Need some help computing vector velocity

Could anyone help me figure out what is wrong with my velocity calculations below?

v=0; for k=1:4226; v1 = Qdat.first_definition.RigidBodies.Positions(5, :, k); v = v1 - v;

end

Matlab does not want to recognize the v (velocity) vector and I'm not sure how to proceed.

Thanks!

[Joseph Cheng]

Can you expand on what you mean by "Matlab does not want to recognize the v (velocity) vector?" I created a dummy Qdat.first_definition.RigidBodies.Positions which is a 5x3x4226 matrix filled with zeros and I can get matlab to execute the code. What is the specific error that matlab describes or the issue with the v varible.

[Max]

Matlab compiles and computes the code but says that v = Na Na Na even though it gives me numbers for v1