Vectorize product into new dimension
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I often run into for-loops like the following (minimal working example):
A = randn(1000);
b = randn(1,1000);
lA = size(A,1);
lb = length(b);
B = zeros(lA,lA,lb);
for k=1:lb
B(:,:,k) = A*b(k);
end
Is there a good way to vectorize this to make it faster? I tried writing
B = reshape(kron(b,A),lA,lA,lb)
but this is not always faster and sometimes even substantially slower than the for-loop.
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Answers (1)
Steven Lord
on 13 Dec 2021
Using slightly smaller arrays so these lines can run in Answers:
A = randn(100);
b = randn(1,100);
lA = size(A,1);
lb = length(b);
tic
B = zeros(lA,lA,lb);
for k=1:lb
B(:,:,k) = A*b(k);
end
toc
tic
B2 = A.*reshape(b, 1, 1, []);
toc
max(abs(B-B2), [], 'all')
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