[Haskell-cafe] Re: hmatrix
Alberto Ruiz
aruiz at um.es
Mon Jun 2 05:37:14 EDT 2008
Anatoly Yakovenko wrote:
> do you have any plans to provide an interface for inplace updates?
Yes, I will try to write a simple version of Data.Array.ST...
> On Sun, Jun 1, 2008 at 3:20 AM, Alberto Ruiz <aruiz at um.es> wrote:
>> Anatoly Yakovenko wrote:
>>> What is the most efficient way to update a position in a matrix or a
>>> vector? I came up with this:
>>>
>>> updateVector :: Vector Double -> Int -> Double -> Vector Double
>>> updateVector vec pos val = vec `add` v2
>>> where
>>> v2 = fromList $ (replicate (pos) 0.0) ++ ((val - (vec @>
>>> pos)):(replicate ((dim vec)- pos - 1) 0.0))
>>>
>>> but this seems pretty inefficient to me.
>>>
>>> thanks,
>>> Anatoly
>>>
>> It is probably more efficient to use subVector and join (implemented by
>> copyArray):
>>
>> updateVector' v pos val
>> | pos == 0 = join [b,c]
>> | pos == dim v -1 = join [a,b]
>> | otherwise = join [a,b,c]
>> where a = subVector 0 pos v
>> b = fromList [val]
>> c = subVector (pos+1) (dim v -pos-1) v
>>
>>> updateVector' (fromList [1,2,3,4,5]) 2 57
>> 5 |> [1.0,2.0,57.0,4.0,5.0]
>>
>> (The three cases are required because empty vectors are not currently
>> allowed.)
>>
>> Something similar can be done for matrices using flatten and reshape.
>>
>> Although vectors and matrices in this library are immutable and intended to
>> be manipulated as a whole by higher level functions, this kind of update
>> functions may often be useful. I will include them soon.
>>
>> Alberto
>>
>
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