How to find all double expressions added to a very long formula?

Given a large formula how do i find all expressions multiplied by two terms such as.

syms m1 m2 l1 l2 thetadotdot1 thetadotdot2 thetadot1 thetadot2 theta1 theta2 g; n=1;

t1=m2*l2^2*(thetadotdot1+thetadotdot2)+m2*l1*l2*cos(theta2)*(2*thetadotdot1+thetadotdot2)+(m1+m2)*l1^2*thetadotdot1-m2*l1*l2*sin(theta2)*(thetadot2^2)-2*m2*l1*l2*sin(theta2)*(thetadot1*thetadot2)+m2*l2*g*cos(theta1+theta2)+(m1+m2)*l1*g*cos(theta1);

The output should be

-2*m2*l1*l2*sin(theta2)

Where the coeficient is

(thetadot1*thetadot2)

Currently I am using this method:

  feval(symengine, 'coeff', t1, thetadot1,n)/thetadot2;

But I want a better bethod that this, as this might not work for all cases.

Answer 1

[Ayush Modi]

Hi Yusuf,

I understand you want to get the coefficient values of all the terms which contains thetadot1*thetadot2.

You can achieve this using "coeffs" function. Here is the code to get you started:

syms m1 m2 l1 l2 thetadotdot1 thetadotdot2 thetadot1 thetadot2 theta1 theta2 g; n=1;

t1=m2*l2^2*(thetadotdot1+thetadotdot2)+m2*l1*l2*cos(theta2)*(2*thetadotdot1+thetadotdot2)+(m1+m2)*l1^2*thetadotdot1-m2*l1*l2*sin(theta2)*(thetadot2^2)-2*m2*l1*l2*sin(theta2)*(thetadot1*thetadot2)+m2*l2*g*cos(theta1+theta2)+(m1+m2)*l1*g*cos(theta1)

t1 = 

target_term = thetadot1 * thetadot2;
t1_factored = factor(t1);
[coeffs_array, terms_array] = coeffs(t1_factored, [thetadot1, thetadot2]);
index = find(terms_array == target_term);
coefficient = coeffs_array(index)

coefficient = 

Refer to the following MathWorks documentation for more information on "coeffs" function:

https://www.mathworks.com/help/releases/R2019b/symbolic/coeffs.html?s_tid=doc_ta