Inverse of sorting arrangement

Question

Paolo Binetti 2018-10-4

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评论： Stephen23 2023-6-27

[B,I] = sort(A) output is such that B = A(I). What would be the most efficient way to rather get B and J, such that A = B(J)?

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Answer 1

Stephen23 2018-10-4

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编辑：Stephen23 2023-6-27

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Assuming that A is a vector:

[B,I] = sort(A);
[~,J] = sort(I);
B(J)

If you have a matrix/array, then you first need to define what the intended effect is: to sort elements along one dimension (e.g. separately sort each column or row of a matrix) or to sort the rows atomically (i.e. using SORTROWS).

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Mohamed Eldin 2023-6-27

I used the command "sortrows" and it worked perfectly

as in the following example:

[Indata,id] = sortrows(InData,1); % sort InData

[~,ids] = sortrows(id,1); % sort the index

InData_returne = Indata(ids,:); % return InData again

Stephen23 2023-6-27

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@Mohamed Eldin: you do not need the 2nd SORTROWS, a simple SORT is quite sufficient:

[B,I] = sortrows(A,1);
[~,J] = sort(I);
A = B(J,:);

Also note that if you are only sorting one column instead of SORTROWS you could use indexing & SORT:

[B,I] = sort(A(:,1));

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Answer 2

Matt J 2018-10-4

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https://ww2.mathworks.cn/matlabcentral/answers/422153-inverse-of-sorting-arrangement#answer_339714

编辑：Matt J 2018-10-4

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 [B,I] = sort(A); 
 J=1:numel(I);
 J(I)=J;

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Robert 2020-7-28

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Surprisingly, even this works:

 [B,I] = sort(A); 
 J(I) = 1:numel(I);

Matt J 2020-7-28

Yes, but it can have unintended effects if J is a pre-existing variable in the workspace.

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Answer 3

Bruno Luong 2023-6-27

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https://ww2.mathworks.cn/matlabcentral/answers/422153-inverse-of-sorting-arrangement#answer_1263183

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If A has distinct elements such as

A = rand(1,10)
A = 1×10
    0.0113    0.2350    0.1021    0.4269    0.9673    0.4399    0.3871    0.0288    0.5912    0.1876

a single line command is

[B,~,J] = unique(A);

Check

J
J = 10×1
     1
     5
     3
     7
    10
     8
     6
     2
     9
     4
B(J)
ans = 1×10
    0.0113    0.2350    0.1021    0.4269    0.9673    0.4399    0.3871    0.0288    0.5912    0.1876