Use two bsxfun for row and column vector to span matrix without putting in memory

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T
T 2017-9-29
回答： Jan 2017-9-30
Hello, I have a vector A of size 1xN and a vector B of size MxN I can span a MxN matrix temp with temp=bsxfun(@times,A,B);. Now the tricky part. I create this temp matrix to multiply the following to my data matrix:
data=data.*exp(1i.*temp);
Is there a way I can use bsxfun to directly apply this operation on data, without creating the large temp matrix in the memory?
Thanks
  1 个评论
dpb
dpb 2017-9-29
bsxfun created the temp in memory behind scenes anyway, so it didn't actually save memory in computation, just was transparent in not having the explicit temp variable.
Beginning w/ R2016b implicit expansion was introduced that will replace most uses of bsxfun with more efficient memory usage. If you have a release of that vintage or newer investigate that feature.

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回答（1 个）

Jan
Jan 2017-9-30
A = rand(1, 1e5);
B = rand(100, 1e5);
data = ones(size(B));
tic, for k = 1:5
temp = bsxfun(@times, A, B);
data = data .* exp(1i .* temp);
end, toc
tic, for k = 1:5
temp = bsxfun(@times, 1i * A, B);
data = data .* exp(temp);
end, toc
tic, for k = 1:5
data = data .* exp((1i * A) .* B); % Auto-expanding in >= R2016b
end, toc
Elapsed time is 2.421372 seconds.
Elapsed time is 2.857152 seconds.
Elapsed time is 2.136714 seconds.
Observation from the profiler:
data = data .* exp(1i .* temp);
takes the same time as
data = data .* exp(temp);
but
temp = bsxfun(@times, 1i * A, B);
or
temp = 1i * bsxfun(@times, A, B);
have 100% more runtime than
temp = bsxfun(@times, A, B);
If this piece of code is the bottleneck of your code, create C-Mex function.

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