Multiplying a 2d matrix with each slice of 3d matrix
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What is the fastest way multiply a 2d matrix with each slice of a 3d matrix?
x = rand(1000,200);
F = rand(100,1000,10);
b = zeros(100,200,10);
for i = 1:10
b(:,:,i) = F(:,:,i)*x;
end
2 Comments
Accepted Answer
Andrei Bobrov
on 2 Aug 2014
f1 = size(F);
x1 = size(x);
b = reshape(sum(bsxfun(@times,reshape(F,[f1(1),1,f1(2:3)]),...
reshape(x',1,x1(2),[])),3),f1(1),x1(2),[]);
2 Comments
Ray M
on 11 Oct 2021
Great Solution, the solution you have provided is equivalent to:
for i = 1:whatever
b(:,:,i) = F(:,:,i) * x;
end
How would you produce the trasnpose multiplication effect (assuming correct sizes)?
for i = 1:whatever
b(:,:,i) = x * F(:,:,i);
end
More Answers (4)
Steven Lord
on 11 Oct 2021
In release R2020b we introduced the pagemtimes function for this purpose.
3 Comments
Ray M
on 11 Oct 2021
Edited: Ray M
on 11 Oct 2021
I am not sure if this meant to be a reply to my question under Andrei Bobrov solution, but it does not appear so. I am simply interested in performing a single matrix multiplication by slices of a 3D matrix with the use of bsxfun as follows:
mat_mxn * 3Dmat_nxkxl
and
3Dmat_mxnxk * mat_nxh
Thanks
Steven Lord
on 12 Oct 2021
@Image Analyst Your bsxfun call is calling times not mtimes. times can work with implicit expansion.
A = int16(magic(4));
B = repmat(A, 1, 1, 3);
C = A.*B
mtimes isn't fully defined for integer arrays. If mtimes doesn't work pagemtimes probably shouldn't.
D = A*A % errors
Edric Ellis
on 4 Aug 2014
1 Comment
Nils Melchert
on 19 May 2020
Edited: Nils Melchert
on 19 May 2020
Is there the possibility to get a minimal example for exactly this use case? I am struggling with the same thing.
Image Analyst
on 2 Aug 2014
You can do it easily if the number of rows and columns in your 3D and 2D match, which they don't in your example:
rows = 1000;
columns = 200;
slices = 10;
x = rand(rows, columns);
F = rand(rows, columns, slices);
b = zeros(rows, columns, slices);
for slice = 1 : slices
b(:,:, slice) = F(:,:, slice) .* x; % Use dot star, not just star.
end
If the number of rows and columns are different you need to make some decisions about exactly where you want to multiply, if one is smaller than the other, or one extends out past the other.
1 Comment
Image Analyst
on 2 Aug 2014
Alternate way. Not sure which is faster:
% Mask the 3D image called "image3D" with 2D image called "mask".
masked3DImage = bsxfun(@times, image3D, cast(mask, class(image3D)));
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