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# Finding unique rows using "uniquetol" from top

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J AI on 27 Jul 2020
Edited: Bruno Luong on 27 Jul 2020
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!

#### 9 Comments

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Bruno Luong on 27 Jul 2020
Here we go, small example
>> A = 1 + eye(2)*2^-40
A =
1.0000 1.0000
1.0000 1.0000
>> [Au,matlabidx] = uniquetol(A,1e-6,'byrows',true)
Au =
1.0000 1.0000
matlabidx =
2
Bruno Luong on 27 Jul 2020
Even smaller (smallest example)
>> A = 1 + [1;0]*2^-40
A =
1.0000
1.0000
>> [Au,matlabidx] = uniquetol(A,1e-6,'byrows',true)
Au =
1
matlabidx =
2
J AI on 27 Jul 2020
The data I was trying to work on is not random at all, rather it is from a well-known experiment in machine learning called the "Tiger example" often found in POMDP literature. Since several examples already have been provided by Bruno already, I am going to pass.

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### Accepted Answer

Bruno Luong on 27 Jul 2020
Edited: Bruno Luong on 27 Jul 2020
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,:);

#### 2 Comments

J AI on 27 Jul 2020
Thanks a lot!
Bruno Luong on 27 Jul 2020
Actually I was stupid; you can use the third output of UNIQUETOL, no need fot ISMEMBERTOL
[Au,matlabidx,I] = uniquetol(A, 1e-6, 'byrows', true);
firstidx = accumarray(I, (1:size(A,1))', [], @min)
% Check
norm(A(matlabidx,:)-A(firstidx,:),Inf)

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