F# 从序列中获取项目

发布于 2024-11-03 08:21:02 字数 561 浏览 3 评论 0原文

我正在尝试学习 F#

我想做的是下载一个网页，将其拆分为一个序列，然后找到一个项目的索引，并获取其后的下 3 个项目。

这是代码——有人可以告诉我我做错了什么吗？

let find = "<head>"
let page = downloadUrl("http://www.stackoverflow.com")
let lines = seq (  page.Replace("\r", System.String.Empty).Split([|"\n"|],   StringSplitOptions.RemoveEmptyEntries)  )
let pos = lines |> Seq.findIndex(fun a -> a == find) // getting a Exception of type 'System.Collections.Generic.KeyNotFoundException' was thrown.
let result = // now to get the next 3 items
printfn "%A" (Seq.toList result);;

原文

Im trying to learn F#

What I would like to do is download a webpage, split it into a sequence then find the index of an item and take the next 3 items after it.

Heres the code -- can someone show me what Im doing wrong please?

let find = "<head>"
let page = downloadUrl("http://www.stackoverflow.com")
let lines = seq (  page.Replace("\r", System.String.Empty).Split([|"\n"|],   StringSplitOptions.RemoveEmptyEntries)  )
let pos = lines |> Seq.findIndex(fun a -> a == find) // getting a Exception of type 'System.Collections.Generic.KeyNotFoundException' was thrown.
let result = // now to get the next 3 items
printfn "%A" (Seq.toList result);;

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相思碎 2024-11-10 08:21:02

您正在执行一些 F# 文本处理。以下是一些可能出现的问题：

下载 HTML 页面后，您没有进行任何预处理，例如删除所有 HTML 标记。
page.Replace("\r", System.String.Empty).Split([|"\n"|] 是有问题的，因为我猜你想将项目/单词分开。此行仅拆分行。
let pos =lines |> Seq.findIndex(fun a -> a == find) 更改 ==<。 /code> 到 =。在 F# 中，= 是用于比较的布尔运算符
。 let result =lines |> Seq.take pos 仅采用前 pos 项，您应该跳过这些项，然后将 pos 项作为。在：

：

lines
|> Seq.skip (pos+1)
|> Seq.take 3

So you are doing some F# text processing. Here are some possible problems:

After you downloaded the HTML page, you didn't do any preprocessing, say remove all HTML tags.
page.Replace("\r", System.String.Empty).Split([|"\n"|] is problematic because I guess you want to split the items/words out. This line only splits lines out.
let pos = lines |> Seq.findIndex(fun a -> a == find) change == to =. In F#, = is the boolean operator for comparison.
let result = lines |> Seq.take pos only takes the first pos items. You should skip these items and then take pos items as in:

lines
|> Seq.skip (pos+1)
|> Seq.take 3

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非要怀念 2024-11-10 08:21:02

let result = lines |> Seq.take pos

此行会跳过找到的项目之前的所有内容，而不是其后的 3 个项目。

编辑： 如果搜索的项目不存在，Seq.findIndex 将失败。您需要Seq.tryFindIndex：

match lines |> Seq.tryFindIndex(fun a -> a == find) with
| Some pos -> let result = // now to get the next 3 items
              printfn "%A" (Seq.toList result)
| None     -> ()

let result = lines |> Seq.take pos

This line skips everything before the found item, not takes the 3 items after it.

EDIT: Seq.findIndex fails if the item searched for doesn't exist. You want Seq.tryFindIndex:

match lines |> Seq.tryFindIndex(fun a -> a == find) with
| Some pos -> let result = // now to get the next 3 items
              printfn "%A" (Seq.toList result)
| None     -> ()

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