Haskell函数用于查找字母数字 [英] Haskell function for finding letter number

问题描述

Haskell有一个函数可以让你找到一个单词的特定元素的数量，例如[char]，所以FROG中的'F'的元素编号是0.另外，那个字母出现的单词更多这个函数可以返回一个字母位置列表，即TEETER中的'E'将会是[1,2,4]吗？

解决方案您要查找的两个函数位于 Data.List 中，它们是 elemIndex 和 elemIndices 。

  elemIndices :: Eq a => a  - > [a]  - > [Int] 
 elemIndex :: Eq a => a  - > [a]  - >也许Int 
 
 
λ> elemIndex'F'FROG
只需0 
λ> elemIndices'E'TEETER
 [1,2,4] 
  

寻找像这样的东西，写下你想要的类型通常是有帮助的，然后（如果可能的话）推广它。您可以通过这种方式经常在标准库中找到东西。

Does Haskell have a function that lets you find the number of a particular element of a word i.e. [char] so the element number of 'F' in "FROG" is 0. Also, what about words where that letter appears more than once, can this function return a list of letter positions i.e. 'E' in "TEETER" would be [1,2,4]?

解决方案

The two functions you're looking for are in Data.List, they are elemIndex and elemIndices.

elemIndices :: Eq a => a -> [a] -> [Int]
elemIndex :: Eq a => a -> [a] -> Maybe Int


λ> elemIndex 'F' "FROG"
Just 0
λ> elemIndices 'E' "TEETER"
[1,2,4]

When looking for something like this it's often helpful to write down the type you want and then (if possible) generalize it. You can often find things in the standard library this way.

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