On 25 Sep, 09:22, Yingjie Lan <lany...@yahoo.com> wrote: > Hi, > > I noticed that in python3k, multiplying a sequence by a negative integer is > the same as multiplying it by 0, and the result is an empty sequence. It > seems to me that there is a more meaningful symantics. > > Simply put, a sequence multiplied by -1 can give a reversed sequence. > > Then for any sequence "seq", and integer n>0, we can have > > "seq * -n" producing "(seq * -1) * n". > > Any thoughts? > > Yingjie
If [1, 2]*-1 is correct, then, arguably, so should be -[1, 2] :) Some answers have invoked mathematics to weigh the value of this proposal, e.g. likening lists to vectors. But the obvious mathematical analogy is that the set of all lists forms a monoid under the operation of concatenation, which (unfortunately?) is performed with the "+" operator in Python. So it is natural that "*" represents repeated concatenation. Now under concatenation, non-empty lists do not have an inverse, i.e. for any non-empty list l, there does not exist a list l' such that l + l' == []. So there is no natural interpretation of -l and therefore of l*-1. However, by using "+" for list (and string) concatenation, Python already breaks the mathematical pledge of commutativity that this operator implies. -- Arnaud -- http://mail.python.org/mailman/listinfo/python-list
