递归函数的空列表 [英] empty list on recursive function

问题描述

我试着在上寻求一些乐趣，但我现在被困在一个问题上，想要继续前进但似乎不能让我的功能工作。我试图计算给定整数的主因子。该功能适用​​于较小的数字，如13195：

 > primeFactor 13195 
 [29,13,7,5] 
  

但是当我运行更大号码，例如600851475143：

 > primeFactor 601851475143 
 [] 
  

这对我来说似乎很奇怪。我知道haskell是一种懒惰的语言，但我不认为它应该是懒惰的...

  primeFactor':: Int  - > [Int] 
 primeFactor'n = [p | p 其中素数':: [Int]  - > [Int]  - > [Int] 
 primes'ys [] = ys 
 primes'ys（x：xs）| x`notMultipleOf` ys = primes'（x：ys）xs 
 |否则= primes'ys xs 
 
  - 辅助函数到primeFactor'
 notMultipleOf :: Int  - > [Int]  - > Bool 
 notMultipleOf n [] = True 
 notMultipleOf n xs = and [n`mod` x / = 0 | x < -  xs] 
  

解决方案

Int 有32位，您不能存储该数字（使用 Integer ）。

<另一方面，您可以使用 Data.Numbers.Primes （和查看代码）：

 > primeFactors 601851475143 
 [3,3,23,1009,2881561] 
> primeFactors 600851475143 
 [71,839,1471,6857] 
  

Im trying do some work on Project Euler for fun, but I got stuck now on an problem and want to move on but cant seem to get my function working. Im trying to get count the primefactors of a given integer. The function works on smaller numbers such as 13195:

> primeFactor 13195
[29,13,7,5]

But when I run a bigger number such as 600851475143:

> primeFactor 601851475143
[] 

This seems really weird to me. I know haskell is a lazy language, but I don´t think it should be that lazy...

primeFactor' :: Int -> [Int]
primeFactor' n = [ p | p <- primes' [] [2 .. n], n `mod` p == 0 ]
   where primes' :: [Int] -> [Int] -> [Int]
         primes' ys [] = ys
         primes' ys (x:xs) | x `notMultipleOf` ys = primes' (x:ys) xs
                           | otherwise            = primes' ys xs                                                                                         

-- helper function to primeFactor'
notMultipleOf :: Int -> [Int] -> Bool
notMultipleOf n [] = True
notMultipleOf n xs = and [n `mod` x /= 0 | x <- xs]

解决方案

Int has 32 bits you can't store that number (use Integer).

On the other hand, you can use Data.Numbers.Primes (and see code):

> primeFactors 601851475143
[3,3,23,1009,2881561]
> primeFactors 600851475143
[71,839,1471,6857]

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