why does mathematica not return the result of the multiplication of a symbolic matrix and a symbolic vector?

Question

I am really stuck at a simple problem. Maybe, it is not totally a problem. However, I don't understand why.

I am trying to take the dot product of two expressions.

Here is the code:

a = {{Ixx, 0, 0}, {0, Iyy, 0}, {0, 0, Izz}}
b = {{0}, {0}, {Theta1'[t]}}

a.b should work according to the documentation. I am expecting the result

{{0}, {0}, {Izz*Theta1'[t]}}

and after // MatrixForm, it will be [0; 0; something].

But, after the a.b production, I just get this:

Thanks in advance.

Just don't use MatrixForm for defining matrices. It does not define a matrix; instead it is only for displaying purposes. — Henrik Schumacher, Apr 04 '20 at 20:41
thanks for your reply. i understand the problem. apparently, i should be careful by using MatrixForm. — yoeruek, Apr 04 '20 at 21:01

Nasser · Accepted Answer · 2020-04-04T20:51:51.200

MatrixForm is wrapper. It prevent future evaluations. You must have added a MatrixForm around a and b before, that is why.

Try

a = {{Ixx, 0, 0}, {0, Iyy, 0}, {0, 0, Izz}}; b = {{0}, {0}, {Theta1'[t]}};
(a.b) // MatrixForm

It looks like you did this before

a = MatrixForm@{{Ixx, 0, 0}, {0, Iyy, 0}, {0, 0, Izz}}; 
b = MatrixForm@{{0}, {0}, {Theta1'[t]}};
(a.b)

To see the matrix using MatrixForm and also not have to worry about it going into it and causing problems later on, just use () like this

(a = {{Ixx, 0, 0}, {0, Iyy, 0}, {0, 0, Izz}}) // MatrixForm
(b = {{0}, {0}, {Theta1'[t]}}) // MatrixForm
(c = a.b) // MatrixForm

Perfect explanation. It directly works. Thanks a lot. – yoeruek Apr 04 '20 at 20:58 — yoeruek, Apr 04 '20 at 20:58

1 Answers1