On Fri, Sep 16, 2022 at 3:37 PM Yang Liu <micat...@gmail.com> wrote:
>
> Hi Aron,
>
> Many thanks for your answers. I previously use say f.args[0] to access input
> like \nabla*f to check. But has() is more error free.
>
> Is there any more robust way to get the specific args symbol? For instance,
> if the input is like (expr=x*y*z). But I want to get the x from this
> expression. I would say expr.arg[0]. But if I change the expression order.
> This doesn't work.
expr.free_symbols will give you a set of all the symbols in an
expression. Does that give you what you need?
If not, I'm not clear what you mean by "the x from this expression".
Note that if you just want a symbol named "x" you can create another
one with Symbol('x'). SymPy treats any two symbols with the same name
and assumptions as equal, so recreating a symbol with the same name is
equivalent to reusing the same symbol object.
Aaron Meurer
>
> Thanks!
>
> On Friday, September 16, 2022 at 9:27:10 PM UTC+2 asme...@gmail.com wrote:
>>
>> You can name a symbol any string you want, so Symbol(r'\nabla') will
>> create a symbol named \nabla.
>>
>> To test if a symbol is in an expression, use .has():
>>
>> >>> (x + 1).has(x)
>> True
>> >>> (y + 1).has(x)
>> False
>>
>> Aaron Meurer
>>
>> On Fri, Sep 16, 2022 at 6:26 AM Yang Liu <mica...@gmail.com> wrote:
>> >
>> > Hi,
>> >
>> > I want to represent the \nabla symbolically without calculation. Is there
>> > a way to declare such a symbol? Also for \triangle?
>> >
>> > Moreover, I want to write a function say F(\nabla u) take \nabla*u as an
>> > input. Is there a way to detect if I pass the "\nabla" into the function?
>> > Pseudo code would be something like:
>> >
>> > F(expression):
>> > if \nabla in expression:
>> > do something
>> > else:
>> >
>> > Many Thanks!
>> >
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>
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