[Matlab Grader] How to check if the matrix has upper triangular form?

Hi,

I want to build a test that checks if a matrix has an upper triangular form. But somehow it didn't work in Matlab Grader.

I tried to use the matlab function istriu

A = [1.6537    1.1405    1.2570;
     0.0000    0.9661    0.6341;
    -0.0000    0.0000    0.8816;
    -0.0000   -0.0000   -0.0000;
    -0.0000    0.0000         0]

isUpper = istriu(A)

but it gives me always zero back, even though the matrix is upper triangular. When I am doing it locally, it gives me the right answer.

Does someone know why is this happening and how can I solve it?

Answer 1

[James Tursa]

This looks like a display issue. The numbers in the lower triangle are not exactly zero, they just display that way out to 4 digits. Take a look at this result:

A == 0

and you will likely see that some of those lower triangular elements are not exactly 0.

[Biljana Gjeroska]

Thanks a lot. I was thinking that the numerical deviation is already taken into account. But here I need to build it manually :)