Matrix Product optimization with Bsxfun

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Davide Agostinelli
Davide Agostinelli 2022-11-5
评论： Bruno Luong 2022-11-5
Dear community,
i am looking to do:
A % --> 128 x 128 (matrix)
B % --> 1 x 128 (vector)
And i need to calculate (repeated 1e5 times):
D = A*B'
% that is not the element wise A.*B
For speed, i have problem by doing the fast:
D = bsxfun(@mtimes, A, B');
That should be the same but gives me error.
How can i fix this regarding dimensions?
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Steven Lord
Steven Lord 2022-11-5
Note that this question on Stack Overflow was asked ten years ago. Much has changed in MATLAB in the past decade, including the introduction of implicit expansion in release R2016b (as one of the answers on that question mentions.)
If you're computing the matrix product of a 128-by-128 matrix and a 128-by-1 vector, you need neither bsxfun nor implicit expansion. Just use normal matrix multiplication. If that's not what you're trying to do, please explain in more detail why you're asking about bsxfun or implicit expansion.

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Bruno Luong
Bruno Luong 2022-11-5
编辑：Bruno Luong 2022-11-5
You simply cannot invent something that does not exist (support) in MATLAB, the function mtimes is not supported by bsxfun see fun — Binary function to apply
https://fr.mathworks.com/help/matlab/ref/bsxfun.html
Beside that I have the hard time to figure out what you would ;do with matrix multiplication with expansion
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Rik
Rik 2022-11-5
Ran in:
Note that there is a big difference between ' and .' for complex data, and only you know which one you mean:
A=[1 1+i];
A' , A.'
ans =
1.0000 + 0.0000i 1.0000 - 1.0000i
ans =
1.0000 + 0.0000i 1.0000 + 1.0000i
Bruno Luong
Bruno Luong 2022-11-5
The runtime should be about the same even for complex data between matrix vector multiplcation with ' and .', since it takes care by the Blas and there is no conjugate explicitly performed.

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