How to define a symbolic function q such that diff(q, t) gives me q_dot

So I want to define a symbolic variable q that is a function of t

and want the script to give me

I do not know what the function q is !! I just know that q is a function of t and I want to symbolically output qdot for differentation of q w.r.t t

% My approach that did not work
% Define the number of degrees of freedom
n_dof = 2;

syms t

% Define the generalized coordinates and their derivatives
q = sym('q',[n_dof,1],'real');
q_dot = sym('q_dot',[n_dof,1],'real');
q_ddot = sym('q_ddot',[n_dof,1],'real');

diff(q, t)

% I want diff(q, t) to give me q_dot 
% and diff(q_dot, t) to give me q_ddot in symbolic form

Answer 1

[Walter Roberson]

The only way to get diff(q, t) to return q_dot is if you define

q = int(q_dot, t)

I would suggest to you that instead you do things like

dq = diff(q, t)
f = some expression
f_pretty = subs(f, dq, q_dot)

[Walter Roberson]

Note that you will need to define q as a function or else the diff() will return 0.

Also I recommend that you read through the first example in the documentation for odeFunction()