#20028: sorting of number field elements
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Reporter: cremona | Owner:
Type: defect | Status: new
Priority: major | Milestone: sage-7.1
Component: number fields | Resolution:
Keywords: sort number field | Merged in:
elements | Reviewers:
Authors: | Work issues:
Report Upstream: N/A | Commit:
Branch: u/cremona/20028 | f9f6b99b09fc6c5183d2fe7a82e4f27e957f059c
Dependencies: | Stopgaps:
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Comment (by gjorgenson):
Replying to [comment:19 cremona]:
> Sure, rem_indices is not used. But also: the line P=self(P) is executed
man ytimes (once for each j) and in the next line should it not be if
p++points[j]? Otherwise j is not used. I am also worried about the
effect of popping points[i] while in the i-loop, but perhaps that's OK
given that the i loop goes backwards. A comment to explain what this
block of code is doing would help, since I suspect that someone might come
up with a slicker solution. (If we only wanted to remove duplicates and
did not care about the order then something like points=list(set(points))
would work.)
That code doesn't try to remove duplicates (the list of points returned by
rational_points in line 2998 is assumed to contain no duplicates already),
but is trying to weed out periodic points of the map {{{self}}} that have
periods smaller than n so that only those with minimal period n are
returned. This is done by iterating each point in the list by {{{self}}}
(in the j-loop) n-1 times and checking whether any of those iterates is
the same point as the 0th iterate (points[i]). j is not used for indexing
but is needed to keep track of how many times to iterate each point before
it can be concluded that the point has minimal period n.
The problems associated with popping elements from the list are avoided
since the i-loop moves backwards. I'm not sure of a way to do this more
efficiently, but it would definitely be good to at least add some more
documentation to the code. Would that be appropriate to do in this ticket,
or would it be cleaner to leave it to a separate ticket?
--
Ticket URL: <http://trac.sagemath.org/ticket/20028#comment:23>
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