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Piment
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how to index a matrix by using a index matrix that has same size?

Asked by Piment
on 30 Mar 2013

I have a m by n data matrix and a m by n index matrix which rearranges the order of the n elements in each row of the data matrix. How can I get an indexed data matrix without using loop? Thanks.

  1 Comment

But matrices already are indexed arrays or vectors... right?

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3 Answers

Answer by Walter Roberson
on 30 Mar 2013
Edited by Walter Roberson
on 30 Mar 2013
 Accepted answer

Use sub2ind()

  3 Comments

typo: sub2ind

To illustrate Walter's answer, if M is the matrix of data and I the matrix of column indices, you can do

 rId = (1:size(M,1)).' * ones(1,size(M,2)) ;    % Matrix of row indices.
 M_reordered = M(sub2ind(size(M), rId, I)) ;

Oops, yes, I was concentrating on not putting an "s" in and so left out the "2" :(

I thought that you had forgotten the "s" and I ended up realizing that it was a "2" ;-)


Answer by Anand
on 30 Mar 2013

If A is your original matrix and idx is the matrix of indices, you can use logical indexing: A(idx).

Here's an example:

>> A = rand(3)
A =
      0.8147    0.9134    0.2785
      0.9058    0.6324    0.5469
      0.1270    0.0975    0.9575
>> idx = [9 8 7;6 5 4;3 2 1]
idx =
       9     8     7
       6     5     4
       3     2     1
>> A(idx)
ans =
      0.9575    0.5469    0.2785
      0.0975    0.6324    0.9134
      0.1270    0.9058    0.8147

  1 Comment

This is linear indexing actually; the idx matrix that you defined is essentially what we get after calling sub2ind() with a third arg that is a matrix of column indices for each row (different from linear index).


Answer by Piment
on 30 Mar 2013
Edited by Piment
on 30 Mar 2013

to be more specific, it's something like(it's actually 300 by 5500 in my case):

A =

    0.9649    0.4854    0.9157
    0.1576    0.8003    0.7922
    0.9706    0.1419    0.9595
    0.9572    0.4218    0.6557

idx =

     1     3     2
     3     1     2
     2     1     3
     3     2     1

  2 Comments

Walter's answer and my illustration should work; just replace M with A and I with idx.

Piment
on 30 Mar 2013

thank you both, it really does work.


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