Am 28.09.12 03:22, schrieb Darren Grant:
Hi all,
I'm working on project euler problem 14, whose solution is the maximum Collatz chain length for all natural numbers less than 1-million. [...]
So I wrote the following instead: [...] collatz :: (Int, CollatzSubMap) -> ([Int], CollatzSubMap) [...]
result = maximum [length $ fst $ collatz (x, M.empty) | x <- [1..999999] :: [Int]]
Where I'm currently stumped is in feeding the resulting map from one call to collatz into the next iteration in the list comprehension; that M.empty should carry the end result of previous iterations.
Whenever you want to carry something along while walking through a list, you can use a fold: cmax :: ([Int], CollatzSubMap) -> Int -> ([Int], CollatzSubMap) cmax (xs, m) y = (if length ys > length xs then ys else xs, m') where (ys, m') = collatz (y, m) *Main> length $ fst $ foldl cmax ([], M.empty) [1..1000] 179 But I think I wouldn't do it this way. There is no need to keep all those lists when you are only interested in their lengths. Also I have a strong feeling a monad might be a good way to carry along the state while focusing the code on the actual computation, but I still don't fully understand those. Harald