Re: RE: Filling sparse matrix by A(i, i)=... .Fast for small matrices, slow for large.Is there a better way to fill the matrix?

Hi there,
try using previously allocated memory.

Everytime you add some value probably memory is
allocate one more time  
( .. how about std::vector<T>.reserve() mechanism 
dear sirs )


Beautifull exampe of it is in MTL's 
online example file.

Go there :

http://www.osl.iu.edu/research/mtl/show_src.php3?file=./examples/sparse_matrix.cc

Cheers.
JO


 --- Guilherme Parreira <[email protected]> wrote:

> The MTL documentation says to use iterators in order
> to speed up computation
> in matrices, but I don't know how.
> 
> Regards, 
> ==============================
> Eng. Guilherme F. Parreira
> Electrical Engineering Graduate Student
> GOPAC - UFMG - Brazil
> ==============================
> -----Original Message-----
> From: [email protected]
> [mailto:[email protected]] On
> Behalf Of Tony
> Sent: domingo, 24 de abril de 2005 10:23
> To: [email protected]
> Subject: MTL: Filling sparse matrix by A(i, i)=...
> .Fast for small
> matrices,slow for large.Is there a better way to
> fill the matrix?
> 
> Hi
> 
> I have a sparse matrix
> 
> #define N   10000
> 
> typedef matrix< Type, 
>   rectangle<>, 
>   compressed<>,
>   row_major >::type Matrix;
> 
> Matrix A(N,N);
> 
> and I am filling it up something like
> 
> for(i=0; i < N ; i++){
>     A(i,i) = ....
>        lots of other stuff in here setting up other
> elements of A 
>     
> }
> 
> In my case I have quite a complex set of if
> statements in the loop
> setting up the A matrix and I have a maximum of 12
> non zero entries on
> each row.
> 
> The code works fine for small matrix sizes, say N
> less than 50,000. 
> But when I set N larger than this the code runs very
> slowly.  In my
> case I would like to run the code with N=1500000. 
> But I need to leave
> the computer on for several days for A to be
> constructed.
> 
> Question:
> Can I fill the elements of A in a faster way?  I am
> wondering if if
> would be faster adding a whole row at a time to A,
> but I don't know
> how to do this, if this is a possibility can someone
> please show me
> how to do this and/or any other ways I can speed up
> filling up the
> elements of A
> 
> Thank you for your time
> 
> Tony
> 
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> 
> 
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