How to edit sparse matrix entries inside mex file?

Hi all,

I have a large sparse matrix that I pass to a mex file I have written. At some stage in this mex file, I want to update some entries in this large sparse matrix. How do I access specific elements inside this sparse matrix and change them?

Thanks!

Siddharth

[SiddharthKrishnamoorthy]

To be a little bit more specific - I only want to edit the diagonal terms - so a method that doesn't involve looping through all the entries would be better...

[James Tursa]

Manipulating sparse matrices in a mex routine can be very tricky. Are the diagonal elements currently non-zero? Are you changing any non-zero elements to zero or vice-versa? If so, you will need to rewrite the entire matrix into a new area of memory (i.e., changes can't be done in-place). Let us know what exactly you are doing and then I can supply some example mex code to do it if it can be done in-place. If it can't be done in-place then the code will be much more involved.

[SiddharthKrishnamoorthy]

I'm certain that any elements that I will edit are already non-zero.

[James Tursa]

OK. I will work up a short example and post it later on.

[SiddharthKrishnamoorthy]

Thank you very much for your help! One way I was thinking of doing it (It's not exactly elegant), is to write a matlab function that changes the (i,j)th element of an array, given the array, and the indices. Then use mexCallMATLAB to call this function.

[James Tursa]

Note that mexCallMATLAB always returns a deep copy ... it cannot be used for any in-place calculations. So if your intention is to write something that only changes a non-zero value to a different non-zero value quickly in-place, you definitely do not want to use mexCallMATLAB.

[SiddharthKrishnamoorthy]

Yes, I found that out the hard way. Also the whole thing was really expensive (and wrong). I've now implemented a new way where I use the IR and JC matrices to arrive at the diagonals and edit the entry in the corresponding pr matrix.

Answer 1

[Steven Lord]

You will need to manipulate the ir, pr (and pi if the data is complex), and jc. See this documentation page and this other documentation page for a description of the jc and ir pointers and the fulltosparse example on this third documentation page for an example of a similar type of manipulation.

You're also going to want to be very careful that your modifications don't result in an invalid sparse matrix.

[SiddharthKrishnamoorthy]

Thank you, I will look into this. I'm fairly certain that my modifications don't alter the structure of the sparse matrix, only the non-zero entries.