Thread Subject: A problem about sparse matrix

I now have a matrix It's a matrix I want to do the following thing:
A is a sparse matrix.for example n by n
rowcolumindex is an vector which store some index of A.My code is like this:
A(rowcolumindex,:)=0;
A(rowcolumindex,rowcolumindex)=speye(length(rowcolumindex));%You can understand I want to set the rows rowcolumindex be a eye matrix.
But I find it's no efficiency.Because after the first row code that A(rowcolumindex,:) now become a full matrix,I think. Am I right?Any efficiency implementation

"zedong
" <zdongwu@gmail.com> wrote in message <gjidh6$e25$1@fred.mathworks.com>...
> I now have a matrix It's a matrix I want to do the following thing:
> A is a sparse matrix.for example n by n
> rowcolumindex is an vector which store some index of A.My code is like this:
> A(rowcolumindex,:)=0;
> A(rowcolumindex,rowcolumindex)=speye(length(rowcolumindex));%You can understand I want to set the rows rowcolumindex be a eye matrix.
> But I find it's no efficiency.Because after the first row code that A(rowcolumindex,:) now become a full matrix,I think. Am I right?Any efficiency implementation


I would prefer to do it this way:

complement=1:n;
complement(rowcolumnindex)=[];

Anew=speye(n);
Anew(:,complement)=A(complement,:).'; %filling columns of sparse matrices is faster.
Anew=Anew.';