Using find on matrices

[harishankar ramachandran]

Hi all,

If I have a matrix "A", and I apply a condition on it, I can extract the 
indices for which the condition is true, and I can then use that vector to 
change those values. For eg:

indx=find(A>4);
A(indx)=5;

There is also a double index version of find, where I can get the (i,j) 
coordinates of the points. However, I do not seem to be able to use the 
following code:

[ii,jj]=find(A>4);
A(ii,jj)=5;

This instead assigns 5 to a submatrix of A defined by rows ii and columns jj.

What is the proper way to vectorially assign values to elements of a matrix A, 
if I have the element locations in the form [ii,jj]? Without a for loop, that 
is.

Another question: Suppose I have a set of disjoint conditions with which I 
wish to partition a matrix. How do I extract the corresponding elements with 
a single command? This seems like a very useful capability, but I don't find 
anyway to do this without using a for loop. Eg:

A=int(rand(5,5)*12);
v=0:2:12;
l=list()
for k=1:length(v)-1
  l(k)=find(A>=v(k) & A<v(k+1));
end

This yields:
A  =

    2.    7.     6.    2.     3.
    9.    10.    7.    2.     11.
    0.    8.     8.    2.     2.
    3.    10.    2.    10.    3.
    7.    0.     6.    7.     4.

 l  =
    l(1) = 3.    10.
    l(2) = 1.    4.    14.    16.    17.    18.    21.    23.    24.
    l(3) = 25.
    l(4) = 5.    6.    11.    12.    15.    20.
    l(5) = 2.    8.    13.
    l(6) = 7.    9.    19.    22.

But can I do it faster, without for loops? I would like to have a command that 
does:

A=int(rand(5,5)*12);
v=0:2:12;
l=findmany(A>=v(1:$-1) & A<v(2:$));

or something like that. Is there some such thing?

Thanks in advance

hari ramachandran
-- 
Dr. Hari Ramachandran, EE Dept, IIT-Madras