sorting values of a matrix column when the other column has the same value

Hi all,

I have a matrix Athat has two columns as following:

A = [0.1 0.1 ; 0.1 -0.3 ; 0.1 0.5 ; 0.1 0; 0.1 -0.5; 0.1 -0.1; 0.1 0.4;
    0.2 0.1 ; 0.2 -0.3 ; 0.2 0.5 ; 0.2 0; 0.2 -0.5; 0.2 -0.1; 0.2 0.4; 
    0.3 0.1 ; 0.3 -0.3 ; 0.3 0.5 ; 0.3 0; 0.3 -0.5; 0.3 -0.1; 0.3 0.4];

And I want to sort the second column of this matrix for every uniqe value of the first column so that the result looks like:

Asorted = [0.1 -0.5 ; 0.1 -0.3 ; 0.1 -0.1 ; 0.1 0; 0.1 0.1; 0.1 0.4; 0.1 0.5;
    0.2 -0.5 ; 0.2 -0.3 ; 0.2 -0.1 ; 0.2 0; 0.2 0.1; 0.2 0.4; 0.2 0.5;
    0.3 -0.5 ; 0.3 -0.3 ; 0.3 -0.1 ; 0.3 0; 0.3 0.1; 0.3 0.4; 0.3 0.5];

My "failing" attempt of doing this is the following:

A1 = A(:,1); %forst column
A2 = A(:,2); %second column
Auni = unique(A1); %unique value of the first column

for i = 1:size(A,1) %go through all the points in the first column
    for j = 1:numel(Auni) %go throguh the unique values of the first column
        if A1(i) == Auni(j) %if the point equals one of the uniqe values
           [A2 , sortdx] = sort(A2); %sort out the second column of matrix A
           A1 = A1(sortidx); %sort the first column accordingly
           Asorted = [A1 A2]; %combine both results.
        end
    end
end

Also, how can I reorder another matrix B based on the sorting done previously on matrix A?

Any help would be appreicted.

Thanks.

Answer 1

[Star Strider]

Use the sortrows function, sorting the first column then the second column —

A = [0.1 0.1 ; 0.1 -0.3 ; 0.1 0.5 ; 0.1 0; 0.1 -0.5; 0.1 -0.1; 0.1 0.4;
    0.2 0.1 ; 0.2 -0.3 ; 0.2 0.5 ; 0.2 0; 0.2 -0.5; 0.2 -0.1; 0.2 0.4; 
    0.3 0.1 ; 0.3 -0.3 ; 0.3 0.5 ; 0.3 0; 0.3 -0.5; 0.3 -0.1; 0.3 0.4];

Asorted = sortrows(A, [1 2])

Asorted = 21x2

0.1000 -0.5000 0.1000 -0.3000 0.1000 -0.1000 0.1000 0 0.1000 0.1000 0.1000 0.4000 0.1000 0.5000 0.2000 -0.5000 0.2000 -0.3000 0.2000 -0.1000

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Asorted = [0.1 -0.5 ; 0.1 -0.3 ; 0.1 -0.1 ; 0.1 0; 0.1 0.1; 0.1 0.4; 0.1 0.5;
    0.2 -0.5 ; 0.2 -0.3 ; 0.2 -0.1 ; 0.2 0; 0.2 0.1; 0.2 0.4; 0.2 0.5;
    0.3 -0.5 ; 0.3 -0.3 ; 0.3 -0.1 ; 0.3 0; 0.3 0.1; 0.3 0.4; 0.3 0.5]

Asorted = 21x2

0.1000 -0.5000 0.1000 -0.3000 0.1000 -0.1000 0.1000 0 0.1000 0.1000 0.1000 0.4000 0.1000 0.5000 0.2000 -0.5000 0.2000 -0.3000 0.2000 -0.1000

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.

[LH]

Many thanks!

[Star Strider]

As always, my pleasure!