Haskell从列表中过滤3个倍数到子列表 [英] Haskell Filter Multiples of 3 from a List to a Sublist

问题描述

我仍然试图掌握Haskell和Functional Programming的工作方式，我需要帮助理解为什么我的函数不工作。我正在尝试创建一个函数，它将整数列表作为参数，并从第一个列表中过滤/返回一个包含3的倍数的子列表。这里是我的代码：

 模块Main其中
 
 sublist = [] 
 
 myFunc :: [Int]  - > [Int] 
 
 myFunc [] = [] 
 
 myFunc [t] = do 
 if t`mod` 3 == 0 
 then t ：sublist 
 else myFunc [] 
 
 myFunc（h：t）= do 
 if h` mod` 3 / = 0 
 then myFunc t 
否则做
h：子列表
 myFunc t 
  

这只返回一个包含传递给函数的最后一个值，仍然是sublist = []。感谢您的任何建议，您可以提前给我。

解决方案

我想你需要先转换到功能风格。 / p>

例如，从列表中获取偶数

pre $ code> >过滤即使[1..10]
[2,4,6,8,10]

<如果没有使用现有的函数，您可以实现相同的功能。

  filter'::（a  - > Bool） - > ; [a]  - > [a] 
 filter'_ [] = [] 
 filter'condition（x：xs）= if condition x 
 then x：filter'condition xs 
 else filter'condition xs 
 
 divisibleBy3 n = mod n 3 == 0 
  

现在，您程序可以写成

  filter'divisibleBy3 inputList 
  

I am still trying to grasp the way Haskell and Functional Programming works, and I need help understanding why my function is not working. I am trying to create a function that takes a list of integers as a parameter and filters out/returns a sublist which contains any multiples of 3 from the first list. Here is my code:

module Main where

sublist = []

myFunc :: [Int] -> [Int]

myFunc [] = []

myFunc [t] = do
    if t `mod` 3 == 0
        then t : sublist
    else myFunc []

myFunc (h:t) = do
    if h `mod` 3 /= 0
        then myFunc t
        else do
            h : sublist
            myFunc t

This only returns a list containing the last value passed to the function, and still sublist = []. Thanks for any advice you can give me in advance.

解决方案

I think you need to first switch over mentally to functional style.

for example, this is to get even numbers from a list

> filter even [1..10]
[2,4,6,8,10]

without using the existing functions you can implement the same functionality

filter' :: (a -> Bool) -> [a] -> [a]
filter' _ [] = []
filter' condition (x:xs) = if condition x
                              then x : filter' condition xs
                              else     filter' condition xs

divisibleBy3 n = mod n 3 == 0

now, your program can be written as

filter' divisibleBy3 inputList   

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