Hey everyone,
> Here's how you can get an infinite loop in two lines with Permutation.
> >
> > sage: p =
> >
> Permutation([(1,10,11,13,9,3,16,0,22),(8,2,15,18,24,26,20,21,4),(5,6,17,12,25,7,23,14,19)])
>
>
> > sage: p.to_cycles()
> >
> > Of course it has to be because my permutation goes from 0 to n-1 while
> the
> > code expects 1-n.
> >
> > It is all very nice. Now, if the code does not work, we should just
> refuse
> > to build the corresponding permutations, and say it only works with
> > integers. Otherwise it has to be fixed quicjky or all this code is
> totally
> > useless.
>
> An other nice feature
> {{{
> sage: Permutation([-1,1])
> [-1, 1]
> sage: Permutation([-1,1]).to_cycles()
> [(1, -1, False)]
> }}}
>
For this issue, I started work on cleaning up the input for permutations
(there's 3 different trac tickets on this: #8392, #8918, #9831!), but have
currently stalled. I was planning on getting back to this during the sage
days at ICERM in February. There's a partial patch called
trac_8392-check_permutation-ts.patch in the queue. I can get back on this
now if desired. (Feel free to rebase this or let me know I need to rebase.)
>
> > By the way : is there any r&%$&$&$&%$ reason why Permutations (with a
> > terminal s) does not contain Permutation object but LISTS ?
> >
> > sage: list(Permutations(["a","b","c"]))
> > [['a', 'b', 'c'], ['a', 'c', 'b'], ['b', 'a', 'c'], ['b', 'c', 'a'],
> ['c',
> > 'a', 'b'], ['c', 'b', 'a']]
> > sage: map(type,list(Permutations(["a","b","c"])))
> > [<type 'list'>, <type 'list'>, <type 'list'>, <type 'list'>, <type
> 'list'>,
> > <type 'list'>]
>
> Permutations are the possible permutations of a (multi)set and should
> not be considered as bijection (as the permutations in the python
> library itertools). In particular, the following does work
> {{{
> sage: Permutations('aabb').list()
> [['a', 'a', 'b', 'b'], ['a', 'b', 'a', 'b'], ['a', 'b', 'b', 'a'],
> ['b', 'a', 'a', 'b'], ['b', 'a', 'b', 'a'], ['b', 'b', 'a', 'a']]
> }}}
> In particular, the parent Permutations does *not* contains
> Permutation. A bit confusing, but I think that's it!
>
> > I know, it would be slower otherwise. But it is bad code. This kind of
> > things should be done with an optional flag if you need it.
>
> +1
>
I completely agree.
Best,
Travis
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