Python equation solver, I'm getting a floating point error

[Arman777]

I am trying to solve the equation like this,

from sympy.solvers import solve
from sympy import Symbol
import math

x = Symbol('x')

A, B, C, D = 0.59912051, 0.64030348, 263.33721367, 387.92069617

print(solve((A * x) + (B * math.sqrt(x**3)) - (C * math.exp(-x / 50)) - D, x , numerical = True))

Traceback (most recent call last):
  File "c:/Users/***/Desktop/Python Stuff/***", line 9, in <module>
    solve((A * x) + (B * math.sqrt(x**3)) - (C * math.exp(-x / 50)) - D, x , numerical = True)
  File "C:\Users\***\AppData\Local\Programs\Python\Python37\lib\site-packages\sympy\core\expr.py", line 280, in __float__
    raise TypeError("can't convert expression to float")
TypeError: can't convert expression to float

Why this is happening ?Thanks

 

[Mark44]

It's possible that what solve returns isn't something that print can handle. I would try this:

solve((A * x) + (B * math.sqrt(x**3)) - (C * math.exp(-x / 50)) - D, x)

Also, from my reading of the docs for solve, you don't need to include the numerical = True part, since that is the default.

 

[Arman777]

I tried that however its still giving me the same error.

 

[BvU]

Error goes away if I remove 'math.sqrt' and 'math.exp'

I find one reference saying you shouldn't use 'math' in sympy, but it doesn't give a workaround.

 

[StoneTemplePython]

Sympy is there for symbolic computations. The math library is there for numerics (typically bad ones, basically anything you'd want in there is in numpy)

so import sqrt and exp from sympy, get your final symbolic expression, then use .evalf() at the end to get it as floating point.

 

[Arman777]

I tried and it worked. However the python couldn't solve the equation..

 

[BvU]

I still get an error

from sympy.solvers import solve
from sympy import Symbol
from sympy import sqrt
from sympy import exp

x = Symbol('x')

A, B, C, D = 0.59912051, 0.64030348, 263.33721367, 387.92069617

print(solve((A * x) + (B * sqrt(x**3)) - C*exp(-0.02*x) - D, x , numerical = True))

line 10:
    print(solve((A * x) + (B * sqrt(x**3)) - C*exp(-0.02*x) - D, x , numerical = True))
  File "H:\Program Files\Python36\lib\site-packages\sympy\solvers\solvers.py", line 1171, in solve
    solution = _solve(f[0], *symbols, **flags)
  File "H:\Program Files\Python36\lib\site-packages\sympy\solvers\solvers.py", line 1742, in _solve
    raise NotImplementedError('\n'.join([msg, not_impl_msg % f]))
NotImplementedError: multiple generators [x, exp(x/50), sqrt(x**3)]
No algorithms are implemented to solve equation 59912051*x/100000000 + 16007587*sqrt(x**3)/25000000 - 38792069617/100000000 - 26333721367*exp(-x/50)/100000000

If I leave out the exp, there is a result (66.6 and two complex numbers).

Is there a way to force x real ?

 

[StoneTemplePython]

Is there a way to force x real ?

x = Symbol('x', real = True)

you can also do things like positive = True, if so inclined

 

[Arman777]

I kind of find the condition set however I am not sure how that can help

import sympy as sym
x = sym.Symbol('x')

A, B, C, D = 0.59912051, 0.64030348, 263.33721367, 387.92069617
r = sym.solveset((A * x) + (B * sym.sqrt(x**3)) - (C * sym.exp(-x / 50)) - D, x)
print(r)

ConditionSet(x, Eq(0.40998854650011*x**3*exp(x/25) - 0.35894538550266*x**2*exp(x/25) + 315.541451511899*x*exp(x/50) + 464.822490657851*x*exp(x/25) - 204307.910508669*exp(x/50) - 150482.466517017*exp(x/25) - 69346.4881034792, 0), Complexes(Reals x Reals, False))

or in pprint form

⎧                                 x                         x
⎪                                 ──                        ──
⎨                              3  25                     2  25
⎪x | x ∊ ℂ ∧ 0.40998854650011⋅x ⋅ℯ   - 0.35894538550266⋅x ⋅ℯ   + 315.541451511
⎩

       x                        x                      x
       ──                       ──                     ──
       50                       25                     50
899⋅x⋅ℯ   + 464.822490657851⋅x⋅ℯ   - 204307.910508669⋅ℯ   - 150482.466517017⋅ℯx                        ⎫
──                       ⎪
25                       ⎬
   - 69346.4881034792 = 0⎪
                         ⎭

 

[Arman777]

The problem can be solved by using binary search. I solved it