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

_______________________________________________
This list is archived at http://www.osl.iu.edu/MailArchives/mtl-devel/