Symbolic math- substitute a derivative function for a variable - subs

I want to substitute in the following matrix result: A=[diff(theta(t),t) -diff(psi(t),t) -diff(theta(t),t) diff(psi(t),t)] I want to substitute diff(theta(t),t) for a variable called thetap and diff(psi(t),t) for a variable called psip in orden to obtain the following matrix A=[thetap -psip -thetap psip]

then I tried to do this:

theta2 = sym('theta2(t)') psi2 = sym('psi2(t)'

A=[diff(theta(t),t) -diff(psi(t),t) -diff(theta(t),t) diff(psi(t),t)]

dtheta = diff(theta,t) dpsi = diff(psi,t)

Ap_s = subs(A,{dtheta,dpsi},{thetap,psip})

but it doesn't work. It returns again the dependent expression diff(theta(t),t) and diff(psi(t),t) without doing the substitution. I've tried: Ap_s = subs(A,{diff(theta(t),t),diff(psi(t),t)},{thetap,psip}) but it doesn't work. Can anybody help me?

Answer 1

[Jaskirat]

I see that you are using the “subs” function of the Symbolic Math Toolbox from MATLAB.

The given substitution of differential expressions in a matrix with other symbols can be achieved by using “subs” and “str2sym”.

Here is an example code which shows the process of substitution-

syms theta(t) t psi(t) 

A=[diff(theta(t),t) -diff(psi(t),t); -diff(theta(t),t) diff(psi(t),t)] 

subs(A,[str2sym("diff(theta(t),t)"),str2sym("diff(psi(t),t)")],[str2sym('theta2(t)'),str2sym('psi2(t)')]) 

The following documentation links can be referred to get more information about “subs” function :

https://www.mathworks.com/help/symbolic/sym.subs.html