MATLAB: How to pre-allocate an empty matrix

pre-allocate

here is part of my code…
...
for l = 1:length(WtLabDt)                                               
    appnd_data = [];
    for m = 1:length(s.labs)                                          
        appnd_data = [appnd_data;e.(s.labs{m})(:,l)];                  
    end
    OVR_MEAN(:,l) = mean(appnd_data);                                 
end
Matlab is showing light red lines under 'appnd_data' (in line 4 on lhs) asking me to pre-alloacte it for speed. Does line 2 not pre-allocate it ?

Best Answer

A pre-allocation would be:
    size1      = cellfun('size', s.labs, 1);
    appnd_data = zeros(sum(size1), 1);
    iPos       = 1;
    for m = 1:length(s.labs)                                          
       fPos = iPos + size1(m);
       appnd_data(iPos:fPos-1) = e.(s.labs{m})(:,l);
       iPos = fPos;
    end
But is there any reason to create this large vector, when all you want is to get its mean?
    allSize1 = sum(cellfun('size', s.labs, 1));
    S = 0;
    for m = 1:length(s.labs)                                          
       S = S + sum(e.(s.labs{m})(:,l));
    end
    OVR_MEAN(:, l) = S / allSize1;
Perhaps I did not understand, why the vector OVR_MEAN(:, l) is assigned, although the result is a scalar. But the ideas should got clear now:
  1. Pre-allocation means reserving memory for the final array at once instead of letting the array grow iteratively
  2. If you can avoid to create large temporary arrays, do this.
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