Sympy returns a ConditionSet object when solving an equation, while matlab returns a single float when the same equation is solved

问题

I have 4 input variables (floats):

Xmax Xmin percentage mode and I want to solve the following (rather long) equation for s:

> (1/2+1/2*erf((log(Xmax)-(log(mode)+s**2))/(sqrt(2)*s))-(1/2+1/2*erf((log(Xmin)-(log(mode)+s**2))/(sqrt(2)*s))) - percentage == 0

I want to use sympy to solve the equation, but it returns a ConditionSet Object (meaning that it cannot solve the equation)

My code is as follows:

from sympy import *

def CalcScaleParam(mode, CIfact, percentage):
    s = Symbol('s', Real=True)
    eqn = (1/2+1/2*erf((log(4)-(log(2)+s**2))/(sqrt(2)*s))-(1/2+1/2*erf((log(1)-(log(2)+s**2))/(sqrt(2)*s)))) - 0.95
    sigma = solveset(eqn, s)
    print(sigma)

CalcScaleParam(2,2, 0.9545)

My Matlab code is as follows:

function sigma = Test()

syms s
eqn =(1/2+1/2*erf((log(4)-(log(2)+s^2))/(sqrt(2)*s))-(1/2+1/2*erf((log(1)-(log(2)+s^2))/(sqrt(2)*s)))) - 0.95 == 0;
sigma = solve(eqn,s);

end

And returns sigma=0.335

I initially thought it was an issue with the math but since the equation is solved successfully in matlab I suspect that the problem is coming from sympy.

回答1:

The fractions are using Python's integer division, which gives 1/2 == 0. The entire expression reduces to -0.95. Declare it using 1./2 or 0.5, or maybe Rational(1,2). You might also have to express the percentage 0.95 as Rational(19,20).

来源：https://stackoverflow.com/questions/37818218/sympy-returns-a-conditionset-object-when-solving-an-equation-while-matlab-retur