On Thu, 2008-10-02 at 05:45 +0100, Matthew Williams wrote:
Hi Guys,
I'm new to Haskell and I was wondering if you can help me:
One of the first program's I tend to write when I'm looking at a new language is a program to generate a list of perfect numbers:
--My First Perfect Number Generator factors :: Integer -> [Integer] factors x = [z | z <- [1..x-1], x `mod` z == 0]
Hi Matthew, A big optimization for larger numbers is that you only need to go up to the square root of x here and add both z and x/z to the list. (Where x is a perfect square you need to avoid adding the root twice.) It's late and there is probably a better way to do this, but: import List semi_factors :: Int -> [Int] semi_factors x = takeWhile (\n -> n * n <= x) [z | z <- [2..x-1], x `mod` z == 0] factors n = let xs = semi_factors n in nub (1 : (xs ++ (map (n `div`) xs)))
is_perfect :: Integer -> Bool is_perfect x = if sum(factors x) == x then True else False
"if <something> then True else False" should ring alarm bells! Immediately replace with simply "<something>": is_perfect x = sum (factors x) == x you could also use is_perfect x = foldl' (+) 0 (factors x) == x (strict foldl from Data.List)
do_perfect :: [Integer] -> [Integer] do_perfect x = [z |z <- x, is_perfect z ]
Then to run it:
do_perfect [1..9000]
I think more idiomatic would be: do_perfect x = filter is_perfect [2..x] All this speeds it up a bit. But I can't think any more - time to sleep. thanks Ben
I'm using GHC to run it. My problem / question is this: It's running
quite a lot slower than equivalent programs in erlang and python. I suspect it's down to the way I've written it. Any thoughts (or comments in general)
Many thanks
Matt