On Thursday 10 April 2008 01:20:49 pm Matt Amos wrote:
I'm trying to break out of my imperative mind-set by learning Haskell in small snippets. After a few successes I've hit a bit of a roadblock with one of the classic dynamic programming problems, the longest increasing subsequence:
http://en.wikipedia.org/wiki/Longest_increasing_subsequence
The most efficient algorithm relies on destructive updates, so a simple translation doesn't seem possible. I've been trying to think of a way of removing the need for destructive updates while keeping the efficiency, but so far without much luck. Ideally, I'd like to avoid threading the whole thing with a state monad, since then it would just be an imperative algorithm in disguise.
Any suggestions would be very gratefully appreciated.
Memorization is a nice way to implement dynamic programming algorithms in Haskell. Basically, you arrange it so that the underlying runtime does the destructive updates for you. http://www.haskell.org/haskellwiki/Memoization
Cheers,
Matt