how to reduce the size of a matrix?

Suppose I have 4 by 4 matrix or of any size and I want to make it a 2by2 matrix by taking the average of the sub matrixes.

Answer 1

[Image Analyst]

You can do this easily with conv2:

m = randi(9, 4, 4) % Sample data.

m = 4×4

5 1 8 2 9 2 1 1 8 5 6 7 4 8 4 3

sums = conv2(m, ones(2,2)/4, 'same');
means = sums(1:2:end, 1:2:end)

means = 2×2

4.2500 3.0000 6.2500 5.0000

Answer 2

[Torsten]

n = 4;
A = rand(n,n);
A_reduced = zeros(n/2,n/2);
for i=1:n/2
    for j=1:n/2
        Ahelp = A(2*(i-1)+1:2*i,2*(j-1)+1:2*j);
        A_reduced(i,j) = mean(Ahelp(:));
    end
end
A

A = 4×4

0.6054 0.9179 0.5640 0.4081 0.0255 0.3535 0.6808 0.4845 0.6431 0.2362 0.2967 0.8546 0.4578 0.2333 0.1625 0.2299

A_reduced

A_reduced = 2×2

0.4756 0.5344 0.3926 0.3859