Am Donnerstag 18 März 2010 05:03:28 schrieb Alexander Solla:
On Mar 17, 2010, at 8:33 PM, zaxis wrote:
`allPairs list = [(x,y) | x <- list, y <- list] ` is not what `combination` does !
let allPairs list = [(x,y) | x <- list, y <- list] allPairs [1,2,3]
[(1,1),(1,2),(1,3),(2,1),(2,2),(2,3),(3,1),(3,2),(3,3)]
Yeah, I know that. I said so specifically. combination computes the power set of a list. You said your goal was to compute a set of "two groups". You don't need the power set in order to compute a set of pairs. Moreover, computing the power set is a slow operation. Indeed, it is the source of your slowness.
I think you've been fooled by the names. If you look at the code (or run it on a smaller sample), you'll see that allTwoGroup computes the list of all partitions of sample into two complementary sets. If sample were [1,2,3], the result would be (apart from the order) [([1,2,3],[]),([1,2],[3]),([1,3],[2]),([2,3],[1]),([1],[2,3]),([2],[1,3]), ([3],[1,2]),([],[1,2,3])] with sample = [1 .. 100], it's a list of 2^100 elements, that'll take a long time to compute no matter how.