group collects equal elements in sublists. Thus, unique can be implemented as: unique = map head . group i.e. taking the first of each group of equal elements.
(Yet) another way of doing this, is a modified quicksort: qsort [] = [] qsort (x:xs) = qsort (filter (<x) xs) ++ [x] ++ qsort (filter (>x) xs) At each step, this will ignore all elements equal to the pivot value. On 18/05/07, Dan Weston <[EMAIL PROTECTED]> wrote:
OK, I looked up "group" and didn't see any Ord constraint analog. I give up. What is the common idiom? I was stupidly using nub even though I already have an Ord constraint. I have rewritten it to: unique :: Ord a => [a] -> [a] unique (x:y:z) = (if x < y then (x:) else id) (unique (y:z)) unique xyz = xyz uniqueSort = unique . sort but I would much rather use the "common idiom" than this dreck. Help a poor guy out? :) Dan Paul Johnson wrote: > Andrew Coppin wrote: >> It occurs to me that if you want a sorted list of only unique >> elements, it would seem (to me) to be efficient to do the sorting and >> the uniquing at the same time. Does any library function do this? (I >> imagine it wouldn't be hard to write it yourself...) > Yes, although it only works on instances of Ord then, because of the > sorting. > > Its actually quite a common idiom, and worth figuring out for yourself. > Hint: look at "group". > > Paul. > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe@haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > > _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
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