MATLAB: Multiply a set of matrices (3D array) by a vector of values

indexing

I have a 3D array (a) such that size(a) = [n,n,m]. I also have a vector (v) of length(v) = m. I would like to multiply each element from my vector with each 2D matrix in my array. Is there a way to do this without a for loop. To clarify, using a for loop it would be:

for i = 1:m
  b(:,:,i) = v(i)*a(:,:,i);
end

Best Answer

% -------Example-----------------
M=randi(3,3,2,4) 
v=1:4  
%--------------------------------
bsxfun(@times,M,reshape(v,1,1,4))

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MATLAB: Saving arrays in a For loop (3D array)

Use a cell array to store your matrices

Example

for k=1:3
   M{k}=rand(randi(10),3)
end
%To get your matrices
M{1}
M{2}
M{3}

MATLAB: Is there a way to do these matrix vector multiplications without loops

First, you would choose a different ordering. Instead of

A = rand(m,n,4,4);
v = rand(m,n,4);

you would have

A = rand(4,4,m,n);
v = rand(4,m,n);

which will permit faster data access.

As for the multiplication, if you have the Parallel Computing Toolbox, you can do this directly with pagefun

Agpu=gpuArray(reshape(A,4,4,[]));
vgpu=gpuArray(reshape(v,4,1,[]));
u=reshape(pagefun(@mtimes,Agpu,vgpu), size(v) );

If you do not have the Parallel Computing Toolbox, you can instead do,

 u = sum(  A.*reshape(v,1,4,m,n)   ,2);

Best Answer

Related Solutions

MATLAB: Saving arrays in a For loop (3D array)

MATLAB: Is there a way to do these matrix vector multiplications without loops

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