Haskell中的项目euler问题3 [英] Project euler problem 3 in haskell

问题描述

我是Haskell的新手，尝试从 http://projecteuler.net/ 解决3个问题。

  13195的主要因素是5,7,13和29. 
 
什么是最大的素数数字的因子是600851475143？ 
  

我的解决方案：

  import Data.List 
 
 getD :: Int  - > Int 
 getD x = 
  - 找到deviders 
 let deriveList = filter（\y  - >（x`mod` y）== 0）[1 .. x] 
 filteredList =过滤器isSimpleNumber派生列表
中的最大过滤列表
 
  - 支票是简单的
 isSimpleNumber :: Int  - > Bool 
 isSimpleNumber x = let deriveList = map（\y->（x`mod` y））[1..x] 
 filterLength = length（filter（\ z  - > z == 0）deriveList）
 in 
 case 
 2的filterLength  - > True 
 _  - >假
  

我尝试运行例如：

  getD 13195 
> 29 
  

但是当我尝试时：

  getD 600851475143 
  

我得到错误Exception：Prelude.maximum：empty list为什么？

谢谢@Barry Brown，我认为我必须使用：

  getD :: Integer  - >整数
  

但是我得到错误：

 无法匹配实际类型为'Integer'的预期类型'Int'
期望类型：[Int] 
实际类型：[Integer] 
在filter'的第二个参数，即`deriveList'
在表达式中：filter isSimpleNumber deriveList 
  

Thank you。

解决方案

您的类型签名将整数值限制为大约2 ^ 29。尝试将> Int 更改为整数。

编辑：

我看到你已经意识到你需要使用Integer而不是Int。你需要改变getD和isSimpleNumber的类型，否则你会得到一个类型不匹配。

另外一般情况下，如果您遇到类型问题，只需删除类型声明并让Haskell告诉你正确的类型。

  Main> ：t getD 
 getD :: Integral a => a  - > a 
 
 Main> ：t isSimpleNumber 
 isSimpleNumber :: Integral a => a  - > Bool 
  

I'm new in Haskell and try to solve 3 problem from http://projecteuler.net/.

The prime factors of 13195 are 5, 7, 13 and 29.

What is the largest prime factor of the number 600851475143 ?

My solution:

import Data.List

getD :: Int -> Int
getD x = 
  -- find deviders
  let deriveList = filter (\y -> (x `mod` y) == 0) [1 .. x]
      filteredList = filter isSimpleNumber deriveList
  in maximum filteredList

-- Check is nmber simple
isSimpleNumber :: Int -> Bool
isSimpleNumber x = let deriveList = map (\y -> (x `mod` y)) [1 .. x]
                       filterLength = length ( filter (\z -> z == 0) deriveList)
                       in 
                          case filterLength of
                            2 -> True
                            _ -> False

I try to run for example:

getD 13195
> 29

But when i try:

getD 600851475143

I get error Exception: Prelude.maximum: empty list Why?

Thank you @Barry Brown, I think i must use:

getD :: Integer -> Integer

But i get error:

Couldn't match expected type `Int' with actual type `Integer'
Expected type: [Int]
  Actual type: [Integer]
In the second argument of `filter', namely `deriveList'
In the expression: filter isSimpleNumber deriveList

Thank you.

解决方案

Your type signature limits the integer values to about 2^29. Try changing Int to Integer.

Edit:

I see that you already realised that you need to use Integer instead of Int. You need to change the types of both getD and isSimpleNumber otherwise you will get a type mismatch.

Also in general, if you are having trouble with types, simply remove the type declarations and let Haskell tell you the correct types.

Main> :t getD
getD :: Integral a => a -> a

Main> :t isSimpleNumber
isSimpleNumber :: Integral a => a -> Bool

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