Sort Index - Bug

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Rainer Ng
Rainer Ng on 19 Sep 2019
Commented: Walter Roberson on 14 Apr 2021
[a, sort_index]=sort([100;20;30;40])
The sort_index should return
4
1
2
3
But it does not.

Accepted Answer

Walter Roberson
Walter Roberson on 19 Sep 2019
>> [a, sort_index]=sort([100;20;30;40])
a =
20
30
40
100
sort_index =
2
3
4
1
This output is correct. It tells you that a(1) == 20 came from element #2 of the input, and that a(2) == 30 came from element 3 in the input, and that a(3) == 40 came from element 4 of the input, and that a(4) == 100 came from element 1 of the input.
To get the output you want, use
b(sort_index) = 1:length(sort_index);
  4 Comments
Franziska Höppe
Franziska Höppe on 13 Apr 2021
Thank you for you detailed answer! :-)
The issue is if you let run the loop, you get a different result. You get:
2
5
2
Do you know why?
Walter Roberson
Walter Roberson on 14 Apr 2021
I never got that output; which code were you using, and which MATLAB version?

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More Answers (1)

James Tursa
James Tursa on 19 Sep 2019
The sort index gives the location in the original array of the sorted values. I.e., the sort results "a" are "original_array(sort_index)"
>> x = [100;20;30;40];
>> [a,sort_index] = sort(x)
a =
20
30
40
100
sort_index =
2
3
4
1
>> x(sort_index)
ans =
20
30
40
100
>> isequal(x(sort_index),a)
ans =
1
  1 Comment
Rainer Ng
Rainer Ng on 19 Sep 2019
I apologize. I should have understood this. Thank you.

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