Build huge matrix with vector components

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Kevin
Kevin 2014-7-10
评论： Kevin 2014-7-11
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

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采纳的回答

Jos (10584)
Jos (10584) 2014-7-11
n = 4 % a small example
m = 3
[a, b] = ndgrid(1:n,1:m)
pos = [0 0 ; b(:) a(:)]

更多回答（2 个）

James Tursa
James Tursa 2014-7-11
编辑：James Tursa 2014-7-11
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
Image Analyst
Image Analyst 2014-7-11
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!

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