Hi,
This function is really useful, but I'm having problems with combinations
of sqrt and pow: is there any way to keep the sqrts as they are, but expand
integer pows?
Thanks,
Stewart
On Thursday, 28 June 2012 15:53:57 UTC+1, Spencer Ogden wrote:
>
> Aaron,
>
> Thanks for your lead on evaluate=False and the example function. Very
> close for iPad typing and no error checking. I switched around the list
> comprehensions a bit and got it working this way:
>
> from sympy import Symbol, Mul, Pow, pprint, Matrix, symbols
>
> a = Symbol('a')
> exp = a**2
> print(exp)
>
> def pow_to_mul(expr):
> """
> Convert integer powers in an expression to Muls, like a**2 => a*a.
> """
> pows = list(expr.atoms(Pow))
> if any(not e.is_Integer for b, e in (i.as_base_exp() for i in pows)):
> raise ValueError("A power contains a non-integer exponent")
> #repl = zip(pows, (Mul(*list([b]*e for b, e in i.as_base_exp()),
> evaluate=False) for i in pows))
> repl = zip(pows, (Mul(*[b]*e,evaluate=False) for b,e in
> (i.as_base_exp() for i in pows)))
> return expr.subs(repl)
>
> print(pow_to_mul(exp))
>
> This outputs:
> a**2
> a*a
>
> Thanks again, just what I needed,
>
> Spencer
>
> On Wednesday, June 27, 2012 9:41:04 PM UTC-4, Aaron Meurer wrote:
>>
>> I don't think there's a built-in function to do this, but it shouldn't be
>> too hard to write your own, using .atoms to find the Pow's and .xreplace
>> (or .subs if you are not using the development version) to replace them.
>> Note that SymPy automatically converts a*a to a**2, so to keep it as a*a,
>> you have to use Mul(a, a, evaluate=False).
>>
>> evaluate=False is somewhat of a hack, so be aware that it is fragile.
>> Some functions will reevaluate the expression, converting it back to Pow.
>> Other functions will break because some expected invariant will be broken
>> by the evaluate=False expression (e.g., I doubt factor() would work
>> correctly). So I would not do this until the very end, before you send it
>> to SQL.
>>
>> Something like this should work:
>>
>> def pow_to_mul(expr):
>> """
>> Convert integer powers in an expression to Muls, like a**2 => a*a.
>> """
>> pows = list(expr.atoms(Pow))
>> if any(not e.is_Integer for b, e in i.as_base_exp() for i in pows):
>> raise ValueError("A power contains a non-integer exponent")
>> repl = zip(pows, (Mul(*([b]*e for b, e in i.as_base_exp()),
>> evaluate=False) for i in pows)
>> return expr.subs(repl)
>> # Or, in the development version, a better way is
>> # return expr.xreplace(dict(repl))
>>
>> Disclaimer: I typed the above function on my iPad without even checking
>> if the syntax was correct. It should work, though, assuming I remembered
>> all my interfaces correctly and didn't forget something.
>>
>> Aaron Meurer
>>
>> On Jun 27, 2012, at 7:09 PM, Spencer Ogden
>> <[email protected]<javascript:>>
>> wrote:
>>
>> I'm trying to use sympy to expand matrix math into valid SQL code. SQL
>> doesn't support ** of course, so I would like to expand terms like a**2 to
>> a*a. In the general context of a CAS, this is a silly thing to do, so I
>> haven't been able to locate a function that would do this (expand, replace,
>> rewrite).
>>
>> Is this possible?
>>
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