映射功能应用于列表的特定元素 [英] Map function applied to specific elements of the list

问题描述

我有一个功能:

mapAtOriginal :: (a -> a) -> [Int] -> [a] -> [a]
mapAtOriginal f is xs = helper 0 is xs
  where
    helper _ [] xs = xs
    helper c (i:is) (x:xs)
      | c < i     =   x : helper (c+1) (i:is) xs
      | otherwise = f x : helper (c+1)    is  xs

它是这样的:

mapAtOriginal (*2) [0,3] [1,2,3,4]   -- == [2,2,3,8]

所以我想重写它，但是要使用 map 函数.我了解 map 适用于列表中的每个元素，但是，我只需要将其应用于特定索引.

So I want to rewrite it but using a map function. I understand that map applies to every element of the list, however, I need it applied only for specific indices.

我该怎么办?

推荐答案

地图不知道它在列表中的位置".因此，您首先需要将该信息编码为元素本身.可以使用 zip [0 ..] 完成，它基本上用每个元素的出现位置对其进行注释.

map doesn't know "where in the list" it is. So you first need to encode that information into the elements themselves. This can be done with zip [0..], which basically annotates each element with position at which it occurs.

然后，在您的 map 函数中，您只需要在注释元组上进行模式匹配，并使用 if 决定是否应用其他元组元素的操纵器功能.

Then, in the function you map, you just need to pattern-match on the annotation-tuple, and use an if to decide whether or not to apply the manipulator function to the other tuple element.

请注意， zip 和 map 的组合始终等同于单次通过

Note that the combination of zip and map is always equivalent to a single pass zipWith, so that's what you should preferrably use.

这篇关于映射功能应用于列表的特定元素的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！