extracting elements

[Mike Miller]

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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