On Jan 23, 7:34 am, "Stephen C. Gilardi" <squee...@mac.com> wrote: > There was a recent suggestion here: > > http://groups.google.com/group/clojure/msg/32d323e15f8ce624 > > about the proper value of: > > (clojure.contrib.lazy-seqs/combinations) > > (and perhaps by extension (clojure.contrib.lazy-seqs/combinations '()) > (or any number of empty seq arguments))
No no no :). (combinations '()) should be nil/empty coll, but (combinations) should be a [[]] (or any coll/seq containing only an nil/empty coll, I don't care which types). This can be argued several ways, but the clearest is to think about the length of the sequence returned by combinations. It should be the product of the lengths of the input sequences. Thus, if any of the inputs to combinations are empty lists, (* ? 0 ? ?) ==> 0, the return value should be nil. But, if the input is *no lists at all*, then we have (*) ==> 1 and the output should be a list containing the empty list. > Does a decision to use empty collections in the powerset case fit well > with the suggestion that > > (clojure.contrib.lazy-seqs/combinations) return ([]) > > or are they in conflict or unrelated? Well, you can reason in the same way about power-set; if the input has n elements, the output should have 2^n elements. Thus, (power-set nil) should return [[]] as well (or, again, any coll/seq containing only an empty coll/seq, I don't care which type). Or, you could just look at Wiki: "the power set of the empty set is the set containing the empty set" http://en.wikipedia.org/wiki/Power_set -Jason --~--~---------~--~----~------------~-------~--~----~ You received this message because you are subscribed to the Google Groups "Clojure" group. To post to this group, send email to clojure@googlegroups.com To unsubscribe from this group, send email to clojure+unsubscr...@googlegroups.com For more options, visit this group at http://groups.google.com/group/clojure?hl=en -~----------~----~----~----~------~----~------~--~---
