[Haskell-begin] Maximum of a List?
Steve Klabnik
steve.klabnik at gmail.com
Sat Jul 26 17:54:57 EDT 2008
Hello everyone-
I'm working on some project Euler problems today, and I'm stuck on one. It's
not the problem itself that's the problem, it's that finding the maximum of
a list makes me run out of heap space!
http://projecteuler.net/index.php?section=problems&id=14
my code:
import Data.List
f :: Int -> Int -> Int
f acc x
| x == 1 = acc
| even x = f (acc + 1) (x `div` 2)
| otherwise = f (acc + 1) (3 * x + 1)
answer = (foldl' max 0) $ map (f 1) [1 .. 999999]
I tried using 'foldl' max ' instead of 'maximum' because I thought that
foldl' was supposed to work better than foldl or something...I could be
confused on that point. Anyway, here's what I get...
/ _ \ /\ /\/ __(_)
/ /_\// /_/ / / | | GHC Interactive, version 6.4, for Haskell 98.
/ /_\\/ __ / /___| | http://www.haskell.org/ghc/
\____/\/ /_/\____/|_| Type :? for help.
Loading package base-1.0 ... linking ... done.
Prelude> :l ..\test.hs
Compiling Main ( ..\test.hs, interpreted )
Ok, modules loaded: Main.
*Main> answer
GHC's heap exhausted: current limit is 268435456 bytes;
Use the `-M<size>' option to increase the total heap size.
I also tried writing a simple 'max' function that I thought was tail
recursive, and that that would help. Same error. I'm finding it hard to
believe that finding a maximum of a list of a million small integers would
cause this kind of overflow...is it really that big of a problem?
Any help would be appreciated. I have a feeling I'll run into this problem
again in the future.
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