#14772: Remove CombinatorialClass from Permutations
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Reporter: tscrim | Owner: sage-combinat
Type: enhancement | Status: needs_review
Priority: major | Milestone: sage-5.11
Component: combinatorics | Resolution:
Keywords: days49 | Merged in:
Authors: Travis Scrimshaw | Reviewers:
Report Upstream: N/A | Work issues:
Branch: | Dependencies: #8386 #14519 #14808
Stopgaps: |
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Comment (by darij):
Great!
I'll look into the multisets issue, but as for now here is a fix for
another bug:
{{{
sage: Permutations(descents=[1,3,4])
}}}
gave an error. It turns out that, lacking an n-value in the input, the
code would automatically set n to be the maximum entry of the list {{{[1,
3, 4]}}} plus 1 (btw, that causes another bug when the list is empty;
fixed as well), but whoever wrote that must have missed the fact that the
positions start with 0 (so a presence of 3 in the list really means that
the permutations will have a descent at 4, i. e., their value at 4 will be
larger than their value at 5), so the "correct" n would be the maximum
entry of the list {{{[1, 3, 4]}}} plus 2. I think this is a completely
stupid convention to make, and the syntax (giving the set of descents as a
list without the n, having Sage automatically choose the n) should not be
supported (no doctests in permutation.py use this syntax). Note that it
only allows constructing permutations where the penultimate position is a
descent; all the others require the long syntax with the n given! Also, it
is misleading, because {{{descents=[1,3,4]}}} sounds like it should give
the permutations with descent composition {{{[1,3,4]}}}, but the role of
{{{[1,3,4]}}} here is absolutely not that of a composition.
Would you agree with axing the syntax?
--
Ticket URL: <http://trac.sagemath.org/ticket/14772#comment:44>
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