Build huge matrix with vector components

how can I avoid double for in matlab in order to build matrices like this code do:

pos=[0 0];
   for i=1:m;
   for j=1:n;
   pos=[pos; i j];
   end
end

m and n are numbers like 500 and 900.

I have to find a better solution in order improve computation time . Thank you so much

Vins

Answer 1

[Jos (10584)]

n = 4 % a small example
m = 3 
[a, b] = ndgrid(1:n,1:m)
pos = [0 0 ; b(:) a(:)]

[Kevin]

Thank you so much! This is great!

Answer 2

[James Tursa]

One way:

c1 = ones(n,1)*(1:m);
c2 = (1:n)'*ones(1,m);
pos = [0 0; c1(:) c2(:)];

Or a similar method using repmat:

c1 = repmat(1:m,n,1);
c2 = repmat((1:n)',1,m);
pos = [0 0; c1(:) c2(:)];

You could have dramatically decreased the run time of your double loop simply by pre-allocating pos. E.g.,

pos = zeros(m*n+1,2);
k = 1;
for i=1:m;
    for j=1:n;
        k = k + 1;
        pos(k,:) = [i j];
    end
end

[Kevin]

Thanks a lot! Really helpfull tips!

Answer 3

[Image Analyst]

You don't have to use double. You can use int16 if you want

for i = uint16(1:500)

and so on. It will be one 8th the size, though a 450,000 by 2 array is far from "huge". That's like 7 MB if it's double and less than 2 MB if uint16. Do you know how big an ordinary run of the mill point and shoot camera's digital image is? It's way bigger than that!