CODEWITHSUNDEEP
pythonsympy
Uwe.Schneider
Uwe.Schneider

Reputation: 1415

Evaluating the solution of sympy dsolve numerically

I would like to evaluate the solution of a differential equation against some x_test array

from sympy import *
init_printing()

from __future__ import division
from sympy import *
x, y, z, t = symbols('x y z t')

# Constants
C, R, u_rest = symbols('C R u_rest')

f, g, h = symbols('f g h', cls=Function)

solution = dsolve(C*Derivative(f(x), x) + (1/R)*(f(x)  - u_rest ),f(x))

x_text = np.array(range(0,100))

but I fail

# 1. attempt with evalf(Not working)
solution.args[1].evalf(subs={x: 3.14})

# 2. attempt with lambdify(Not working)
lambdify(x,solution.args[1])(3.14)

What is the right way to do it?

Upvotes: 0

Views: 266

Answers (1)

smichr
smichr

Reputation: 19057

If you look at your solution.args[1] value you will see that it is an expression of several variables, including x. It will not evaluate to a number until you supply values for all variables. Your first attempt doesn't fail, but you don't explain why it is not giving you what you hoped for:

>>> solution.args[1].evalf(subs={x: 3.14})
u_rest*(1.0 - exp((C1 - 3.14/R)/C))

Upvotes: 1

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