On Sat, May 17, 2008 at 8:19 PM, Jeroen
Hi, I know there's been quite some performance/optimization post lately, so I hope there still room for one more. While solving a ProjectEuler problem (27), I saw a performance issue I cannot explain. I narrowed it down to the following code (never mind that 'primes' is just [1..], the problem is the same or worse with real primes):
primes :: [Int] primes = [1..]
isPrime :: Int -> Bool isPrime x = isPrime' x primes where isPrime' x (p:ps) | x == p = True | x > p = isPrime' x ps | otherwise = False
main = print $ length (filter (== True) (map isPrime [1..5000]))
$ time ./experiment1 5000
real 0m4.037s user 0m3.378s sys 0m0.060s
All good, but if I change isPrime to the simpeler
isPrime x = elem x (takeWhile (<= x) primes)
it takes twice as long:
time ./experiment2 5000
real 0m7.837s user 0m6.532s sys 0m0.141s
With real primes, it even takes 10 times as long. I tried looking at the output of ghc -ddump-simpl, as suggested in a previous post, but it's a bit over my head (newby here...).
Any suggestions?
Just a thought: in your first approach you compare any element of the list once. In second---twice: once to check if <= x and second time to check if it is equal to x. That's a hypothesis, but another implementation of isPrime: isPrime x = (== x) $ head $ dropWhile (< x) primes seems to behave closer to your first version than to the second. Note that that does unnecessary comparison as well the find first element
= x.
yours, anton.