Extract part of a sparse matrix

Dear users,

I'm trying to find a way to extract a block from a big sparse matrix without being computationally costly. Here what I have

A - sparse matrix n*n i - a vector with the indices I want to extract (i = [5556 68424 85345 925325], for example)

I can do what I want like this:

A_new = A(i,i).

However, this indexing that is used in full matrices is not effective for sparse. how can I do this operation in an effective way?

Thank you in advance,

Pedro Montenegro University of Porto, Portugal

Answer 1

[Azzi Abdelmalek]

i = [5556 68424 85345 925325];
ii=sub2ind(size(A),i,i)
A_new = A(ii)

[Pedro Montenegro]

Hello Azzi,

Thank you for your answer. However, the result that I want is not the same. Lets do it with a simple full matrix for better understanding:

a=magic(5)

a =

    17    24     1     8    15
    23     5     7    14    16
     4     6    13    20    22
    10    12    19    21     3
    11    18    25     2     9

i=[2 3];

a(i,i) =

     5     7
     6    13

This is the answer that I want. If i run your code I'll have:

ii=sub2ind(size(a),i,i)

ii =

     7    13

a(ii) =

     5    13

Which is different from what i want.

Thank you

[Azzi Abdelmalek]

[ii,jj]=ndgrid(i,i)
idx=sub2ind(size(a),ii(:),jj(:))
a(idx)