I'm trying to use Sympy to rearrange simple expressions; although it seems that Sympy refuses to keep the result as a symbolic expression, and instead evaluates the logarithm numerically.
Is there a way to force Sympy to return the result symbolically?
Here is a minimum working example:
import sympy as sy
sy.init_printing()
def rearrange(expression, lhs):
rhs = sy.solve(expression,lhs)[0]
return sy.Eq(lhs, rhs)
a, b = sy.symbols('a, b', real=True, positive=True)
eqn = sy.Eq(sy.exp(-a**2/b**2), 0.5)
rearrange(eqn, a)
which returns
a=0.832554611157698b
This is the correct answer of course, although I would have preferred to be told the symbolic algebra result, which (when derived manually) is:
a = sqrt(log(2)) * b
where log is base e (i.e. natural log, ln).
How can I obtain this result from Sympy please?
not a sympy expert, but given the limited accuracy of float, sympy can't be sure 0.5 == 1/2. i rearranged the equation a bit so only integers come into play (Fractions or something similar will certainly also do the trick)
eqn = sy.Eq(2*sy.exp(-a**2/b**2), 1)
with the result
a == b*sqrt(log(2))
but as soon as you have any floats in your equation, you will likely get floats back. also in plain python: 2 * 0.5 -> 1.0.
just tried: this also works:
eqn = sy.Eq(sy.exp(-a**2/b**2), Fraction(1, 2))