Create the output with a maximum size at first to avoid the extremely time-consuming iterative growing of an array:
data = zeros(N, 2);
index = 0;
for k = 1:N
if FLAG % Apply your condition here
index = index + 1;
data(index, 1:2) = rand(1,2); % Your values to be stored
end
end
data = data(1:index, :); % Crop unused elements
This is much faster, when the size of the output is over a certain limit. Notice that for the iterative growing of a 1000x2 array, you have to obtain sum(1:1000)*2*8 bytes from the operating system and copy the same amount of data: more than 800kB althought the result has 16kB only.
