Block matrix inner products

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Dario Cortese
Dario Cortese 2019-2-20
I have been using the following custom function to perform the multiplication of a vector by a matrix, in which each element of the vector multiplies a 3x3 block within a (3xN)x(3) matrix:
function [B] = BlockScalar(v,A)
N=size(v,2);
B=zeros(3*N,3);
for i=1:N
B(3*i-2:3*i,:) = v(i).*A(3*i-2:3*i,:);
end
end
Similarly, when I want to multiply a collection of 3x3 matrices by a collection of 3x3 vectors, I use the following
function [B] = BlockMatrix(A,u)
N=size(u,2);
B=zeros(N,3);
for i=1:N
B(i,:) = A(3*i-2:3*i,:)*u(:,i);
end
end
Since I call them very often, these unfortunately, slow down the running of my code significantly. I was wondering if there was a more efficient (perhaps vectorised) version of the above operations.

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