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How to simplify a code to remove a for loop and make it faster?

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carlos Uribe
carlos Uribe 2014-8-14
回答： Andrei Bobrov 2014-8-14
Hello,
I'm still learning vectorization and haven't figured out how to simplify this code:
B=zeros(1,a^2);
for j=1:a^2;
B(:)=B(:)+C(:,j)*D(j);
end;
in which B is a vector of 1 row by a^2 columns, C is an a x a matrix, and D is a vector of 1 row by a^2 columns
So basically what is happening is I take all the columns of C (one by one) and multiply them by D and then add them to B but this is repeated column by column on C.
I appreciate if anyone can show me how to do this.
Thank you very much.

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回答（3 个）

Iain
Iain 2014-8-14
One of these ought to do it:
B = C' * D;
or
B = C' * D';
You might need to transpose B to get what you want....
Azzi Abdelmalek
Azzi Abdelmalek 2014-8-14
编辑：Azzi Abdelmalek 2014-8-14
Edit
a=4
C=rand(a^2);
D=rand(1,a^2);
B=sum(bsxfun(@times, C,D),2)
Andrei Bobrov
Andrei Bobrov 2014-8-14
B = D*C';

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