Also, "unique" permutations require a notion of equality, and which is hard to define in general (classic essay is [1], about Common Lisp, but applies to Julia mutatis mutandis). At least Julia has bits types for numbers, which makes life a bit easier.
Whether one picks is, isequal, or == for comparison, it easy to come up with cases which go against user expectations (at least for some users). For example, given type Foo end type Bar x end what should be the unique permutations of [0.0,-0.0,NaN,NaN,Foo(),Foo(),Bar(NaN),Bar(NaN)] ? Best, Tamas [1] http://www.nhplace.com/kent/PS/EQUAL.html On Sun, Nov 22 2015, Ratan Sur <ratan.r....@gmail.com> wrote: > I think julia more than other languages has a tendency to stick with > mathematical consistency over some user preferences, which is good. For > that reason, I would be in favor of permutations remaining as is but having > multiset_permutations renamed to something more intuitive, like > uniqueperms, or unique_permutations. > > On Sun, Nov 22, 2015 at 2:16 AM Glen O <gjo1...@gmail.com> wrote: > >> While it is true that an interpretation of arrays is multisets, that's not >> the only reasonable interpretation. And while the strict interpretation of >> "permutations" suggests it should include duplicates, you have to consider >> what the user would most likely expect it to do. Most would think that a >> list of the permutations would include unique permutations only. >> >> So perhaps both functionalities should be available in the same function >> with a keyword argument. At the very least, the description of the function >> should directly inform the user that it's going to give duplicate >> permutations if the array contains repeat elements. >> >> On Saturday, 21 November 2015 04:24:51 UTC+10, Jiahao Chen wrote: >>> >>> The current behavior of permutations is correct and should not be >>> changed. Combinatorially, arrays are multisets, not sets, since they allow >>> for duplicate entries, so it is correct to produce what look like identical >>> permutations. The redundancy is important for operations that can be >>> expressed as sums over all permutations. >>> >>> Combinatorics.jl currently provides multiset_permutations for generating >>> only distinct permutations: >>> >>> >>> https://github.com/JuliaLang/Combinatorics.jl/blob/3c08c9af9ebeaa54589e939c0cf2e652ef4ca6a0/test/permutations.jl#L24-L25 >>> >>
