Starting off in haskell

chris-kun · 2011-01-23 21:30:08

So I've been learning haskell this past week, and I thought it might be a good idea to get some feedback from guys who know what they're doin' . I just finished euler #4 here, so if anyone can see anything that could be implemented in a better way, stylistic changes, etc. or maybe my code is just completely unreadable and I should work on that first xD.

But yeah, much appreciated

Ramses de Norre · 2011-01-23 21:47:53

I didn't try your code and it might be more efficient, but I solved it with a lot less code:

main = 
  print $ maximum $ filter isPalindrome' [ x*y | x<-[1..999], y<-[1..999]]

isPalindrome' :: (Integral a) => a -> Bool
isPalindrome' = isPalindrome . digitsRev

isPalindrome :: (Eq a) => [a] -> Bool
isPalindrome x = x == reverse x

digitsRev :: (Integral a) => a -> [a]
digitsRev 0 = []
digitsRev n = remainder : digitsRev quotient
  where (quotient, remainder) = quotRem n 10

This was a while ago though, and I made it pretty quick and didn't bother optimizing as it runs in less than a second

chris-kun · 2011-01-23 22:00:47

I tried a similar solution to yours (although it was in lua) where I just ran through every possibility and just took the largest palindrome, but it took at least a minute to run, so I had to figure out a way to list all the products in order, and return the first one found.

actually your code above runs for a minute and produces a stack overflow for me. maybe my machine just sucks balls, lol.

Last edited by chris-kun (2011-01-23 22:04:44)

Ramses de Norre · 2011-01-23 22:04:17

chris-kun wrote:

I tried a similar solution to yours (although it was in lua) where I just ran through every possibility and just took the largest palindrome, but it took at least a minute to run, so I had to figure out a way to list all the products in order, and return the first one found.
actually your code above runs for a minute and produces a stack overflow for me. maybe my machine just sucks balls, lol.

Really? Did you compile it with -O2 ? It runs in about 0.40 seconds on my core 2 duo at 1.8GHz, not such an exceptionally powerful machine I'd think...

chris-kun · 2011-01-23 22:09:16

ah you're right compiled it runs in 2.3 secs for me, although mine still runs in 0.043

Ramses de Norre · 2011-01-23 23:09:03

Yeah your solution is probably way more efficient
I just tried the brute force method first and it was fast enough

dagle · 2011-01-29 20:56:55

nvm, speedbump in my brain...

Last edited by dagle (2011-01-29 21:05:46)

Daenyth · 2011-02-16 19:49:04

I rather like my approach: https://github.com/Daenyth/Project-Eule … /euler4.hs

chris-kun · 2011-02-16 19:52:37

yeah I get that you can bruteforce it.. I was trying to add in the functionality of ordering all of the products from largest to smallest, though

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#1 2011-01-23 21:30:08

Starting off in haskell

#2 2011-01-23 21:47:53

Re: Starting off in haskell

#3 2011-01-23 22:00:47

Re: Starting off in haskell

#4 2011-01-23 22:04:17

Re: Starting off in haskell

#5 2011-01-23 22:09:16

Re: Starting off in haskell

#6 2011-01-23 23:09:03

Re: Starting off in haskell

#7 2011-01-29 20:56:55

Re: Starting off in haskell

#8 2011-02-16 19:49:04

Re: Starting off in haskell

#9 2011-02-16 19:52:37

Re: Starting off in haskell

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