#18940: Polynomials ignore the step argument in __getitem__
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Reporter: pbruin | Owner:
Type: defect | Status: new
Priority: major | Milestone: sage-6.8
Component: algebra | Resolution:
Keywords: | Merged in:
Authors: Peter Bruin | Reviewers:
Report Upstream: N/A | Work issues:
Branch: | Commit:
u/pbruin/18940-getitem_step | 97f95822da2ae740aa9a19bcc75f73c216879aeb
Dependencies: #19409 | Stopgaps:
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Changes (by pbruin):
* commit: => 97f95822da2ae740aa9a19bcc75f73c216879aeb
* branch: => u/pbruin/18940-getitem_step
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
> }}}
New 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__()`.
--
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Ticket URL: <http://trac.sagemath.org/ticket/18940#comment:4>
Sage <http://www.sagemath.org>
Sage: Creating a Viable Open Source Alternative to Magma, Maple, Mathematica,
and MATLAB
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