如何计算数字列表中的偶数和奇数 [英] How to count even and odd numbers in a list of numbers

问题描述

我想编写一个函数，告诉我列表中偶数和奇数的出现次数。例如，如果我的列表是[1,2,3,4,5]，我希望它返回（2,3）。

这就是我目前的情况

  counteven :: [整数]  - >整数
 counterteven [] = 0 
 counterteven（x：xs）
 |（x`div` 2）== 0 = 1 + counterteven xs 
 | otherwise = counteven xs 
 
 countodd :: [Integer]  - >整数
 countodd [] = 0 
 countodd（x：xs）
 |（x`div` 2）/ = 0 = 1 + countodd xs 
 |否则= countodd xs 
 
 oddsAndEvens :: [Integer]  - > （整数，整数）
 oddsAndEvens（x：xs）=（（counteven（x：xs）），（countodd（x：xs）））
  

它在我的文本编辑器上运行，但是我得到了奇怪的结果。

有人可以帮我解决问题吗？

解决方案

您有两个问题。

问题1： div vs mod 。

div 将数字分开。 mod 取其余数。例如， div 6 2 = 3 和 mod 6 2 = 0 。

您错误地使用 div 。这应该是 mod ，所以例如你的第一个函数应该是：

  counteven :: [Integer]  - >整数
 counterteven [] = 0 
 counterteven（x：xs）
 | x`mod` 2 == 0 = 1 + counterteven xs  - 注意：`mod`不是`div`！ 
 |否则= counteven xs 
  

我相信你可以修正 countodd


问题二：过度限制性模式匹配

您的最终功能也有问题：

  oddsAndEvens :: [Integer]  - > （整数，整数）
 oddsAndEvens（x：xs）=（counteven（x：xs），countodd（x：xs））
  

（x：xs）只有模式匹配非空列表，所以 oddsAndEvens [] 会产生错误，因为您尚未定义其行为。

  oddsAndEvens :: [Integer]  - > （Integer，Integer）
 oddsAndEvens xs =（counteven xs，countodd xs）
  

由于 xs 匹配任何东西，函数现在可以用于空列和非空列表。

I want to write a function that will tell me the number of even and odd number occurences within a list. For example, if my list was [1,2,3,4,5], I want it to return (2,3).

This is what I have so far

counteven :: [Integer] -> Integer
counteven [] = 0
counteven (x:xs) 
     |(x `div` 2) == 0 = 1 + counteven xs
     |otherwise = counteven xs

countodd :: [Integer] -> Integer
countodd [] = 0
countodd (x:xs) 
     |(x `div` 2) /= 0 = 1 + countodd xs
     |otherwise = countodd xs

oddsAndEvens :: [Integer] -> (Integer,Integer)
oddsAndEvens (x:xs) = ((counteven(x:xs)),(countodd(x:xs)))

It does run on my text editor but I am getting strange results.
Can someone please help me what the problem might be?

解决方案

You have two problems.

Problem 1: div vs mod.

div divides numbers. mod takes their remainder. For example, div 6 2 = 3, and mod 6 2 = 0.

You incorrectly use div. That should be mod, so for instance your first function should be:

counteven :: [Integer] -> Integer
counteven [] = 0
counteven (x:xs)
  | x `mod` 2 == 0   = 1 + counteven xs -- NOTE: `mod` not `div`!
  | otherwise        = counteven xs

I'm sure you can fix countodd on your own given this example.

Problem two: over-restrictive pattern matching.

Your final function has an issue too:

oddsAndEvens :: [Integer] -> (Integer, Integer)
oddsAndEvens (x:xs) = (counteven (x:xs), countodd (x:xs))

The argument (x:xs) only pattern matches on non-empty lists, so oddsAndEvens [] will produce an error, since you haven't defined its behaviour.

It should be:

oddsAndEvens :: [Integer] -> (Integer, Integer)
oddsAndEvens xs = (counteven xs, countodd xs)

Since xs matches anything, the function will now work for empty and nonempty lists.

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