How to order a matrix?

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?

[Adam]

What is wrong with just using

B = unique( A, 'rows' )

or

B = unique( A, 'rows', 'stable' )

though your chosen output ordering does not match either unique's sorted output nor its stable output which retains the original ordering.

Your version using 2:3 won't do this though either.

You can sort the output to your desired ordering afterwards though.

[Sky Scrapper]

My original matrix is 1000x20

what i have given in the question is the 1st four columns.I need to keep the corresponding whole columns. so what I did,

[~,idy,idx] = unique(A(:,2:3),'rows','stable');

B =A(idy,:);

then it's showing 1st row but it's also showing the repeted rows for 300 0 0 and so on.

Could you please clarify where is the mistake?

[Adam]

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]

I could solve my problem. thanks for helping me!

Answer 1

[Alex Mcaulley]

Try this:

[~,ia,~] = unique(A(:, 2:3),'rows','stable');
B = A(ia,:);

[Sky Scrapper]

Yes, i have tried but it is not working for my original matrix.

[Sky Scrapper]

I think this should work but it's showing the same result!

[Sky Scrapper]

If I am doing,

[~,ia,~] = unique(A(:, 2:3),'rows','stable');

B = A(ia,:);

again it's removing 1st column. if i am doing,

[~,ia,~] = unique(A(:, 1:3),'rows','stable');

B = A(ia,:);

then it's showing the repeated column also!

[Alex Mcaulley]

A = [ 100  5  0 ;   200  0  -5  ; 300  0  0 ; 300  5  0 ; -100  0  5  ; 300 0 0; 300 0 5];
[~,ia,~] = unique(A(:, 2:3),'rows','stable');
B = A(ia,:)
B =

   100     5     0
   200     0    -5
   300     0     0
  -100     0     5
  

It keeps the first column! And remove the repeated rows

[Sky Scrapper]

Yes, I got the solution of my problem. i need to take the tolerance of my values because i have some fractional values.

Thank you very much for your help.

[Alex Mcaulley]

Using round

https://es.mathworks.com/help/matlab/ref/round.html

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]

I have already done this. Again thanks a lot!

Answer 2

[Guillaume]

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, :);

[Sky Scrapper]

Yes, i have already done that.

I had to take tolerance=0.02

Thank you very much for your kind support.