On Fri, May 6, 2011 at 9:59 AM, Steven D'Aprano <steve+comp.lang.pyt...@pearwood.info> wrote: > On Mon, 02 May 2011 10:33:31 -0700, Raymond Hettinger wrote: > >> I think it is time to give some visibility to some of the instructive >> and very cool recipes in ActiveState's python cookbook. > [...] >> What are your favorites? > > > I'm not sure if favourite is the right word, but I'm amazed by this one: > McCarthy's "amb" (ambiguous) operator. > > http://code.activestate.com/recipes/577368 > > Here's how I might use it to solve the problem if making change. In > Australian currency, I have 5, 10, 20, 50 cent and $1 and $2 coins. > Ignoring the dollar coins, how can I make up change for any multiple of > five cents up to a dollar? > > Suppose I have more 5 cent coins that I can deal with, and I want to make > sure I hand out at least three of them. Here's how to make 45 cents worth > of change: > >>>> amb = Amb() >>>> a = amb(range(3, 21)) # number of 5 cent coins >>>> b = amb(range(11)) # number of 10 cent coins >>>> c = amb(range(6)) # number of 20 cent coins >>>> d = amb(range(3)) # number of 50 cent coins >>>> for _ in amb(lambda a,b,c,d: 5*a+10*b+20*c+50*d == 45): > ... print a, b, c, d > ... > 3 1 1 0 > 3 3 0 0 > 5 0 1 0 > 5 2 0 0 > 7 1 0 0 > 9 0 0 0 > > > Suppose I have some words, and want to put them together so that there > are a certain number of vowels: > >>>> amb = Amb() >>>> a = amb(['quick', 'slow', 'hungry', 'wise-old']) >>>> b = amb(['fox', 'hare', 'turtle', 'kangaroo']) >>>> c = amb(['lazy', 'stupid', 'sleepy', 'confused']) >>>> d = amb(['dog', 'aardvark', 'sloth', 'wombat']) >>>> >>>> def test(a, b, c, d): > ... s = "The %s brown %s jumped over the %s %s." % (a, b, c, d) > ... num_vowels = sum(s.count(c) for c in 'aeiou') > ... return num_vowels in (12, 18, 19) > ... >>>> for _ in amb(test): > ... print a, b, c, d > ... > quick fox lazy sloth > quick fox lazy dog > quick kangaroo stupid aardvark > [...more solutions cut for brevity] > hungry kangaroo confused aardvark > > > > As written, amb is just a brute-force solver using more magic than is > good for any code, but it's fun to play with.
I use a similar technique *a lot* for various kinds of constraint solvers, but I have yet to come up with a really satisfying spelling for it. Have you looked at the way this is done in Sage? Geremy Condra -- http://mail.python.org/mailman/listinfo/python-list
