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呼叫Seq.skip和Seq.take in F#

sequences f#

9

Moonlight · 技术社区 · 16 年前

let aBunch = 1000
let offset = 0

let getIt offset =
  MyIEnumerable
  |> Seq.skip aBunch * offset
  |> Seq.take aBunch
  |> Seq.iter ( .. some processing ...)

我试图理解为什么Seq.Skip和Seq.take都没有根据在线文档生成异常 FSharp Collections

版本:(Visual Studio 2010)Beta 1

5 回复 | 直到 15 年前

1

24

Joel Mueller 15 年前

我知道这是一个老问题,但如果有人在搜索时遇到我这样的问题:

你可以用 Seq.truncate 如果你最多想要 n个项目。如果少于 n个

2

7

Dustin Campbell 16 年前

如果调用的值大于序列,Seq.skip和Seq.take都将引发此异常。您可以检查Seq.fs中的源代码以了解原因:

let skip count (sequence: seq<_>) =
    { use e = sequence.GetEnumerator() 
      let latest = ref (Unchecked.defaultof<_>)
      let ok = ref false
      for i in 1 .. count do
          if not (e.MoveNext()) then 
              raise <| System.InvalidOperationException "the input sequence had insufficient elements" 
      while e.MoveNext() do
          yield e.Current }

let take count (sequence : seq<'T>)    = 
    if count < 0 then invalidArg "count" "the number of elements to take may not be negative"
    (* Note: don't create or dispose any IEnumerable if n = 0 *)
    if count = 0 then empty else  
    { use e = sequence.GetEnumerator() 
      for i in 0 .. count - 1 do
          if not (e.MoveNext()) then
              raise <| System.InvalidOperationException "the input sequence had insufficient elements" 
          yield e.Current }

3

krdx 11 年前

skip 您可以将自己的版本添加到Seq模块,如下所示:

module Seq =
    let skipSafe (num: int) (source: seq<'a>) : seq<'a> =
        seq {
            use e = source.GetEnumerator()
            let idx = ref 0
            let loop = ref true
            while !idx < num && !loop do
                if not(e.MoveNext()) then
                    loop := false
                idx := !idx + 1

            while e.MoveNext() do
                yield e.Current 
        }

结合 Seq.truncate (这是一个例外 Seq.take 相当于-它将在不引发异常的情况下使用尽可能多的可用项)。

[1..10] 
|> Seq.skipSafe 20
|> Seq.truncate 5

(* returns empty seq *)

4

3

user3908642 9 年前

下面是一个使用内置函数的略短的“skipSafe”实现:

module Seq =
    let skipSafe num = 
        Seq.zip (Seq.initInfinite id)
        >> Seq.skipWhile (fun (i, _) -> i < num)
        >> Seq.map snd

或者,如果您希望直接将其嵌入到当前管道中,请替换

|> Seq.skip num

具有

|> Seq.zip (Seq.initInfinite id)
|> Seq.skipWhile (fun (i, _) -> i < num)
|> Seq.map snd

5

1

Sergii 9 年前

module Seq = 
    let trySkip count source  =
        source |> Seq.indexed |> Seq.filter(fst >> (<=) count) |> Seq.map snd