Make this matrix multiplication more efficient

2 views (last 30 days)
Hello,
I would like to do the following matrix multiplication much efficiently:
m=1000;n=500;
a=zeros(n,1);
b=rand(n,1);
A=rand(m,n);
B=rand(m,m);
for i=1:n
a(i)=b'*(A'*B(i,:)'*B(i,:)*A)*b;
end
Thanks in advance

Accepted Answer

James Tursa
James Tursa on 25 Nov 2015
a = (B(1:n,:)*(A*b)).^2;
You dimensions for B look a little strange to me, since your calculations do not use all of the rows of B (hence the B(1:n,:) reduction above).
  1 Comment
Ignacio Echeveste
Ignacio Echeveste on 26 Nov 2015
Yes, the dimensions were wrong. Thank you, it is much more efficient.

Sign in to comment.

More Answers (1)

Richa Gupta
Richa Gupta on 25 Nov 2015
Hi Ignacio,
The code below reduces the time from 2.6 secs to 0.06 secs on my machine:
m = 1000; n = 500;
a = zeros(n,1);
b = rand(n,1);
A = rand(m,n);
B = rand(m,m);
for i=1:n
temp =(B(i,:)*A)*b;
a(i) = temp'*temp;
end
Hope this helps.

Community Treasure Hunt

Find the treasures in MATLAB Central and discover how the community can help you!

Start Hunting!

Translated by