On 11/19/2012 01:21 AM, Dominic Steinitz wrote:
Dmitry Malikov
writes: Playing around with repa arrays and got some questions.
1) How I can get list of indexes of array that suffice some predicate?
> a1 AUnboxed (Z :. 3) (fromList [False,False,True]) it :: Array U (Z :. Int) Bool
Indexes of element that satisfying specific predicate could be obtained like that:
> (\a p → Data.List.map (subtract 1 . snd) $ filter (p . fst) $ zip (toList a) [1..]) a1 (== False) [0,1]
Looks ugly. How REPA users used to do filtering like that without converting to list?
I hope someone will correct me if I am wrong and furthermore I was not entirely clear what you were trying to do but it seems to me that if you want to filter out an unknown number of elements from a collection then repa is the wrong abstraction to use.
You can however filter out a known number of elements e.g.
xs = Repa.fromListUnboxed (Z :. 3) [1, 2, 3]
removeOne ix xs = Repa.fromFunction (Z :. dx - 1) (\(Z :. jx) -> xs ! (Z :. f jx)) where Z :. dx = Repa.extent xs f jx | jx < ix = jx | otherwise = jx + 1
test = Repa.computeP $ removeOne 1 xs :: IO (Array U DIM1 Float)
Does that help?
Dominic.
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Does that help? Yep, kinda, this is a nice example of fromFunction usage, but my question was about retrieving indexing of elements that satisfy some predicate.
Thanks for response. -- Best regards, dmitry malikov !