spy doesn't work - why?

Hello,

I am calling spy on a sparse matrix J, within a subfunction, but the result is not displayed when I run my main.

The matrix is first created with the command sparse (m, n) as a null matrix of dimensions m*n. Then I fill it in as needed with a for, line by line, by taking subsections of the line and assigning to that subsection a vector of ones.

What could I be doing wrong that makes it so that the result of spy isn't displayed?

Basic pseudocode (the actual indexes are more complex, they are for a Jacobian pattern):

J = sparse (m, n);
for i=1:m
    J(i, 5:9) = ones(5, 1);
end
spy(J)

[Walter Roberson]

By the way, I recomend that you use spalloc() instead of sparse() for that case. spalloc() permits you to indicate the number of non-zero entries you expect to have. That makes a difference because the sparse matrix must be rebuilt every time it gets "full"

[Walter Roberson]

The code you posted works for me on its own.

You mention sub-function, so I have to wonder if you are using a shared variable improperly?

[Sulaymon Eshkabilov]

It does work ok:

m=13; n=23;
J = sparse (m, n);
for i=1:m
    J(i, 5:9) = ones(5, 1);
end
spy(J) ; shg

[Clothilde Breger]

I don't know, the code was given to me for a uni project, I only filled in the matrix and used spy(J) according to what we saw in class (but it was already created using sparse, not spalloc, to avoid making the question too easy I suppose).

I'll ask my teacher about it, since it could be linked to how I use the rest of the code.

Thank you for your help :)