c = sorted(set(a)-set(b)) although for me :~( that is another step more obscure than
c = list(set(a)-set(b)) c.sort() Bags don't seem to be built in to my copy of Python, and although I'm interested in why lists don't support the difference operation, I don't want to get away from standard Python. Steve. "Scott David Daniels" <[EMAIL PROTECTED]> wrote in message news:[EMAIL PROTECTED] > bambam wrote: >>> The reason that lists don't have set-like methods is because >>> lists aren't sets -- lists can contain duplicate elements > and they are ordered. I'd have used sets if I was sure you > meant [1,2,3] to mean the same thing as [3,1,2] and no duplicates. > >> Interesting point -- if that's all there is in it, then lists should >> have difference and intersection methods. Not because they >> are the same as sets -- because they are slightly different than >> sets. In this case it doesn't matter - my lists don't contain >> duplicate elements this time - but I have worked with lists in >> money market and in inventory, and finding the intersection >> and difference for matching off and netting out are standard >> operations. > Here you seem to be talking about multisets (also called bags). > They have more fully defined algebraic properties analogous to sets. > > bag([1,2,3,3,4]) == bag([3,1,2,4,3]) != bag([1,2,3,4]) > bag([1,2,2,3]) - bag([1,2]) == bag([2,3]) > bag([1,2,3]) - bag([3,4]) == bag([1]) > >>>> Excellent. By symmetry, I see that "list" casts the set back into a >>>> list. > Some will say 'sorted' is a better conversion of a set to list, since > the result is well-defined. > > --Scott David Daniels > [EMAIL PROTECTED] -- http://mail.python.org/mailman/listinfo/python-list
