sequences

问题描述

背景:

我有一系列连续的，带时间戳的数据.数据序列在其中数据不连续的地方有间隙.我想创建一种将序列拆分为序列序列的方法，以使每个子序列包含连续数据(将输入序列拆分为空白).

I have a sequence of contiguous, time-stamped data. The data-sequence has gaps in it where the data is not contiguous. I want create a method to split the sequence up into a sequence of sequences so that each subsequence contains contiguous data (split the input-sequence at the gaps).

约束:

• 返回值必须是一个序列序列，以确保仅根据需要生成元素(不能使用列表/数组/缓存)
• 解决方案不得为O(n ^ 2)，可能排除了Seq.take-Seq.skip模式(参见)
• 对功能惯用方法的加分(因为我想精通函数式编程)，但这不是必需的.

• The return value must be a sequence of sequences to ensure that elements are only produced as needed (cannot use list/array/cacheing)
• The solution must NOT be O(n^2), probably ruling out a Seq.take - Seq.skip pattern (cf. Brian's post)
• Bonus points for a functionally idiomatic approach (since I want to become more proficient at functional programming), but it's not a requirement.

方法签名

let groupContiguousDataPoints (timeBetweenContiguousDataPoints : TimeSpan) (dataPointsWithHoles : seq<DateTime * float>) : (seq<seq< DateTime * float >>)= ... 

从表面上看，这个问题对我来说似乎微不足道，但是即使使用Seq.pairwise，IEnumerator <_>，序列理解和yield语句，解决方案也使我难以理解.我确信这是因为我仍然缺乏结合F#惯用语的经验，或者可能是因为我还没有接触过某些语言构造.

On the face of it the problem looked trivial to me, but even employing Seq.pairwise, IEnumerator<_>, sequence comprehensions and yield statements, the solution eludes me. I am sure that this is because I still lack experience with combining F#-idioms, or possibly because there are some language-constructs that I have not yet been exposed to.

// Test data
let numbers = {1.0..1000.0}
let baseTime = DateTime.Now
let contiguousTimeStamps = seq { for n in numbers ->baseTime.AddMinutes(n)}

let dataWithOccationalHoles = Seq.zip contiguousTimeStamps numbers |> Seq.filter (fun (dateTime, num) -> num % 77.0 <> 0.0) // Has a gap in the data every 77 items

let timeBetweenContiguousValues = (new TimeSpan(0,1,0))

dataWithOccationalHoles |> groupContiguousDataPoints timeBetweenContiguousValues |> Seq.iteri (fun i sequence -> printfn "Group %d has %d data-points: Head: %f" i (Seq.length sequence) (snd(Seq.hd sequence)))

推荐答案

我认为这可以满足您的要求

I think this does what you want

dataWithOccationalHoles 
|> Seq.pairwise
|> Seq.map(fun ((time1,elem1),(time2,elem2)) -> if time2-time1 = timeBetweenContiguousValues then 0, ((time1,elem1),(time2,elem2)) else 1, ((time1,elem1),(time2,elem2)) )
|> Seq.scan(fun (indexres,(t1,e1),(t2,e2)) (index,((time1,elem1),(time2,elem2))) ->  (index+indexres,(time1,elem1),(time2,elem2))  ) (0,(baseTime,-1.0),(baseTime,-1.0))
|> Seq.map( fun (index,(time1,elem1),(time2,elem2)) -> index,(time2,elem2) )
|> Seq.filter( fun (_,(_,elem)) -> elem <> -1.0)
|> PSeq.groupBy(fst)
|> Seq.map(snd>>Seq.map(snd))

感谢您提出这个很酷的问题

Thanks for asking this cool question

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