Preallocation of Array that is storing histograms in Matlab -

x = []; y = [];  = 1:m       qimg = imread(fullfile(dir2, queryframes(i).name),fmt);       image1 = rgb2gray(qimg);     x = [x, imhist(image1)];        end  j= 1:n       rimg = imread(fullfile(dir1, refframes(j).name),fmt);      image2 = rgb2gray(rimg);      y = [y, imhist(image2)];      end 

can show me how can preallocate x , y, 2 arrays storing histograms. matlab suggests preallocation both arrays speed. help.

by default grayscale images (which have), imhist creates histogram using 256 bins (docs here) , outputs column vector. therefore, unless change number of bins in iterations want x have 256 rows. number of columns dictated number of images processing, here m images x variable , n images y variable.

therefore want initialize both variables follows:

x = double(256,m); y = double(256,n); 

also, not recommended use i or j loop iterators since can confused imaginary unit.