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3 matrix product without dimension error

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Amirhosein hematyar
评论： Yazan 2021-7-2
Hi all
I have a x with 2*72 dimensions and want to product the x(i)'*W*x(j) that W is inv(cov(x)) and 72*72.
I write it:
H=zeros(n,n);
for i=1:n
for j=1:n
H(i,j)=x(:,i)'*W*x(:,j);
H(j,i)=H(i,j);
end
end
but i receive dimension errors
please help me.

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回答（2 个）

Walter Roberson
Walter Roberson 2021-7-2
x is 2x72. indexing one column of it gives 2x1. Transpose gives 1x2. That does not match the 72x72 of W.
Yazan
Yazan 2021-7-2
编辑：Yazan 2021-7-2
You can multiply the matrices directly without a loop.
% random matrices
x = randn(2, 72); y = randn(72, 72);
% multiplication, result is 2-by-72
res = x*y;

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