MATLAB: Dot Product Doubt, don’t get the result I expected.

dot producteyeMATLABmoment of inertiapythontranspose

Hello I am new to Matlab and I am stuck in doing some maths(dot product) problem today. Please help me out.
I can write the correct code and format in python format, but i got some issues in the matlab. The python code is the following —
% This is the python code
RIMatrix_sph1 = RIMatrix_sph + mass_sph * (np.dot(q1.T, q1) * np.eye(3) - np.dot(q1, q1.T))
The error I encounter in matlab is in this specific part of the code —
(np.dot(q1.T, q1) * np.eye(3) - np.dot(q1, q1.T))
Here is the code I wrote in the Matlab
% This is the MATLAB code
mass_sph = 23.4572;
RIMatrix_sph = [0.0938 0 0; 0 0.0938 0; 0 0 0.0938]
RIMatrix_sph1 = RIMatrix_sph + mass_sph * (dot(transpose(q1),q1) * eye(3) - dot(q1,transpose(q1)))
I would be appreciated if you coulde help me with the problem.
PS.
This is the correct answer return by my python code —
RIMatrix_sph1 = [1.0321 0 0; 0 1.0321 0; 0 0 0.0938]
This is the wrong answer return by my MATLAB code —
RIMatrix_sph1 = [0.0938 -0.938 -0.9382; -0.9382 0.0938 -0.9382; -0.9382 -0.9382 0.0938]
Thank you Again!

Best Answer

You are using the wrong operator. You need basic matrix multiplication, not the dot product:
>> q1 = [0;0;0.2];
>> q1.'*q1
ans =  0.040000
>> q1*q1.'
ans =
   0.000000   0.000000   0.000000
   0.000000   0.000000   0.000000
   0.000000   0.000000   0.040000
As the dot product of two vectors by definition returns a scalar, the numpy.dot implementation is a bit strange. Reading the numpy documentation clarifies that "If both a and b are 2-D arrays, it is matrix multiplication..." , which is what your python code is actually doing (not a dot product). Note that the numpy documentation also recommends using an explicit matrix multiplication, rather than this bizarre overloaded "dot" operator: "..using matmul or a @ b is preferred".
Also note that your screenshot shows (implicit) matrix multiplication of those terms, not the dot product.
Related Question