Thread Subject:

rearrange matrix or vector elements by finding the index

Hi,
I need to find the Index of a matrix that rearranges it like in the following example:
I have this Vector: A = [5 6 4 4 5 4 5 7];
and B = sort(A) is equal to: B = [4 4 4 5 5 5 6 7];
I need to find the Index X so that A = B(X)
Is there a function for that?

Thanx Jorge

"Jorge " <jorge.ebinger@db.com> wrote in message <iamon0$lrr$1@fred.mathworks.com>...
> Hi,
> I need to find the Index of a matrix that rearranges it like in the following example:
> I have this Vector: A = [5 6 4 4 5 4 5 7];
> and B = sort(A) is equal to: B = [4 4 4 5 5 5 6 7];
> I need to find the Index X so that A = B(X)
> Is there a function for that?
>
> Thanx Jorge

Look at the second output argument to sort!
doc sort

 [A, ix] = sort([3 4 5 7 6])
%{
ix = [1 2 3 5 4]
%}

"Jorge " <jorge.ebinger@db.com> wrote in message <iamon0$lrr$1@fred.mathworks.com>...
> Hi,
> I need to find the Index of a matrix that rearranges it like in the following example:
> I have this Vector: A = [5 6 4 4 5 4 5 7];
> and B = sort(A) is equal to: B = [4 4 4 5 5 5 6 7];
> I need to find the Index X so that A = B(X)
> Is there a function for that?
>
> Thanx Jorge

help sort

Mira

[B,Index]=sort(A);

"Jorge " <jorge.ebinger@db.com> wrote in message <iamon0$lrr$1@fred.mathworks.com>...
> Hi,
> I need to find the Index of a matrix that rearranges it like in the following example:
> I have this Vector: A = [5 6 4 4 5 4 5 7];
> and B = sort(A) is equal to: B = [4 4 4 5 5 5 6 7];
> I need to find the Index X so that A = B(X)
> Is there a function for that?
======

Sorry, we all read your post to fast. You want the indexing X that inverts the sort operation.

One way is


[B,idx] = sort(A);
[~,X]=sort(idx);

This might be faster than applying sort() twice:

P=speye(length(A));
P=P(idx,:);
[~,X]=max(P,[],1)


Not sure, though.

"Jorge " <jorge.ebinger@db.com> wrote in message
news:iamon0$lrr$1@fred.mathworks.com...
> Hi, I need to find the Index of a matrix that rearranges it like in the
> following example:
> I have this Vector: A = [5 6 4 4 5 4 5 7];
> and B = sort(A) is equal to: B = [4 4 4 5 5 5 6 7];
> I need to find the Index X so that A = B(X)
> Is there a function for that?

A = [5 6 4 4 5 4 5 7]
[B, idx] = sort(A)
clear idx2 % if it already exists
idx2(idx) = 1:numel(idx)
B - A(idx) % should be all zero
A - B(idx2) % should also be all zero

--
Steve Lord
slord@mathworks.com
comp.soft-sys.matlab (CSSM) FAQ: http://matlabwiki.mathworks.com/MATLAB_FAQ
To contact Technical Support use the Contact Us link on
http://www.mathworks.com

"Steven_Lord" <slord@mathworks.com> wrote in message <iamt6k$kje$1@fred.mathworks.com>...

>
> A = [5 6 4 4 5 4 5 7]
> [B, idx] = sort(A)
> clear idx2 % if it already exists
> idx2(idx) = 1:numel(idx)

Steve's method is the best of finding the reverse permutation.

Bruno