# How to order a matrix?

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Sky Scrapper el 23 de Abr. de 2019
Comentada: Sky Scrapper el 23 de Abr. de 2019
Hello,
I have a matrix,
A=[ 100 5 0 ; 200 0 -5 ; 300 0 0 ; 300 5 0 ; -100 0 5 ; 300 0 0; 300 0 5];
I want to get,
A= [100 5 0; 200 0 -5 ; 300 0 0; 300 5 0 ; 300 0 5 ; -100 0 5];
I am writing code:
B= unique(A(:, 2:3),'rows');
then it's removing the first row as 5 0 is also repeted for the 4th row 300 5 0 but I need to keep the 1st row.
Is it possible to do?
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Adam el 23 de Abr. de 2019
It's not really obvious from your example what you are trying to achieve in a more general case. Your example just removes one row (which can be removed by using just a simple unique as mentioned) and then orders the remaining ones in a fairly unintuitive manner with respect to how they started.
What are you actually trying to keep? If you are trying to keep all unique rows then what I wrote works fine, if you are really trying to only run a unique on the 2nd and 3rd columns then your version does that, but from what you say, that isn't what you want to do.
Sky Scrapper el 23 de Abr. de 2019
I could solve my problem. thanks for helping me!

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### Respuestas (2)

Alex Mcaulley el 23 de Abr. de 2019
Editada: Alex Mcaulley el 23 de Abr. de 2019
Try this:
[~,ia,~] = unique(A(:, 2:3),'rows','stable');
B = A(ia,:);
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Alex Mcaulley el 23 de Abr. de 2019
Using round
If you want to round the first column:
A = [ 100 5 0 ; 200 0 -5 ; 300 0 0 ; 300 5 0 ; -100 0 5 ; 300 0 0; 300 0 5];
A(:,1) = round(A(:,1),-2);
[~,ia,~] = unique(A(:, 2:3),'rows','stable');
B = A(ia,:);
Sky Scrapper el 23 de Abr. de 2019
I have already done this. Again thanks a lot!

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Guillaume el 23 de Abr. de 2019
how can i do that?
Use uniquetol instead of unique. Note that the 'rows' option of unique is 'ByRows', true with uniquetol
[~, irows] = uniquetol(A(:, [2 3]), 'ByRows', true); %if you are happy with the default tolerance
B = A(irows, :);
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Sky Scrapper el 23 de Abr. de 2019
Yes, i have already done that.
Thank you very much for your kind support.

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