parsing expressions with multiple subscripts

[Peter Brady]

Hello all,

I'm trying to parse expressions with multiple subscripts and am running into errors.

For example, the following works:

sympy.symbol("a_{2,2}")

But parsing an expression does not:

sympy.S("a_{2,2} = a_{3,1}+2")

Can someone explain how to make this work?

Thanks,

Peter.

[Aaron Meurer]

S/sympify only work with valid Python code. A Symbol name can contain
any string, but it will only be parsable in a larger expression if it
is a valid Python name.

You can try using parse_expr from sympy.parsing.sympy_parser. It is
designed to parse some things that aren't valid Python. However, as
far as I know, it hasn't been designed to work with non-Python
variable names, so it would probably need some extension to work with
this.

The other way that always works is to wrap the name with Symbol, like
sympify("Symbol('a_{2, 2}')"). You also need do some extra things to
parse the =.

Aaron Meurer

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[Peter Brady]

Thanks for clarifying Aaron.  I think I can write the symbols without the comma and brackets since only the second the index can be larger than 10.

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