CODEWITHSUNDEEP
python-3.xmathsympyequation-solving
Daniel
Daniel

Reputation: 311

How to solve an equation in Python 3 with Sympy?

I new with Python and much more with Sympy, I'm trying to solve an equation; the user introduces the equation and the value of 'x'. Example: eq: x**2+x*3+1, x = 4, so the 4 should be replaced on the 'x' of the equation and calculate the result.

This is what I have been trying:

import sympy
from sympy.solvers import solve
from sympy.core.function import Derivative
from sympy.core.relational import Eq
from sympy.core.symbol import Symbol

x = Symbol('x')

info = input("Introduce the equation and the value of /'x' \nEg: x**3+x**2+1 9\n")

infoList = info.split()

equation = infoList[0]
x_value = infoList[1]

expr = equation
eqResult = expr.subs(x, x_value)

print(eqResult)

The error is that the expr, which is an str object, has no attribute subs. Any help is appreciated.

UPDATE

The function eval() works fine, but is there a solution less risky? I have been doing other stuff and tried to calculate the Derivative of the function:

x = Symbol('x')

info = input("Introduce the equation and the value of /'x' \nEg: x**3+x**2+1 9\n")

infoList = info.split()

equation = infoList[0]
x_value = infoList[1]

exprDerivada = Derivative(equation, x)
resultDerivate = exprDerivada.doit().subs({x:x_value})

The code above gives the result of the first derivate of a function, but the equation is accepted as it comes without the function: eval()

Again, any help is appreciated.

Upvotes: 0

Views: 203

Answers (1)

Mayeul sgc
Mayeul sgc

Reputation: 2089

You are sending a string object that indeed doesn't have a subs property
Use eval(expr).subs(.....) to convert the string to the math expression.
However eval() is a risky stuff to use as it will execute pretty much anything. So be sure to properly secure the user input or you could get you system destroyed by malicious user input. See this answer for a detailed explanation.

Upvotes: 1

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