Matlab function NNZ, numerical zero

Question

I am working on a code in Least Square Non Negative solution recovery context on Matlab, and I need (with no more details because it's not that important for this question) to know the number of non zero elements in my matrices and arrays.

The function NNZ on matlab does exactly what I want, but it happens that I need more information about what Matlab thinks of a "zero element", it could be 0 itself, or the numerical zero like 1e-16 or less.

Does anybody has this information about the NNZ function, cause I couldn't get the original script

Thanks.

PS : I am not an expert on Matlab, so accept my apologies if it's a really simple task.

I tried "open nnz", on Matlab but I only get a small script of commented code lines...

it only tells you perfect zeros. You can test this very easily `nnz(eps())` — Ander Biguri, May 21 '19 at 15:12

gnovice · Accepted Answer · 2019-05-21T15:33:34.930

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Since nnz counts everything that isn't an exact zero (i.e. 1e-100 is non-zero), you just have to apply a relational operator to your data first to find how many values exceed some tolerance around zero. For a matrix A:

n = nnz(abs(A) > 1e-16);

Also, this discussion of floating-point comparison might be of interest to you.

edited May 21 '19 at 15:33

answered May 21 '19 at 15:14

gnovice

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Thanks, in this way I can fix any threshold I want. – belkacem oukil May 21 '19 at 15:22

score 2 · Answer 2 · answered May 21 '19 at 15:14

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You can add in a tolerance by doing something like:

nnz(abs(myarray)>tol);

This will create a binary array that is 1 when abs(myarray)>tol and 0 otherwise and then count the number of non-zero entries.

answered May 21 '19 at 15:14

John

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I think your answer is really close to the next answer, and I think that's what I was looking for thanks ! – belkacem oukil May 21 '19 at 15:21
It looks like we answered it about simultaneously. Be careful. It looks like he has the inequality backwards. Otherwise they look to be identical. – John May 21 '19 at 15:23
@John: Thanks, small typo, just fixed it. – gnovice May 21 '19 at 15:24

Matlab function NNZ, numerical zero

2 Answers2