On 19/09/2020 23:46, Oscar Benjamin wrote:
On Sat, 19 Sep 2020 at 23:35, David Bailey<d...@dbailey.co.uk> wrote:
On 19/09/2020 20:50, Davide Sandona' wrote:
Hello David,
in Sympy there are three substitution methods:
1. subs: it is the most generic.
2. xreplace: it only replace the exact expression you provide.
3. replace: this is the most powerful, as it allows for "pattern matching
operations".
In your case, I would do something like this:
f, g = symbols("f, g", cls=Function)
a, b, c = symbols("a:c")
expr = f(a + 2 * b + c)
expr.replace(f, g)
Thanks for that quick response! Unfortunately I don't think your solution
really generalises. I mean suppose I wanted to replace
f(anything) by g(5*anything**2)
Hi David,
You can do this using replace but instead of replacing f with g you
can ask to replace all instances of f with a function that will
process the argument(s) that f had. It is convenient to do this in
Python using a lambda function:
In [7]: f = Function('f')
In [8]: g = Function('g')
In [9]: expr = f(1 + sqrt(2))
In [10]: expr.replace(f, lambda arg: g(5*arg**2))
Out[10]:
⎛ 2⎞
g⎝5⋅(1 + √2) ⎠
I'm not sure how well that displays in the email. It looks pretty
screwy in gmail as I write it but I think it does what you want!
Oscar
Thanks Oscar,
Wow - that is ingenious, and it will take me a while to think out how
much of Mathematica's pattern functionality can be achieved this way.
However, I am curious, What are the patterns produced by the Wild
function actually used for - or are these an attempt to reproduce
Mathematica's pattern matching functionality (which is really neat) that
is yet to be finished?
David
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