On Sun, Apr 24, 2022 at 2:28 AM Andre Bolle <andrebo...@gmail.com> wrote:
>
> So true. I deleted my post soon after I posted it, in case anyone took it
> seriously. But it was intended as a cosmetic work-around. I presume a
> var('a', commutative=True) immediately after would remedy that.
Not quite. Symbols aren't global in SymPy (nothing in SymPy works
globally). If you do
F = symbols('F')
m, a = symbols('m a', commutative=False)
eqn = Eq(F, m*a)
m, a = symbols('m a')
then what would happen is that the expression in eqn would have the
noncommutative symbols and the Python variables 'm' and 'a' would
reference new, separate symbols.
A symbol in SymPy is determined by its name and assumptions, and
commutative is an assumption, so the m and a defined in line 2 would
be different from the m and a defined in line 4.
For example:
>>> m, a = symbols('m a', commutative=False)
>>> expr = m*a
>>> m, a = symbols('m a')
>>> expr == m*a
False
If you wanted to "fix" the expression to not have noncommutatives,
you'd have to keep track of the symbols you wanted to swap out and use
subs, like
>>> m_nc, a_nc = symbols('m a', commutative=False)
>>> m, a = symbols('m a')
>>> expr = m_nc*a_nc
>>> expr.subs({m_nc: m, a_nc: a})
a*m
Aaron Meurer
>
> On Sunday, April 24, 2022 at 5:01:49 AM UTC+1 asme...@gmail.com wrote:
>>
>> I would only do that just for the printing. Telling SymPy that two
>> symbols don't commute means that it will no longer do any
>> simplifications on it that aren't valid without commuting them.
>>
>> Aaron Meurer
>>
>> On Sat, Apr 23, 2022 at 3:32 AM Andre Bolle <andre...@gmail.com> wrote:
>> >
>> > You can always tell Sympy that as far as I am concerned "a" is
>> > non-commutative. Then tradition is preserved.
>> >
>> > >>> var('F m')
>> > (F, m)
>> > >>> var('a')
>> > a
>> > >>> print(Eq(F,m*a))
>> > Eq(F, a*m)
>> > >>> var('a', commutative=False)
>> > a
>> > >>> print(Eq(F,m*a))
>> > Eq(F, m*a)
>> > On Thursday, April 21, 2022 at 8:23:51 PM UTC+1 Andre Bolle wrote:
>> >>
>> >> I think mathematicians prefer the order of symbols preserved.
>> >>
>> >> On Thursday, April 21, 2022 at 7:54:20 PM UTC+1 asme...@gmail.com wrote:
>> >>>
>> >>> On Thu, Apr 21, 2022 at 12:16 PM Oscar Benjamin
>> >>> <oscar.j....@gmail.com> wrote:
>> >>> >
>> >>> > On Thu, 21 Apr 2022 at 18:53, Aaron Meurer <asme...@gmail.com> wrote:
>> >>> > >
>> >>> > > If you want the expression to print in the same way that it was
>> >>> > > originally entered, then it needs to keep track of that information
>> >>> > > when it is created. That's not something that currently happens.
>> >>> > >
>> >>> > > If you just want the printers to have more options in how they sort
>> >>> > > things, that is technically already possible, although it's not
>> >>> > > particularly straightforward and not very well documented.
>> >>> >
>> >>> > It should be easier to do each of these things. At the moment you can
>> >>> > create an Add(y, x, evaluate=False) and the printers do have an
>> >>> > "order" setting (for init_printing) but there isn't a value for the
>> >>> > order parameter that just prints the terms in the exact order that
>> >>> > they appear in the Add.
>> >>> >
>> >>> > This is a very commonly requested option on StackOverflow etc. It
>> >>> > would be good to fix it so that the printers can just print in args
>> >>> > order.
>> >>>
>> >>> The order flag needs to be documented much better. Also I think it
>> >>> would be useful if you could just pass a sort key function directly to
>> >>> the printer, with some examples of how to do that.
>> >>>
>> >>> Aaron Meurer
>> >>>
>> >>> >
>> >>> > --
>> >>> > Oscar
>> >>> >
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