Thread Subject:

Trace array of matrices

Hi,

Is it possible to vectorize the sum of the trace of an array of 2-D matrices?

Currently I am doing it in a loop

A = ones(4,4,N)
sum = 0;
for i = 1:N
 sum = sum + trace(A(:,:,i))
end

Any suggestions?

Thanks a lot..
Purpose - speed...since N is huge!

"Rajgopal " <runraj@gmail.com> wrote in message <h61tsn$lbo$1@fred.mathworks.com>...
> Hi,
>
> Is it possible to vectorize the sum of the trace of an array of 2-D matrices?
>
> Currently I am doing it in a loop
>
> A = ones(4,4,N)
> sum = 0;
> for i = 1:N
> sum = sum + trace(A(:,:,i))
> end
>
> Any suggestions?
>
> Thanks a lot..
> Purpose - speed...since N is huge!

  You might try adding elements along the third dimension first, as in:

 s = trace(sum(A,3));

  By the way, never use the names of functions such as "sum" as the names of your variables. That will get matlab (and you as well) confused.

Roger Stafford

One of many solutions:

d=A(bsxfun(@plus,(0:size(A,3)-1)*16,(1:5:16).'));
st = sum(d(:))

Bruno

"Bruno Luong" <b.luong@fogale.findmycountry> wrote in message <h61vj2$hqj$1@fred.mathworks.com>...
> One of many solutions:
>
> d=A(bsxfun(@plus,(0:size(A,3)-1)*16,(1:5:16).'));
> st = sum(d(:))
>
> Bruno

Thank you so much. That speeded up my code incredibly!