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

Question

LH 2024-7-29

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/2141096-sorting-values-of-a-matrix-column-when-the-other-column-has-the-same-value

评论： Star Strider 2024-7-29

采纳的回答： Star Strider

在 MATLAB Online 中打开

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.

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Answer 1

Star Strider 2024-7-29

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/2141096-sorting-values-of-a-matrix-column-when-the-other-column-has-the-same-value#answer_1491891

在 MATLAB Online 中打开

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;
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; 
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
1000   -0.5000
1000   -0.3000
1000   -0.1000
1000         0
1000    0.1000
1000    0.4000
1000    0.5000
2000   -0.5000
2000   -0.3000
2000   -0.1000
<mw-icon class=""></mw-icon>
<mw-icon class=""></mw-icon>
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;
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;
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
1000   -0.5000
1000   -0.3000
1000   -0.1000
1000         0
1000    0.1000
1000    0.4000
1000    0.5000
2000   -0.5000
2000   -0.3000
2000   -0.1000
<mw-icon class=""></mw-icon>
<mw-icon class=""></mw-icon>