#18940: Polynomials ignore the step argument in __getitem__
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Reporter: pbruin | Owner:
Type: defect | Status: needs_info
Priority: major | Milestone: sage-6.10
Component: algebra | Resolution:
Keywords: | Merged in:
Authors: Peter Bruin | Reviewers:
Report Upstream: N/A | Work issues:
Branch: | Commit:
u/pbruin/18940-getitem_step | 1c56779b3140d12fbe5d0973cdcad0ac9dbb8054
Dependencies: #19409 | Stopgaps:
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Comment (by tscrim):
Replying to [comment:25 jdemeyer]:
> Replying to [comment:24 tscrim]:
> > That depends on if we want a slice of a polynomial returns a
polynomial or a simple list of the coefficients. I am (very) slightly in
favor of returning a polynomial, so with that `[0, 2, 4, 6, 8]`.
>
> Sorry, I asked a bad question so I cannot interpret your answer. Please
try again with this question:
> {{{
> sage: pol = PolynomialRing(QQ, 'x')(range(1,10))
> sage: list(pol[3::2])
> }}}
I would want this:
{{{
sage: list(pol[3::2])
[0, 0, 0, 4, 6, 8]
}}}
(Actually, my answer above should have had 2 leading 0's: `[0, 0, 2, 4, 6,
8]`.)
However, when trying to think about how this would work for Laurent
polynomials, we currently have this behavior for `list`:
{{{
sage: R.<x> = LaurentPolynomialRing(QQ)
sage: p = ~x + 2 + 3*x + 4*x^2
sage: list(p)
[1, 2, 3, 4]
sage: p = 3*x + 4*x^2
sage: list(p)
[3, 4]
}}}
In some ways I would say the iterator for polynomials should be changed to
agree with that of Laurent polynomials, in that it starts from the lowest
degree. Although I understand there are many (good) reasons why we do not
want that behavior. I feel that at some point we just have to choose the
least bad of all options and then put up warning messages about our
choices.
--
Ticket URL: <http://trac.sagemath.org/ticket/18940#comment:26>
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