On Thu, Jul 19, 2007 at 11:39:26PM -0400, Ronald Guida wrote:
In Problem 11, a 20x20 grid of numbers is given, and the problem is to find the largest product of four numbers along a straight line in the grid. The line can be horizontal, vertical, or diagonal.
I found it easier to work with Arrays in this example:
grid :: [[Integer]] grid = readGrid gridText
gridArr :: [[Integer]] -> Array (Int, Int) Integer gridArr = array ((0, 0), (19, 19))
Then you can define a handy function for extracting whatever combination of indices you need:
extractBy :: (Ix i, Ord a) => ((i, e) -> a) -> Array i e -> [[e]] extractBy f = map (map snd) . groupBy (equals f) . sortBy (comparing f) . assocs
And from there on you can work your way out of this problem by replacing ??? with functions that map ((i, j), v) to some value common for same row, col, or diagonal:
rows = extractBy ??? cols = extractBy ??? diags = extractBy ??? adiags = extractBy ???
makeGroups :: Int -> [a] -> [[a]] makeGroups 0 _ = [] makeGroups n xs = let ys = take n xs in if n == length ys then ys : (makeGroups n $ tail xs) else []
The above can be shortened to:
makeGroupsOf n = map (take n) . tails
From here on you should be able to compute products of whatever is required. Good luck and have fun!
Regards, -- Krzysztof Kościuszkiewicz Skype: dr.vee, Gadu: 111851, Jabber: kokr@jabberpl.org Phone IRL: +353851383329, Phone PL: +48783303040 "Simplicity is the ultimate sophistication" -- Leonardo da Vinci