[Haskell-cafe] Re: hmatrix

[Alberto Ruiz]

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