add elements to a matrix

I want to define a 20*100 matrix and add elements to it row by row. I divide a pitcture to 20 blocks containing 15360 pixels and get average value of H of the blocks. The code is shown below:

    Hsv = rgb2hsv(mov);
    H   = Hsv(:, :, 1);
    H   = reshape(H, 120, 4, 128, 5);
    blockMean = 255*reshape(sum(sum(H, 1), 3), 4, 5) / 15360; 
    blockArea = reshape(blockMean, 1, 20);

Now I need to get blockArea of 100 pictures and add them into a 20*100 matrix. How can I achieve it?

Answer 1

[Azzi Abdelmalek]

If your are asking about vertical concatenation

out=[]
for k=1:10
A=rand(5,5)
out=[out;A]
end

[Xiaochao]

Thanks for your help. It does work. But I have a problem that the order of the element blockArea is from 100th picture to 1st picture. How can I make it be from 1st picture to 100th picture.

[Walter Roberson]

See fliplr() and flipud()

[Azzi Abdelmalek]

or try this (switch out=[out;A] to out=[A;out])

out=[]
for k=1:10
A=rand(5,5)
out=[A;out]
end

[Jan]

Pre-allocate!

[Azzi Abdelmalek]

Then the code will change

m=10;n=5
out=zeros(m*n,n)
for k=1:m
A=rand(n)
out(m*n-k*5+1:m*n-(k-1)*n,:)=A;
end

[Xiaochao]

thank you very much. I learn a lot from you guys.

Answer 2

[Jan]

out = zeros(100, 20);  % Pre-allocation
for k = 1:10
  A         = rand(1, 20);
  out(k, :) = A;
end

Pre-allocation is essential. When the array grow in each iteration, you do not allocate for 100*20*8 bytes, but:

sum(1:100) * 20 * 8 bytes

And these are 808kB already.

[Xiaochao]

Thanks, it works well.