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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