From help-request at octave dot org Sun May  1 00:06:11 2005
Subject: Re: extracting elements
From: "Steve C. Thompson" <sct at ucsd dot edu>
To: Mike Miller <mbmiller at taxa dot epi dot umn dot edu>
Cc: Help-Octave List <help at octave dot org>
Date: Sat, 30 Apr 2005 22:03:21 -0700

Mike,

How about this:

I=[1,2,5,4]';              % desired rows
J=[1,3;2,4;1,4;2,3];       % desired columns
X=reshape([1:20],4,5)';    % matrix to pull elements from

X1=X';

[Jr,Jc]=size(J);
[X1r,X1c]=size(X1);

a=(I-1)*X1r*ones(1,Jc);
Y=X1(J+a)

Steve

On Apr 30 16:32PM, Mike Miller wrote:
> Suppose I have an N x M matrix X and I wish to extract particular elements 
> from certain rows of X.  I want to be able to provide a list of rows as a 
> vector, and I want to provide the columns within rows as a matrix like 
> this:
> 
> I = [1,2,5,4]';
> J = [1,3;2,4;1,4;2,3]
> 
>   1  3
>   2  4
>   1  4
>   2  3
> 
> So I want to extract rows 1,2,5,4 in that order, but I want to extract 
> only columns 1,3 in row 1, columns 2,4 in row 2, columns 1,4 in row 5 and 
> columns 2,3 in row 4.
> 
> If X looks like this...
> 
> X=reshape([1:20],4,5)'
> 
>    1   2   3   4
>    5   6   7   8
>    9  10  11  12
>   13  14  15  16
>   17  18  19  20
> 
> ...I can get what I want by doing this:
> 
> for i=1:length(I), Y(i,:)=X(I(i),J(i,:)); end
> 
> Y =
> 
>    1   3
>    6   8
>   17  20
>   14  15
> 
> But I think there must be a way to do this sort of thing without a for 
> loop.  Any suggestions?  Thanks in advance.
> 
> Mike
> 
> -- 
> Michael B. Miller, Ph.D.
> Assistant Professor
> Division of Epidemiology and Community Health
> and Institute of Human Genetics
> University of Minnesota
> http://taxa.epi.umn.edu/~mbmiller/
> 
> 
> 
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Octave is freely available under the terms of the GNU GPL.

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