How to visualize a sparse matrix in MATLAB?

Question

So I have this matrix here, and it is of size 13 x 8198. (I have called it 'blah').

This is a sparse matrix, in that, most of its entries are 0. When I do an imagesc(blah), I get the following image:

Clearly this is worthless because I cannot clearly see the non-zero elements. I have tried playing around with the color scaling, but to no avail.

Anyway, I was wondering if there might be a nicer way to be able to visualize this matrix in MATLAB somehow? I am designing an algorithm and would like to be able to see certain things int teh matrix.

Thanks!

Answer 1

Try spy; it's intended for exactly that.

The problem is that spy makes the axes equal, and your data is 13 x 8198, so the first axis is almost invisible compared to the second one. daspect can fix that.

>> spy(blah)
>> daspect([400 1 1])

---

spy doesn't have an option to plot differently by signs. One option would be to edit the source to add that capability (it's implemented in matlab, and you can get the source by running edit spy). An easier hack, though, is to just spy the positive and negative parts separately:

>> daspect([400 1 1]);
>> hold on;
>> spy(max(blah, 0), 'b');
>> spy(min(blah, 0), 'r');

This has the unfortunate side effect of making places where positives and negatives are close together appear dominated by the second one plotted, here the negatives (e.g. in the top rows of your matrix). I'm not sure what to do about that other than maybe fiddling with marker sizes. You could of course do it in both orders and compare.