Dear Andrew,
On Sun, Aug 19, 2012 at 04:52:45PM -0700, Andrew Mathas wrote:
> Recently when playing around with standard tableaux, which are enumerated
> sets, I rewrote their getitem methods because I thought that they should
> support slices. At the time I thought that this should go into the
> enumerated set code but I wasn't sure where to put it. Modulo the
> documentation, __getitem__ for the standard tableaux classes now looks
> like this:"
> def __getitem__(self, r):
...
> I don't know whether this is generally thought to be a good idea, but I
> like being able to write things like:
>
> sage: StandardTableaux(40)[0:10]
Yes, this is a practical feature for interactive use. On the other
hand, it should not be used in code manipulating a generic enumerated
set. Indeed, for many of those we want to reserve indexed access for
something more mathematically meaningful; think for example a family,
like the basis of a vector space indexed by the partitions of 3:
sage: F = CombinatorialFreeModule(QQ, Partitions(3)).basis()
sage: F
Lazy family (...)_{i in Partitions of the integer 3}
sage: F[Partition([2,1])]
B[[2, 1]]
The default __getitem__ for parents is implemented in Parent. It
really should be in (Finite)EnumeratedSets (see #12955). And
supporting slices could be added there indeed. With an appropriate
warning mentioning that this should be reserved for interactive use or
non generic code.
As far as I know, no one is currently working on #12955.
> Btw, I also think that the InfiniteEnumeratedSet and FiniteEnumerated set
> categories should define an is_finite() method which returns False and
> True, respectively. As far as I could see, there is no easy way to ask
> them whether they are finite or infinite.
They do! With Sage 5.2 with no patches applied:
sage: f = InfiniteEnumeratedSets().example(); f
An example of an infinite enumerated set: the non negative integers
sage: f.is_finite()
False
sage: f.is_finite??
Source File:
/opt/sage-5.2/devel/sage/sage/categories/infinite_enumerated_sets.py
def is_finite(self):
return False
sage: f = FiniteEnumeratedSets().example()
sage: f.is_finite()
True
sage: f.is_finite??
Source File:
/opt/sage-5.2/devel/sage/sage/categories/finite_enumerated_sets.py
def is_finite(self):
return True
Cheers,
Nicolas
--
Nicolas M. Thiéry "Isil" <nthi...@users.sf.net>
http://Nicolas.Thiery.name/
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