MATLAB: Problem with intersect function

Question

I am using the intersect function but it seems to be giving only a subset of the common values in the two vectors being compared. I'm not sure if this is because of the way I have set up the vectors or whether I'm using the function incorrectly.

This is a simpler version of the code but with the same results:

z = -1:0.01:1;

zstart = linspace(-1,0.8,10);

zend = linspace(-0.8,1,10);

zmid = round((zstart+zend)/2,1);

[~,izmid] = intersect(z,zmid);

It should give the 10 indices of z which equal the values of zmid but it is only producing 6 of these.

Answer 1

The intersectt function does not allow tolerances, so floating-point approximation error is going to present problems.

Try this:

z = -1:0.01:1;
zstart = linspace(-1,0.8,10);
zend = linspace(-0.8,1,10);
zmid = round((zstart+zend)/2,1);
% [~,izmid] = intersect(z,zmid); 
izmid2 = ismembertol(z,zmid,1E-2);                      % Use Appropriate Tolerance Value (Here: 1E-2)
idx = find(izmid2);                                     % Indices Corresponding To Logoical Vector ‘izmid2’
z_common = z(idx);                                      % Common Values (± Tolerance Value)

Note the difference in results.

See the documentation on Floatinig-Point Numbers for a discussion of the reason ismembertol works here.

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