for loop question

Tim Chase python.list at tim.thechases.com
Wed Oct 10 22:45:47 CEST 2007


Paul Hankin wrote:
> On Oct 10, 9:12 pm, Tim Chase <python.l... at tim.thechases.com> wrote:
>>  >>> pairs = (test[i:i+2] for i in xrange(len(test)-1))
>>  >>> for a,b in pairs:
>> ...     print a,b
> 
> for a, b in zip(test, test[1:]):
>   print a, b

Very nice!

I second this solution as better than my original.  The only 
"improvement" (in quotes, because it might be more work/opacity 
than the problem merits) might be to use izip/islice from 
itertools to do the evaluation lazily if "test" gets large:

   from itertools import izip, islice
   for a,b in izip(test, islice(test, 1, None)):
     print a,b

[side note/question]
What's with islice having the first optional paramenter expand as 
the stop/third argument by default:

   islice(test, 1) -> stop at 1
   islice(test, 1, 2) -> start at 1, stop at 2

islice (in python2.4) doesn't even take kword params, so you 
can't force it like

   islice(test, start=1)

but instead must specify a stop parameter, even if it's None:

   islice(test, 1, None)

Seems bogus, IMHO.

-tkc






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