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Dmitry Malikov | 11 Nov 2012 00:01
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[repa] beginner questions

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?

2) How can I apply some function `f' to each row of 2D array `a' and 
collect results in single value?

     f ∷ (Shape sh, Source r Bool) ⇒ Array r sh Bool → Bool
     f a = (== toList a) $
           foldl1 (Prelude.zipWith (||)) $
           Prelude.map toList $
           foldl (\l k -> filter (\x -> x ! (Z :. k) == False) l) 
[b1,b2,b3,b4] $
           findWhich (== False) a

     and ∷ [Bool] → Bool
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Dominic Steinitz | 18 Nov 2012 22:21
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Re: [repa] beginner questions

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]
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Dmitry Malikov | 19 Nov 2012 21:47
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Re: [repa] beginner questions

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]
>
(Continue reading)

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