Problem with sparse matrix - HELP

Hello all,

Supose that H = sparse(i,j,s),

where i,j,s is an arrays of values with index of row, index of columns and the value of each position.

Then, i will get the sparse matrix.

But my question is: how can i get the vectors i,j,s from an sparse matrix, or even from the correspondent full matrix !

Thanks in advance,

Answer 1

[Cedric]

 [i,j,s] = find( H ) ;

Note that most often, when building a sparse matrix with vectors of indices/values, it is relevant to specify the size of the matrix:

 H = sparse(i, j, s, m, n) ;

otherwise you might get a matrix which doesn't have the appropriate dimension for further computations.

[rodrigo]

Thanks Cedric.

By the way, how can i maintain the zeros of my matrix in vector s and the correspondent indexes in vectors i and j ?

[Cedric]

You're welcome.

What is the purpose of maintaining zeros? This is precisely what we are trying to avoid when using sparse matrices. Could you describe a bit in which context you need these zeros? If you want to have a vector with all elements of the matrix, just use linear indexing or reshape, e.g.

 >> H = randi( 10, 2, 3 )
 H =
     9     2     7
    10    10     1

 >> s = H(:)
 s =
     9
    10
     2
    10
     7
     1

Note that if s contains all values, you don't need indices. The only thing that you need is to know the initial size (or even just the number of rows or columns and not both) of the matrix (and understand that MATLAB stores numeric arrays column first); using the size and s, you can reshape s into the original matrix:

 >> H2 = reshape( H, 2, [] )
 H2 =
     9     2     7
    10    10     1