keeping track of the original indices of an array after sorting

Question

Md Tanvir Rahman 2015-6-25

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https://ww2.mathworks.cn/matlabcentral/answers/225268-keeping-track-of-the-original-indices-of-an-array-after-sorting

回答： Leelakrishna Vuriti 2019-12-3

I have an array let's say A=[5 4 1 2 3]. Now I have sorted these arrays in ascending order.So the resulting array will now be [1 2 3 4 5]. I use sort(A,'ascend') function to sort this. The question is how can I get the indices of the original array with respect to my new array. originally my indices were 1,2,3,4,5 for corresponding values of 5,4,1,2,3 and now the indices have changed to 3,4,5,2,1. How can I get these indices efficiently in Matlab?

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Stephen23 2015-6-25

编辑：Stephen23 2015-6-25

The documentation clearly explains that the function sort has two outputs... the second output are the indices that you are after.

http://www.mathworks.com/help/matlab/ref/sort.html

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Answer 1

Mischa Kim 2015-6-25

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Md, simply use

[B,I] = sort(A,'ascend') 
B =
     1     2     3     4     5
I =
     3     4     5     2     1

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chandra Naik 2019-2-20

Effective logic, Thank you

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Answer 2

Andy 2017-4-3

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and B = A(I), according to documentation, isn't it?

I get following:

>> x = [ 1 3 2 7; 4 3 2 1]

x =

     1     3     2     7
     4     3     2     1

>> [y,i] = sort(x,2)

y =

     1     2     3     7
     1     2     3     4

i =

     1     3     2     4
     4     3     2     1

>> y1 = x(i)

y1 =

     1     3     4     3
     3     3     4     1

I see y1 is different from y ...

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Stephen23 2017-4-3

编辑：Stephen23 2018-3-21

在 MATLAB Online 中打开

"and B = A(I), according to documentation, isn't it?"

The documentation states "if A is a vector, then B = A(I)." Is your input a vector? NO. However you can esaily read the rest of the documentation, where it explains the output for matrices and arrays: "The index vectors are oriented along the same dimension that sort operates on. For example, if A is a 2-by-3 matrix, then [B,I] = sort(A,2) sorts the elements in each row of A. The output I is a collection of 1-by-3 row index vectors describing the rearrangement of each row of A."

And your data shows this clearly:

 i =
     1     3     2     4
     4     3     2     1

Have a look: do you see that these are the indices along each row, which is exactly what the documentation states that you would get. It means you cannot use A(I) syntax. You could use sub2ind though.