On Aug 29, 2008, at 2:24 AM, Nils Skoruppa wrote:
> On 28 Aug., 15:20, "John Cremona" <[EMAIL PROTECTED]> wrote:
>> Thanks for your comments, David. I am having some success:
>>
>> The one thing I cannot get to work in the additive case is (for
>> example) 2*g where g is a group element. I have tried all possible
>> combinations of __lmul__, _lmul_, __rmul__, _rmul_, but although I
>> have this ok:
>>
>> sage: a,b=A.gens()
>> sage: b._lmul_(20)
>> 2*b
>> sage: 2*b
>>
>> inputting 20*b gives an error:
>> TypeError Traceback (most recent
>> call last)
>
>>>> John
>
> Hi John,
>
> I have a class which derives from AdditiveGroupElement and whose
> parent derives from AbelianGroup. Concerning multiplication I did not
> implement more than __mul__() with two underscores ( and no _lmul_,
> _rmul_, mil_impl-c ... and the like). It seems to work OK:
>
> sage: A.<a,b,c> = FiniteQuadraticModule ('3^3')
> sage: 5*a
> 2*a
> sage: b*7
> b
> sage: -a +
> 2*c
> 2*a + 2*c
>
> I cannot explain why this works, but if you want to have a look:
> http://hg.countnumber.de/fqm-devel/file/98bb736f0c07/cn_group/
> finite_quadratic_module.sage
> (and then line 2132 class
> FiniteQuadraticModuleElement(AdditiveGroupElement).
Here is what should be implemented (and I just tried it out and it
does for me with the latest Sage). Most of the coercion model focus
has been considered in the context of Rings, but groups are fine too.
Let me know if this doesn't work for you.
from sage.groups.group import AbelianGroup
from sage.structure.element import AdditiveGroupElement
class MyAbelianGroup(AbelianGroup):
def __call__(self, n):
return MyAdditiveGroupElement(self, n)
class MyAdditiveGroupElement(AdditiveGroupElement):
def __init__(self, parent, n):
AdditiveGroupElement.__init__(self, parent)
self.n = n
def __repr__(self):
return "My(%s)" % self.n
def _add_(left, right):
# Guaranteed left and right have the same parent.
return MyAdditiveGroupElement(left.parent(), left.n + right.n)
def _rmul_(self, a):
# Guaranteed a is in the basering (probably ZZ)
return MyAdditiveGroupElement(self.parent(), self.n * a)
def _lmul_(self, a):
# Guaranteed a is in the basering (probably ZZ)
return self._rmul_(a) # should this be a default?
sage: G = MyAbelianGroup()
sage: G.base_ring()
Integer Ring
sage: G(3) + G(9)
My(12)
sage: 7*G(3)
My(21)
sage: G(2)*5
My(10)
sage: sum([G(2), G(3), G(7)])
My(12)
sage: G(55) * int(4)
My(220)
One thing that mystifies me is that I though repeated doubling was
supposed to be provided by default if neither _rmul_ nor _lmul_ were
provided. I'll have to look at this, but it's orthogonal to what you
are trying to do.
- Robert
--~--~---------~--~----~------------~-------~--~----~
To post to this group, send email to [email protected]
To unsubscribe from this group, send email to [EMAIL PROTECTED]
For more options, visit this group at http://groups.google.com/group/sage-devel
URLs: http://www.sagemath.org
-~----------~----~----~----~------~----~------~--~---
