how do I vectorize a loop with multi-dimensional arrays/ outer product
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Hi, would someone happen to know how I could vectorize the following loop?
L=4;
for t=1:N;
A(:,:,t)=exp(a*(t-1)).*B(:,:,L);
end
many thanks, Eli
2 comentarios
Respuesta aceptada
Andrei Bobrov
el 21 de Jun. de 2013
B1 = B(:,:,4);
out = reshape(B1(:)*(0:N-1),[size(B1) N]);
3 comentarios
James Tursa
el 22 de Jun. de 2013
Editada: James Tursa
el 22 de Jun. de 2013
Your timing is comparing apples to oranges. You preallocate the answer for your loop but do not include it in your timing, whereas you penalize the reshaped outer product by including its intrinsic allocation in the timing. Your loop timing should be this instead, which includes both the allocation and the operations:
tic
C=NaN(128,256,N);
for t=1:N
C(:,:,t)=B(:,:,4)*a(t);
end
toc
Also, the timings should be run several times to make sure background functions are loaded into memory (I assume you do not want the function loading times to bias your results). So don't clear all between timing runs, just clear the variables you allocate (e.g., C, B1, and C1).
Más respuestas (1)
Iain
el 21 de Jun. de 2013
Editada: Iain
el 21 de Jun. de 2013
A = bsxfun(@times,exp(bsxfun(@times,a,0:(t-1)),B);
You need to make sure that a, 0:t-1 and B are all in the correct dimensions, and that "times", is the correct multiplication for your needs.
A = bsxfun(@times,exp(bsxfun(@times,a,reshape(0:(t-1),1, 1,[]),B);
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