Is this just the same question you have asked twice before? If not, you need to include more code. For example, pre-allocation works when used in a loop, yet I see no loop in what you have posted above. Post the code of the loop where you are doing the concatenation.
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EDIT In response to your comments below...
Yes, I built the array by column, but then reshaped it to a row vector. You get what you want, and this is faster than some complex indexing scheme into C. Just try it before you think it won't end up a row vector!
Idx = 1+5000*looptimes:last+5000*looptimes;
C = zeros(16,last); % Start out this way, fix in last line...
for ii = 1:length(Idx)
C(:,ii) = F((-15+16*(Idx(ii))):16*(Idx(ii)));
end
C = reshape(C,1,16*last); % Make into a row vector!!
