Question

How to create a 2d sparse matrix in a MEX-file written in C. After creating the matrix how to access the elements individually like in C , say mat[i][j]?

I tired using mxCreateNumericArray function but I wasn’t able to access the elements and also make it as a sparse matrix.

Please help

Answer 1

See this page on mxCreateSparse. Then you’ll want to look at mxSetPr, mxSetIr and mxSetJc and the corresponding “get” versions.

Here’s an example of how to allocate a sparse matrix. I realize this is an old link, but to the best of my knowledge, it hasn’t changed.

Basically, how it works is that the ir data contains the row indices. The jr data contains a list of indices into the ir array. For instance, in the link on how to allocate a sparse matrix, the code:

...
static double  static_pr_data[NZMAX] = {5.8, 6.2, 5.9, 6.1};
static int     static_ir_data[NZMAX] = {0, 2, 1, 3};
static int     static_jc_data[COLS+1] = {0, 2, 4};
...

the array static_jc_data tells you that indices static_jc_data[c] through static_jc_data[c+1]-1 of static_pr_data and static_ir_data correspond to the column c of the matrix. Within that range (static_jc_data[c] to static_jc_data[c+1]-1) the entries of static_pr_data gives you the values in the matrix and static_ir_data gives you the correct rows.

For example, the matrix here would be:

A = [ 5.8  0
      0    5.9
      6.2  0
      0    6.1];

To answer your questions about how to access elements individually, you have to search for whether the i,jth element exists and if it does return it, otherwise return 0. To do this, you’d search from static_ir_data[static_jc_data[j]] through static_ir_data[static_jc_data[j+1]-1] to see whether your i exists. If it does, then the corresponding entry in static_pr_data will contain your entry. If it doesn’t, then return 0.

However, typically with sparse matrix usage, if you’re doing a lot of searching through the matrix to see if a certain element exists, you may want to think about how you’re using it. Typically, it’s much better to perform whatever operation you’re doing by only going through the non-zero elements once instead of searching for each i,jth entry.

Oh, and one last thing. Keep in mind that in the MEX code, all your indices are 0 based, but they are 1 based in MATLAB. That should add to the fun.