Concatenate the produced arrays from a for loop?

I am trying to calculate the mean and standard deviation of the R,G,B channels of 50 image files in sequence. I would like to write out the mean of these into a single column vector per color. This would result in 3 50x1 column vectors - one each for R, G, B. I am able to calculate the means, but they are being output separately. Thanks

for j = 1:50
 
fname = sprintf('test (%d).bmp',j);
image4d = imread(fname);
     
Mean = squeeze(mean(image4d, [1 2]));
 
MeanR = Mean(1,:)
MeanG = Mean(2,:)
MeanB = Mean(3,:)

end

My output looks like:

MeanR =

25.1629

MeanG =

3.1821

MeanB =

0

MeanR =

31.6650

MeanG =

3.6093

MeanB =

0

.

.

For all 50 images.

[Sara Boznik]

Maybe you can add one for loop and define k=0 and in each loop is k=k+1, and after you write the

MeanR(k,:) = Mean(1,:)

I am not sure but you can try it.

Wish you best of luck.

[iontrap]

Unfortunately, this yields a 50x1 column of duplicate results for one image.

Answer 1

[Matt J]

Mean=nan(50,3);

for j = 1:50
 
    fname = sprintf('test (%d).bmp',j);
    image4d = imread(fname);
         
    Mean(j,:) = mean( image4d , [1 2]) ;


end

MeanR=Mean(:,1);
MeanG=Mean(:,2);
MeanB=Mean(:,3);

[iontrap]

Thanks a lot, Matt. This works. Could you explain the purpose of this nan(50,3)?

[Matt J]

It just pre-allocates an array of NaNs so that you have somewhere to put the data generated by the loop, e.g.,

>> nan(5,3)

ans =

   NaN   NaN   NaN
   NaN   NaN   NaN
   NaN   NaN   NaN
   NaN   NaN   NaN
   NaN   NaN   NaN

[Stephen23]

"Could you explain the purpose of this nan(50,3)?"

https://de.mathworks.com/help/matlab/matlab_prog/preallocating-arrays.html