# Haskell Quiz/Weird Numbers/Solution Dolio

### From HaskellWiki

< Haskell Quiz | Weird Numbers(Difference between revisions)

(Efficient solution) |
(sharpen cat) |
||

(One intermediate revision by one user not shown) | |||

Line 1: | Line 1: | ||

+ | [[Category:Haskell Quiz solutions|Weird Numbers]] |
||

+ | |||

A first, rather naive solution: |
A first, rather naive solution: |
||

## Latest revision as of 11:03, 13 January 2007

A first, rather naive solution:

module Main where import Control.Monad.Reader import Data.List import System power = filterM $ const [True,False] divisors n = sort $ 1 : join [ [x, n `div` x] | x <- l ] where l = filter ((==0) . (mod n)) . takeWhile ((<n) . join (*)) $ [2..] weird n = sum s > n && all (n/=) (map sum ss) where (s:ss) = power $ divisors n main = print . filter weird . enumFromThenTo 70 72 . read . head =<< getArgs

A much more efficient solution, implementing some of the techniques from the rubyquiz discussion:

module Main where import Control.Monad.Reader import Data.List import System power = filterM $ const [True,False] divisors n = nub $ 1 : l ++ map (div n) (reverse l) where l = filter ((==0) . (mod n)) . takeWhile ((<=n) . join (*)) $ [2..] weird n | a <= 0 = False | a `elem` d = False | sd == a = False | sd < a = True | otherwise = checkSubset a d where d = divisors n sd = sum d a = sd - n checkSubset a [] = True checkSubset a l@(x:xs) | a `seq` False = undefined -- strictness | a < 0 = True | a `elem` l = False | null xs = True | otherwise = checkSubset (a - x) xs && checkSubset a xs main = print . filter weird . enumFromThenTo 70 72 . read . head =<< getArgs