#16025: Calling the .n() method on reals shouldn't increase precision
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Reporter: rws | Owner:
Type: defect | Status: new
Priority: major | Milestone: sage-6.4
Component: calculus | Resolution:
Keywords: | Merged in:
Authors: Jeroen Demeyer | Reviewers:
Report Upstream: N/A | Work issues:
Branch: | Commit:
Dependencies: | Stopgaps:
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Changes (by {'newvalue': u'Jeroen Demeyer', 'oldvalue': ''}):
* author: => Jeroen Demeyer
Old description:
> {{{
> sage: Ei(1.1).n(300)
> 2.16737827956340289858871983597055077552795410156250000000000000000000000000000000000000000
>
> sage: Ei(RealField(300)(1.1))
> 2.16737827956340282358378734233807621497112737591639704719499002090327541763352339357795426
>
> sage: Ei(11/10).n(300)
> 2.16737827956340282358378734233807621497112737591639704719499002090327541763352339357795426
>
> sage: Ei(1.1,prec=300)
> /home/ralf/sage/src/bin/sage-ipython:1: DeprecationWarning: The prec
> keyword argument is deprecated. Explicitly set the precision of the
> input, for example Ei(RealField(300)(1)), or use the prec argument to
> .n() for exact inputs, e.g., Ei(1).n(300), instead.
> See http://trac.sagemath.org/7748 for details.
> #!/usr/bin/env python
> 2.16737827956340306615064476647912607220394065907142504328679588538509331805598360907980986
> }}}
> Not sure what would be correct, though.
New description:
The `.n()` is dangerous, as it can give easily give false precision:
{{{
sage: Ei(1.1).n(300)
2.16737827956340289858871983597055077552795410156250000000000000000000000000000000000000000
sage: Ei(RealField(300)(1.1))
2.16737827956340282358378734233807621497112737591639704719499002090327541763352339357795426
sage: Ei(11/10).n(300)
2.16737827956340282358378734233807621497112737591639704719499002090327541763352339357795426
0394065907142504328679588538509331805598360907980986
}}}
The fact that several doctests in the Sage library have this error shows
that this error is easily made.
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Ticket URL: <http://trac.sagemath.org/ticket/16025#comment:8>
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