## Reduce the computional time to calculate the mutilplying two matrixes

### Thu Nguyen (view profile)

on 17 May 2019
Latest activity Commented on by Thu Nguyen

### Thu Nguyen (view profile)

on 17 May 2019
I calculate this loop for thousands of time.
How can i optimize it to reduce the computational time?
for i=1:n
f(i,:)=w'*Q0(:,:,i)
end
with w =(2000,1) matrix and Q0=(2000,2000,100)
Thanks a lot.

dpb

### dpb (view profile)

on 17 May 2019
1. Preallocate f -- f=zeros(n,numel(w));
2. Reorient w outside the loop
The first may help enough to be noticeable if haven't; the second is very minor aid to the optimizer and likely will make little, if any, difference.
Thu Nguyen

### Thu Nguyen (view profile)

on 17 May 2019
Thank you a lot. I also made preallocate and reorient as you made and only reduce little time. I need reduce more such as ~ 100 times.

### David Goodmanson (view profile)

on 17 May 2019
Edited by David Goodmanson

### David Goodmanson (view profile)

on 17 May 2019

Hi Thu,
try
% N = 1000;
% M = 200;
Q00 = reshape(Q0,N,N*M);
ff = reshape(w'*Q00,N,M)';
This is approximately four times faster on my PC, compared to the first way with f preallocated.

Thu Nguyen

### Thu Nguyen (view profile)

on 17 May 2019
Thank you so much. I found this solution very interesting. But on my PC, it reduce time a little. I want to reduce the computational time about 100 times. Any solution?