Preallocate array in Matlab to improve performance

Question

I am new to Matlab. Mlint tells that I need to preallocate the arrays to improve performance.Can anyone tell me how do I preallocate array size in following code snippet:

for jj= init_frame: nFrames

   im1 = double(mov(jj).cdata);
   color_hist_array(jj,:) = color_histogram(im1, bins);
   [spatio_gram_array(jj,:),mu(jj,:,:),sigma(jj,:,:,:)] = spatiogram(im1, bins);

Preallocation needs to be done in color_hist_array, spatio_gram_array, mu and sigma.

score 4 · Accepted Answer · answered Feb 26 '13 at 16:31

4

Preallocation is actually very easily done in matlab, in your case just by changing one line of code.

Use this and you should be done.

for jj=  nFrames:-1:init_frame

Because you do the loop backwards all variables start at maximum size and space is allocated at once.

answered Feb 26 '13 at 16:31

Dennis Jaheruddin

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@Panther I don't know what the code snippet is supposed to do, but I am quite confident that (if it works) this will solve the warning. – Dennis Jaheruddin Feb 26 '13 at 16:34
Yes it did. Very nice thanks. I was looking for explanation of your code snippet. – MaxSteel Feb 26 '13 at 16:35
@Panther My solution is quite basic. If you want to understand it better how loops work I would recommend you to try `help for` Otherwise google for preallocation. – Dennis Jaheruddin Feb 26 '13 at 16:37
Thanks a lot. Worked for me.Will look for more about preallocation – MaxSteel Feb 26 '13 at 16:38
+1 for sneaky preallocation! – Marc Feb 26 '13 at 20:33

score 3 · Answer 2 · edited May 23 '17 at 12:21

Preallocation means that you first create a variable of the size you will need, and then fill in values. This is much faster than having the variable grow in size on every iteration of a loop, because growing requires allocating new memory of the new size, then copying the old data into the new memory.

Here's an example:

N = 10000;
x = zeros(1,N);
for i = 1:N
    x(i) = someFunction(i);
end

Dennis's answer about looping backward is a convenient trick to preallocate without extra lines of code before the loop.

score 1 · Answer 3 · answered Feb 26 '13 at 16:42

Read this and this link. In simple words, preallocation is just to tell MATLAB, how big your matrix is going to be (before you start using that Matrix). So that MATLAB can allocate sufficient memory. But there is much more analysis to this, which you will find in above links.

Preallocate array in Matlab to improve performance

3 Answers3