sum of product of 2 matrices

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Sreejith
Sreejith 2015-1-10
编辑： John D'Errico 2015-1-10
hi
i have 2 matrices... u is a 2 x 1250 matrix and r is a 1250 x 1250 matrix...
here c = 1
for each j = 1 to 1250 i want to find
how to writ the matlab code for this ?

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回答（2 个）

Zoltán Csáti
Zoltán Csáti 2015-1-10
It is just a standard matrix-matrix product: u*r. So enter
value = u(1,:)*r;
The j-th column of the matrix "value" will contain the sum for the specific j.
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Zoltán Csáti
Zoltán Csáti 2015-1-10
This is what my code does. Write a few indices and you will see.
John D'Errico
John D'Errico 2015-1-10
编辑：John D'Errico 2015-1-10
+1. What you don't seem to understand is that the * operator IS a dot product, i.e., the sum of products of elements. After all, MATLAB is a matrix language, so it is written to do these computations trivially, with the basic operators.
You sum of products is simply a dot product, what * delivers.

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Guillaume
Guillaume 2015-1-10
Use bsxfun:
sum(bsxfun(@times, u(c, :)', r))
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Zoltán Csáti
Zoltán Csáti 2015-1-10
Guillaume provided a vectorized version, that's why there is no matrix index j.
Guillaume
Guillaume 2015-1-10
bsxfun replicates u(c,:)' for each column j of r and sum calculates the sum for each of these columns. Exactly what you asked for.
Just try my code without any modification.
Note, this is equivalent to:
tempu = repmat(u(c, :)', 1, size(r, 2));
p = tempu .* r;
result = sum(p)
Or, if you really want to introduce j and make the calculation much slower:
tempu = repmat(u(c, :)', 1, size(r, 2));
p = tempu .* r;
result = zeros(1, size(r, 2));
for j = 1:size(r, 2)
result(j) = sum(p(:, j));
end
The one line bsxfun does the same.

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