Regarding 1), if you write 1/3, then Python converts this into a float
before it is handed over to sympy. To prevent it, you could import S
(Singleton) from sympy and write S(1)/3.
dexpr_dx = se.Derivative(expr, x, 1).xreplace({y*(x**2 + (-S(1)/3)*y**2) +
(-S(1)/2)*x**2 + (-S(1)/2)*y**2: V(x, y)})
[email protected] schrieb am Dienstag, 25. Januar 2022 um 21:53:27 UTC+1:
> There are a few reasons this is not working.
>
> 1) if you have floats like (1/3) those will not match the rationals like
> Rational(1, 3)
> 2) xreplace will target full nodes, not sub-nodes. So when trying to match
> an Add that is a subexpression, it will fail.
> 3) you have a sign error in the key of the replacement dictionary: the
> first term should be `-y*...`
> 4) the replacement value, `V(x, y)` recreates the thing you are replacing.
>
> Try the following:
>
> >>> d = Derivative(expr, x, 1)
> >>> d.subs(V(x,y), Symbol('V(x,y)'))
> Derivative(1/(Ho - V(x,y)), x)
>
>
> /c
> On Monday, January 24, 2022 at 3:28:50 PM UTC-6 Audrius-St wrote:
>
>> Hello, In the following test code I am attempting to simplify the
>> resulting expression
>> by replacing a subexpression with a function
>>
>> import symengine as se
>> import sympy as sp
>>
>> # Potential V
>> def V(x, y):
>> v = (x*x + y*y)/2 + y*(x*x - (y*y)/3)
>> return v
>>
>> def main():
>>
>> # Symbols
>> x, y, Ho = sp.symbols('x, y, Ho')
>>
>> expr = sp.Pow(Ho - V(x, y), -1)
>> dexpr_dx = se.Derivative(expr, x, 1).xreplace({y*(x**2 + (-1/3)*y**2)
>> + (-1/2)*x**2 + (-1/2)*y**2: V(x, y)})
>>
>> print("dexpr_dx =", dexpr_dx)
>>
>> if __name__ == "__main__":
>> main()
>>
>> Taking in derivative w.r.t. x resulting in V(x, y) being replace the
>> expression
>> dexpr_dx = -(-x - 2*x*y)/(Ho - y*(x**2 + (-1/3)*y**2) + (-1/2)*x**2 +
>> (-1/2)*y**2)**2
>>
>> which I would like to rewrite as -(-x - 2*x*y)/(Ho - V(x, y))**2 as I
>> will be repeating the differentiation in the actual code.
>> In other words, I would like to reduce the output code bloat.
>>
>> I've tried subs(), replace(), and xreplace() with no success. Any insight
>> in how to do this would be appreciated.
>>
>>
>>
>>
>>
>>
>>
>>
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