在列表中将字符串乘以Haskell [英] Multiplying String in a list, Haskell

问题描述

我试图在列表中乘以字符串。例如，当我有一个像 [hello，World] 这样的列表，并且我想将它乘以2时，我希望以 [hello，hello，World，World] 。

下面是我想到的：

  rep :: Int  - > [字符串]  - > [String] 
 rep n [] = [] 
 rep n [x：xs] = replicate n [x] ++ rep n [xs] 
  

但它给了我例外：

（298,1） - （299,44）：函数rep中的非穷举模式

我对这种语言完全陌生，我没有想法如何解决这个问题。你可以帮忙吗？

解决方案

• 你的第一个问题是非空列表的模式是错误的，它应该是（x：xs），而不是 [x：xs] 。


• 此外，复制期待一个单一元素并将其转换为长度 n 的列表，所以它应该是 replicate nx ，而不是 replicate n [x] 。


• 类似的徒劳的 xs 已经是一个列表。

如果您解决了所有这些问题，您最终会得到以下结果，这些结果实际上按预期运行：

  rep :: Int  - > [字符串]  - > [String] 
 rep n [] = [] 
 rep n（x：xs）= replicate nx ++ rep n xs 
  

也就是说有很多不同的方法可以编写它，例如 concatMap ：

  rep n = concatMap（replicate n）
  

或

  rep = concatMap。复制
  




I am trying to multiply strings in a list. For instance, when I have a list like ["hello","World"] and I want to 'multiply' it by 2, I want to end up with ["hello","hello","World","World"].

Here's what I came up with:

rep :: Int -> [String] -> [String]
rep n [] = []
rep n [x:xs] = replicate n [x] ++ rep n [xs]

But it gives me exception:

(298,1)-(299,44): Non-exhaustive patterns in function rep

I am totally new to this language and I do not have any ideas how to sort this out. Can you help?

解决方案

• Your fist problem is that the pattern for non-empty lists is wrong, it should be (x:xs), not [x:xs].
• Furthermore, replicate expects a single element and turns it into a list of length n, so it should be replicate n x, not replicate n [x].
• In a similar vain xs is already a list.

If you fix all of this you'll end up with the following, which actually works as intended:

rep :: Int -> [String] -> [String]
rep n [] = []
rep n (x:xs) = replicate n x ++ rep n xs

That said there are many different ways one could write this, for example with concatMap:

rep n = concatMap (replicate n)

or

rep = concatMap . replicate

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