creating a data array from a matrix

Hi,

I have matrix1 of 39 x 39. I need to create X and Y arrays by reading elements from rows and columns of matrix1 in a certain way.

 X        Y
(2,1)    (1,2)
(3,1)    (1,3)
(4,1)    (1,4)
..
(39,1)   (1,39)
(3,2)    (2,3)
(4,2)    (2,4)
..
(39,2)   (2,39)
(4,3)    (3,4)
..
(39,3)   (3,39)
..
(39,38)  (38,39)

How can I implement in MATLAB? Can somebody help me.

Thanks in advance,

Answer 1

[Geoff Hayes]

Hi Damith - I'd go with a double for loop, pre-allocating memory for the X and Y vectors:

 [m,n] = size(matrix1); % where matrix1 is the 39x39 matrix

 numElems = (m-1)*m/2;  % since ignoring elements on matrix diagonal
 X = zeros(numElems,1);
 Y = X;

 at = 1;

 for i=1:m
   for j=i+1:m          % since not interested in i==j
     X(at) = matrix1(j,i);
     Y(at) = matrix1(i,j);
     at = at + 1;
   end
 end

[Damith]

Thanks a lot. It worked.

Answer 2

[Udit Gupta]

This is simple if you use matlab lower triangle functions. See the example code below please.

A = magic(39)  % Your Martix
mask = tril(true(39),-1);
X = A(mask);
B = A';
Y = B(mask);

[Geoff Hayes]

That's neat!

[Damith]

Great! Both works. Thanks guys.