Question

let aBunch = 1000
let offset = 0

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

使用不同的偏移量调用getIt（）最终会给我一个“无效操作”异常，其中包含“输入序列元素不足”的附加信息

我试着理解为什么，因为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

Answer 1

我知道这是一个老问题，但万一有人在我的搜索方式中遇到过这个问题：

如果您最多需要 n 项，则可以使用Seq.truncate。如果可用的项目少于 n ，则不会抛出异常。

Answer 2

如果使用大于序列的值调用，Seq.skip和Seq.take都将抛出此异常。您可以在Seq.fs中查看源代码以了解原因：

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 }

Answer 3

对于无例外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 
        }

与Seq.truncate（这是一个无例外的Seq.take等价物相结合 - 它可以在不抛出异常的情况下获得尽可能多的项目。）

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

(* returns empty seq *)

Answer 4

这是使用内置函数的稍微更短的“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内嵌到当前管道中，请替换

|> Seq.skip num

与

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

Answer 5

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

在F＃中调用Seq.skip和Seq.take

5 个答案: