MATLAB: Finding unique rows using “uniquetol” from top

Question

It seems like there is no option for finding unique rows from top using uniquetol unlike unique where there is option for the argument "first". Is there a way to do this

[~,idx] = uniquetol(Q2(:,1:ns),'ByRows',true,'first') % the argument "first" picks unique rows from the top

some other way, since the "first" option is not supported by uniquetol?

Thanks!

Answer 1

I suppose you can do something like this. I'm affraid the way UNIQUETOL and ISMEMBERTOL consider the TOL internally, and in some cases the result is not coherently match when the frontier is fuzzy. You might set 'DataScale' to 1 and control TOL to have more robust match.

Fake data

A = ceil(3*rand(1000,2));
A = A + rand(size(A))*1e-10;

Engine

[Au,idx] = uniquetol(A,1e-6,'DataScale',1,'byrows',true);
[tf,I] = ismembertol(A,Au,1e-6,'DataScale',1,'byrows',true);
if ~all(tf)
    error('incompatible tolerance');
end
firstidx = accumarray(I,(1:size(A,1))',[],@min)
if ~all(firstidx) || size(firstidx,1)~=size(Au,1)
    error('incompatible tolerance');
end

Check

A(idx,:)
A(firstidx,:)

If those matching error checking bother you, here is a way to ignore with the risk that the result might be different than UNIQUETOL alone

Au = uniquetol(A,1e-6,'DataScale',1,'byrows',true);
[tf,I] = ismembertol(A,Au,1e-6,'DataScale',1,'byrows',true);
firstidx = accumarray(I(tf),find(tf),[],@min)
Au = A(firstidx,:);