Slowdown in assignment mystery

[John Swensen]

I have a fairly simple example that has me perplexed.  Below are the two 
code listings.  As a little background, the ocw_grab_frame() function 
returns a uint8 matrix.  Also, this computer has 4GB of RAM, and we are 
running this from a Linux box with X11 *not* running so less than 300MB 
is in use before running octave and the swap hasn't been touched.  I 
would expect the first listing to run the fastest.  Since we are 
pre-allocating the space for the images, then it seems like it should be 
a simple memcpy into the new buffer.  Contrary to this assumption, the 
first listing has a mean(tocs)=200ms and std(tocs)=10ms, where the 
second listing has a mean(tocs)=11.2ms and std(tocs)=1.5ms.  I am a 
little confused, so maybe someone has a suggestion.

John Swensen

Listing 1:
imgs = uint8(zeros(480,640,100));
for I=1:100
   tic;
   imgs(:,:,I) = ocw_grab_frame();
   tocs(I) = toc;
end

Listing 2:
for I=1:100
   tic;
   img = ocw_grab_frame();
   tocs(I) = toc;
end