Good Python generators example?

Robert Oschler no_replies at fake_email_address.invalid
Mon Apr 21 00:16:35 CEST 2003


"Andrew Koenig" <ark at research.att.com> wrote in message
news:yu99y9243nn1.fsf at europa.research.att.com...
> Robert> Can someone point me to a nice concise Python 2.2 compatible
> Robert> example of the use of generators?
>
> Here's a program that solves the "8 queens" problem--that is, the
> problem of how to place 8 queens on a chessboard so that none of
> them attacks any of the others.  It prints all the solutions.
>
> from __future__ import generators
>
> class Queens(object):
>     def __init__(self, n):
>         assert n >= 0
>         self.N = n
>         self.board = []
>
>     def generate(self):
>         if len(self.board) == self.N:
>             yield self.board[:]
>         else:
>             for i in range(self.N):
>                 if self.safe(i):
>                     self.board.append(i)
>                     for i in self.generate():
>                         yield i
>                     self.board.pop()
>
>     def safe(self, n):
>         "Is is safe to append a queen to the board in row n?"
>         m = len(self.board)
>         for i in range(m):
>             b = self.board[i]
>             if n == b or b+(m-i) == n or b-(m-i) == n:
>                 return False
>         return True
>
> for i in Queens(8).generate():
>    print i
>
>
> --
> Andrew Koenig, ark at research.att.com, http://www.research.att.com/info/ark


Andrew,

Thanks!  Interesting choice of examples, last time I solved that problem was
with Prolog.  With a stack of Python generators you can pretty well emulate
Prolog backtracking, except for the automatic unbinding of variables.

thx
--

Robert Oschler
Android Technologies, Inc.
http://www.androidtechnologies.com
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