MATLAB Answers

0

Make this matrix multiplication more efficient

Asked by Ignacio Echeveste on 23 Nov 2015
Latest activity Commented on by Ignacio Echeveste on 26 Nov 2015
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

  0 Comments

Sign in to comment.

2 Answers

Answer by James Tursa
on 25 Nov 2015
 Accepted Answer

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

Yes, the dimensions were wrong. Thank you, it is much more efficient.

Sign in to comment.


Answer by 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.

  0 Comments

Sign in to comment.