如何将x元组添加到列表中x次？ [英] How do I add x tuples into a list x number of times?

问题描述

我有一个关于Haskell中的元组和列表的问题。我知道如何将输入添加到特定次数的元组中。现在我想将元组添加到未知次数的列表中;

当我不知道X时，如何将元组添加到列表中x次数？

解决方案

在进行函数式编程时，通常考虑操作的组合而不是单个步骤。所以我们可以通过首先将输入分割成一个字符串列表，然后将每个字符串转换为一个元组来获得它，而不是像想一样将元组添加到列表中。 b $ b

假设元组每行写入一行，我们可以使用 lines 分割输入，然后使用 read 来解析每个元组。为了使它在整个清单上工作，我们使用 map 。

  main = do input<  -  getContents 
 let tuples = map read（lines input）:: [（String，Integer）] 
打印元组
  

$ b $ p

  $ runghc Tuples.hs 
（Hello，2）
（Haskell，4）
  

，我按 Ctrl + D 将EOF发送到程序（或Windows上的 Ctrl + Z ）并打印结果。

  [（Hello，2），（Haskell，4）] 
  

如果你想要更具交互性的东西，你可能需要做自己的递归。参见 Daniel Wagner的回答为例。

I have a question about tuples and lists in Haskell. I know how to add input into a tuple a specific number of times. Now I want to add tuples into a list an unknown number of times; it's up to the user to decide how many tuples they want to add.

How do I add tuples into a list x number of times when I don't know X beforehand?

解决方案

When doing functional programming, it is often better to think about composition of operations instead of individual steps. So instead of thinking about it like adding tuples one at a time to a list, we can approach it by first dividing the input into a list of strings, and then converting each string into a tuple.

Assuming the tuples are written each on one line, we can split the input using lines, and then use read to parse each tuple. To make it work on the entire list, we use map.

main = do input <- getContents
          let tuples = map read (lines input) :: [(String, Integer)]
          print tuples

Let's try it.

$ runghc Tuples.hs
("Hello", 2)
("Haskell", 4)

Here, I press Ctrl+D to send EOF to the program, (or Ctrl+Z on Windows) and it prints the result.

[("Hello",2),("Haskell",4)]

If you want something more interactive, you will probably have to do your own recursion. See Daniel Wagner's answer for an example of that.

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