#15786: floor fails for certain expressions.
-------------------------------------+-------------------------------------
Reporter: fwclarke | Owner:
Type: defect | Status: needs_work
Priority: major | Milestone: sage-6.8
Component: symbolics | Resolution:
Keywords: floor | Merged in:
Authors: David Einstein | Reviewers:
Report Upstream: N/A | Work issues:
Branch: | Commit:
u/deinst/15786-floor | 16e522fb6876ce646af7c52348056b94f3b8dfbc
Dependencies: | Stopgaps:
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Changes (by deinst):
* status: needs_review => needs_work
Comment:
After a bit of thought, it seemed like the best thing to do would be to
move the guts to `_eval_` and replace `__call__` with something like
{{{
def __call__(self, *args, **kwds):
res = super(Function_floor, self).__call__(*args, **kwds)
# Convert to Integer if the output was of type "int" and any of
# the inputs was a Sage Element
if isinstance(res, Expression) and res.is_integer():
return res.pyobject()
else:
return res
}}}
This should please everyone that expects floor to return an integer type
instead of an expression type, while having the functionality routed in
the standard way. Unforturnately, we now run into coersion problems on
the other end, people are calling `floor` with values that
`BuiltinFunction` cannot handle. For example, in `prime_pi.py` there is a
doctest
{{{
sage: prime_pi._eval_(str(-2^100))
}}}
Which ends up taking `floor(str(-2^100))`. This works in the current
implementation, as `str(-2^100)` can be coerced to an element of
`RealIntervalField`, but if we route the call through
`BuiltinFunction.__call__` it dies because `str(-2^100)` cannot be coerced
to an element of `SR` (this somewhat surprises me).
The obvious answer would be to add some coersion at the front of
`Function_floor.__call__`, but I don't know how far we should take
treating `floor` and `ceil` as functions needing special care and
handling.
There is still the problem of a potential infinite loop in the recursive
call
{{{
return floor(SR(x).full_simplify().canonicalize_radical())
}}}
I don't know of any `Expression`s that evaluate to integers, but that
`maxima` cannot simplify, but I'd be surprised if they don't exist. I'm
still rummaging about on the border of `pynac` to figure out how to stop
the recursion. If nothing else I can use the substitute trick, but that
seems too kludgy.
--
Ticket URL: <http://trac.sagemath.org/ticket/15786#comment:14>
Sage <http://www.sagemath.org>
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