Can't susbsitute a variable that is defined

I have solved a system of ODEs using dsolve. They create a set of symbolic variables inside the solutions when integrating as expected. When I check to see the name of these integrationg constants one of these is called "C1". Now, I want to substitue using subs that integration variable for a number.

symvar(Vb_sol) %Check the name of the constants inside my ODE solution
V_sub = subs(Vb_sol,C1,1) %Replace C1 (integration constant) by 1

However, when I try do do the substitution I get the "Undefined function or variable 'C1'" error.

I have attached the variable "Vb_sol", which is a function of C1, and an example script trying to do the substituion.

Thank you

Answer 1

[Paul]

Do

syms C1

before calling subs.

syms y(t) a
eqn = diff(y,t) == a*y;
S = dsolve(eqn)

S = 

syms C1
subs(S,C1,2)

ans = 

[Jacques Barsimantov]

Thanks, that worked!