MATLAB: Sparse Matrix indexing

matrixmatrix manipulationsparse

I need to built a sparse matrix for large matrix in order to avoid entering out of memory. Can a sparse matrix be build like: n=792;m=1000;j=2; A=sparse((n+3)*j+1,(3*n+m-1)*j); %then filling up this zero matrix with ones at certain rows and %columns like A(1:n*j,1:n*j)=eye(n*j,n*j);

I know this will increase the execution time but this will save me lot memory right?. Is there any other method I can perform

Best Answer

Hi Samir,

more or less that's the way to go. Take a look at the function speye, spdiags and the others under "See also" that help build up your matrix (in your example "eye" is a full matrix, whereas you want only to set the diagonal).

Titus

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MATLAB: Generate matrix that is a product of its indices

n = 5;
i = (1:n)';
j = 1:n;
M = i.*j - (i+j)

MATLAB: Construct a large sparse matrix

Nowadays most functions are able to work on sparse matrices and output sparse matrices. If you want to be sure that it is working, use ISSPARSE to check on a small case study, e.g.

 >> A = sparse([2 0 3; 4 5 0; 0 0 6]) ;
 >> B = sparse([0 1 0; 2 0 2; 3 3 0]) ;
 >> C = kron(A, B) ;
 >> issparse(C)
 ans =
     1
 >> C = blkdiag(A, B) ;
 >> issparse(C)
 ans =
     1

There is no advantage in using SPARSE on a matrix that is already sparse.

Note that the situation where you convert a dense version of a large matrix to sparse can/should rarely happen. You generally have to build the matrix directly as a sparse matrix, using a call like

 S = sparse(I, J, V, m, n) ;

where I and J are vectors of respectively row and column indices, V is a vector of values, and m and n define the size.

One of the rare cases where you transform a dense matrix to sparse is when you build a small matrix that must be used as a basis for building a larger one, which seems to be your case.

Best Answer

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MATLAB: Generate matrix that is a product of its indices

MATLAB: Construct a large sparse matrix

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