在列表中查找第一个重复元素

问题描述

对于Haskell我很新。我正在尝试在Haskell中编写代码，该代码从列表中找到第一个重复元素，并且如果它没有重复的元素，则消息不会重复。我知道我可以通过 nub 函数来做到这一点，但我试图在没有它的情况下做到这一点。

解决方案

 将限定数据集合导入为集合
 
 dup :: Ord a => [a]  - >可能是
 dup xs = dup'xs Set.empty 
其中dup'[] _ = Nothing 
 dup'（x：xs）s = Set.member xs 
 then只需x 
 else dup'xs（Set.insert xs）
 
 dupString ::（Ord a，Show a）=> [a]  - > [Char] 
 dupString x = case $ x 
只要x  - > First duplicate：++（show x）
 Nothing  - > 不重复
 
 main :: IO（）
 main = do 
 putStrLn $ dupString [1,2,3,4,5] 
 putStrLn $ dupString [1,2,1,2,3] 
 putStrLn $ dupStringHELLO WORLD
  

这是它是如何工作的：

  * Main>主要
不重复
第一个重复：1 
第一个重复：'L'
  

I am very new to Haskell. I am trying to write code in Haskell that finds the first duplicate element from the list, and if it does not have the duplicate elements gives the message no duplicates. I know i can do it through nub function but i am trying to do it without it.

解决方案

This is one way to do it:

import qualified Data.Set as Set

dup :: Ord a => [a] -> Maybe a
dup xs = dup' xs Set.empty
  where dup' [] _ = Nothing
        dup' (x:xs) s = if Set.member x s
                           then Just x
                           else dup' xs (Set.insert x s)

dupString :: (Ord a, Show a) => [a] -> [Char]
dupString x = case dup x of
                  Just x  -> "First duplicate: " ++ (show x)
                  Nothing -> "No duplicates"

main :: IO ()
main = do
       putStrLn $ dupString [1,2,3,4,5]
       putStrLn $ dupString [1,2,1,2,3]
       putStrLn $ dupString "HELLO WORLD"

Here is how it works:

*Main> main
No duplicates
First duplicate: 1
First duplicate: 'L'

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