how to reduce the size of a matrix?

Question

DARSHAN KUMAR BISWAS 2022-6-12

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回答： Image Analyst 2022-6-12

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.

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Answer 1

Image Analyst 2022-6-12

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在 MATLAB Online 中打开

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

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Answer 2

Torsten 2022-6-12

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编辑：Torsten 2022-6-12

在 MATLAB Online 中打开

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