#18940: Simplify __getitem__ for polynomials and deprecate slicing
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Reporter: pbruin | Owner:
Type: defect | Status: needs_work
Priority: major | Milestone: sage-6.10
Component: algebra | Resolution:
Keywords: | Merged in:
Authors: Peter Bruin, | Reviewers:
Jeroen Demeyer | Work issues:
Report Upstream: N/A | Commit:
Branch: | 1c56779b3140d12fbe5d0973cdcad0ac9dbb8054
u/pbruin/18940-getitem_step | Stopgaps:
Dependencies: #19409 |
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Changes (by {'newvalue': u'Peter Bruin, Jeroen Demeyer', 'oldvalue': u'Peter
Bruin'}):
* status: needs_info => needs_work
* author: Peter Bruin => Peter Bruin, Jeroen Demeyer
Old description:
> Sage should support a `step` different from 1 in the `__getitem__()`
> method of polynomials:
> {{{
> sage: R.<x> = ZZ[]
> sage: f = sum(x^j for j in range(5))
> sage: f
> x^4 + x^3 + x^2 + x + 1
> sage: f[-2:4:2]
> x^3 + x^2 + x + 1 # should be x^2 + 1
> }}}
> We fix this for most classes by implementing a generic `__getitem__()`
> method, which depends on a new Cython method `get_unsafe()` to get single
> coefficients. Overall, this greatly simplifies the code. It is
> conceivable that there is a small performance loss (I have not tested
> this), but it is unlikely that there is any speed-critical code that
> heavily depends on `__getitem__()` with a slice argument. For the
> remaining affected classes (Python classes, sparse polynomials), we fix
> `__getitem__()` in a similar way.
>
> This fix allows us to simplify the method
> `PowerSeries_poly.__getitem__()`. Also, the patch corrects some
> (previously wrong) slice bounds in
> `LaurentPolynomial_univariate.__getitem__()`.
New description:
For polynomials, we implementing a generic `__getitem__()` method, which
depends on a new Cython method `get_unsafe()` to get single coefficients.
Overall, this greatly simplifies the code. Slicing for polynomials is
deprecated, since there is no clear use case and it is not mathematically
well-defined what the answer should be (see also comment [comment:39] and
further below).
This fix allows us to simplify the method
`PowerSeries_poly.__getitem__()`. Also, the patch corrects some
(previously wrong) slice bounds in
`LaurentPolynomial_univariate.__getitem__()`.
--
--
Ticket URL: <http://trac.sagemath.org/ticket/18940#comment:45>
Sage <http://www.sagemath.org>
Sage: Creating a Viable Open Source Alternative to Magma, Maple, Mathematica,
and MATLAB
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