rotate acceleration vector using rotation matrix

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Adam Levschuk on 17 Mar 2020
Commented: James Tursa on 17 Mar 2020
Hello, first time question asker here.
I am trying to orient a smartphone whlist it is in a persons pocket. I collected accelerometer, gyroscope, and magnetometer data from the phone. Next i used the ahrsfilter to output quaternions relative to the NED reference frame.
Next i created a rotation matrix using rotationMatrix = quat2rotm(orientation). this gave me my rotation matrix.
Now here is the problem. My acceleration data which i want to rotate is in a 940x3 (X Y Z) array and my rotation matrix is in a 3x3x940 multidimensional array. So, how can i multiply each of my acceleration points with the appropriate rotation matrix. I suspect i will have to use a loop, however i am unsure about how to do this. How would you go about doing this?
Thank you

James Tursa on 17 Mar 2020
Edited: James Tursa on 17 Mar 2020
You can use a loop, e.g.
result = zeros(size(acc));
for k=1:size(acc,1)
result(k,:) = r(:,:,k) * acc(k,:)';
end
Depending on how r is defined (you didn't specify) you might need this instead:
result(k,:) = acc(k,:) * r(:,:,k);
Or you can explore using one of these FEX submissions (some of which require a C/C++ compiler to create mex routines):

Adam Levschuk on 17 Mar 2020
worked perfectly! thank you for the quick repsonse and great answer. =
I learned a bit more about loops just by using that.
Question: why did create an array of zeros the size of the acceleration matrix?
Thanks again.
James Tursa on 17 Mar 2020
The zeros( ) stuff is siimply pre-allocating the memory for the result before getting into the loop.