[Haskell-cafe] [repa] beginner questions

[Dmitry Malikov]

On 11/19/2012 01:21 AM, Dominic Steinitz wrote:
> Dmitry Malikov <malikov.d.y <at> gmail.com> 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
!