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comp.lang.python

RE: List Combinations

Reedick, Andrew

3/12/2008 3:38:00 PM

> -----Original Message-----
> From: python-list-bounces+jr9445=att.com@python.org [mailto:python-
> list-bounces+jr9445=att.com@python.org] On Behalf Of Shane Geiger
> Sent: Wednesday, March 12, 2008 10:33 AM
> To: Michael Wieher
> Cc: python-list@python.org
> Subject: Re: List Combinations
>
> >
>
>
> def gen(lists):
> out = '[' + ','.join(["v%s" % i for i in range(len(lists))]) +
']'
> comp = ''.join([ " for v%d in lists[%d]" % (i, i) for i in
> range(len(lists))])
> return eval('[ ' + out + comp + ' ]')
>
> a,b,c = [1,2,3],[4,5,6],[7,8,9]
>
> print gen([a, b, c])
>
>
>
>
>
> --
> Shane Geiger
> IT Director
> National Council on Economic Education
> sgeiger@ncee.net | 402-438-8958 | http://ww...
>


It helps to quote your source...
http://www.mail-archive.com/python-list@python.org/msg1...


Start here

http://www.mail-archive.com/python-list@python.org/msg1...
and go through the thread. There are several ways to solve the problem
and we evaluated the performance and 'pythonicity' of each.




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1 Answer

Arnaud Delobelle

3/12/2008 10:55:00 PM

0

On Mar 12, 3:38 pm, "Reedick, Andrew" <jr9...@ATT.COM> wrote:
[...]
> Start here
>
> http://www.mail-archive.com/python-l...@python.org/msg1...
> and go through the thread.  There are several ways to solve the problem
> and we evaluated the performance and 'pythonicity' of each.  

I used a kind of extended cartesian product function a while ago while
writing a parser. If I simplify it down to a simple product function
it goes like this:

def product(*sequences):
i, n = 0, len(sequences)
vals = [None]*n
iters = map(iter, sequences)
while i >= 0:
if i == n:
yield tuple(vals)
i -= 1
else:
for vals[i] in iters[i]:
i += 1
break
else:
iters[i] = iter(sequences[i])
i -= 1

It's neither recursive nor a hack, I haven't tried to measure it
against other approaches (obviously it wouldn't beat the eval(...)
hack). I haven't optimised it for readability (by others) either :)

--
Arnaud