On Jun 4, 12:28 pm, Burcin Erocal <[EMAIL PROTECTED]> wrote:
> On Tue, 3 Jun 2008 22:13:25 -0700 (PDT)
>
>
>
> Josh <[EMAIL PROTECTED]> wrote:
> > I want to add elements of the fraction field of QQ[x,y] to a set,
> > i.e., make sure there are no repeats. However, set.add()ing an
> > element that is equal to a previous element sometimes puts another
> > copy of the element in. Witness the following behavior:
>
> > > R = PolynomialRing(QQ,'x,y')
> > > K = R.fraction_field()
> > > x,y = gens(K)
> > > (2*x)/(2*y) == x/y
>
> > True
>
> > > x/y in set([(2*x)/(2*y)])
>
> > False
>
> > > x/y in [(2*x)/(2*y)]
>
> > True
>
> > Why should these last two behaviors differ?
>
> Python's set uses the hash value of the elements for comparison, and
> list just uses the cmp operator. Since we have mathematically
> equivalent elements hashing to two different values, this fails.
>
> The fraction field code in Sage is needs a makeover. I will probably do
> some of that during (or maybe before) the developer days in Seattle
> between June 13 and 20.
>
> > And how do I get the
> > second behavior out of a set without converting to a list and back?
> > For example, is there some way to force elements of K into a canonical
> > form?
>
> You can construct the fraction field of ZZ[x,y]. The arithmetic is
> slower than QQ[x,y] polynomials, but at least it works.
>
> sage: K = ZZ[x,y].fraction_field()
> sage: x,y = K.gens()
> sage: (2*x)/(2*y)
> x/y
> sage: x/y in set([(2*x)/(2*y)])
> True
>
> Cheers,
>
> Burcin
Hi Burcin,
can you open ticket for this and the following issue [in case it is a
different non-hashing related issue] and post the trac ticket number
here? I would hate it if we lost bug reports.
Cheers,
Michael
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