Haskell-计算列表中每个不同元素出现的次数 [英] Haskell - Counting how many times each distinct element in a list occurs

问题描述

我是Haskell的新手，我只是想编写一个列表理解来计算列表中每个不同值的出现频率，但最后一部分却遇到了麻烦.

I'm new to Haskell and am just trying to write a list comprehension to calculate the frequency of each distinct value in a list, but I'm having trouble with the last part..

到目前为止，我有这个:

So far i have this:

frequency :: Eq a => [a] -> [(Int,a)] 
frequency list = [(count y list,y) | y <- rmdups ]

涉及rmdups的最后一部分出了问题.

Something is wrong with the last part involving rmdups.

count函数先获取一个字符，然后获取一个字符列表，并告诉您该字符出现的频率，代码如下.

The count function takes a character and then a list of characters and tells you how often that character occurs, the code is as follows..

count :: Eq a => a -> [a] -> Int
count x [] = 0
count x (y:ys) | x==y = 1+(count x ys)
               | otherwise = count x ys

先谢谢您.

推荐答案

在计算频率时，您还可以使用关联数组/有限映射存储从列表元素到其计数的关联:

You could also use a associative array / finite map to store the associations from list elements to their count while you compute the frequencies:

import Data.Map (fromListWith, toList)

frequency :: (Ord a) => [a] -> [(a, Int)]
frequency xs = toList (fromListWith (+) [(x, 1) | x <- xs])

示例用法:

> frequency "hello world"
[(' ',1),('d',1),('e',1),('h',1),('l',3),('o',2),('r',1),('w',1)]

请参见 fromListWith 的文档代码> 和 toList .

See documentation of fromListWith and toList.

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