Replace elements of sparse matrix from a full matrix.

Dear Experts

I have a huge matrix, M with zeros. I am converting M into sparse using sparse(M). After, I want to replace few positions (idx) of M from other matrix, N. I am using the following: M(idx) = N(idx). But, this is very very slow. Also MATLAB warns: This sparse indexing expression is slow. How I can do the above task?

Thanks in advance.

[Stephen23]

What size is M?

[KSSV]

[19647672 60] is the size at present am dealing with. It may be more in future.

Answer 1

[Jos (10584)]

What don't you replace the elements before converting it?

[KSSV]

No...I want to replace in sparse matrix. Actually I am using svds for M. M is huge.

[Jos (10584)]

I don't get it. You do start off with a full matrix M which you convert using sparse, don't you?

[KSSV]

Yes..I am starting with a full matrix which has lot of zero's. Then converting it to sparse for svds. And after I have to replace certain zeros of sparse with the result obtained from svds.

[Walter Roberson]

If you know you are replacing zeros, then you can sparse() up the values that are going in as a second sparse matrix, and add that to the first sparse matrix.

[KSSV]

Hi Walter..happy to see you. Yes I tried it, but replacing the values is taking hell lot of time.

[Walter Roberson]

When you create your original sparse matrix, be sure to allow extra nzmax; see http://www.mathworks.com/help/matlab/ref/spalloc.html

[KSSV]

Actually my original matrix has zeros and I want to fill these zeros with numbers later...

[Walter Roberson]

Then spalloc() would be appropriate. If you create a matrix without using it or equivalent then you end up doing a lot of sparse reallocations as you add in new values.