Into itertools

bearophileHUGS at lycos.com bearophileHUGS at lycos.com
Mon Apr 27 11:38:23 CEST 2009


Arnaud Delobelle:

> Some people would write it as:
>
> def leniter(iterable):
>     if hasattr(iterable, '__len__'):
>         return len(iteratble)
>     return sum(1 for _ in iterable)

That's slower than my version.



> > def xpairwise(iterable):
> >     return izip(iterable, islice(iterable, 1, None))
>
> This doesn't work if the iterable is an iterator.

I see. I think I have never used it with an iterator so far :-)
I'll fix it.


> Why not:
>
> def xpairs(seq):
>     for i, el in enumerate(seq):
>         for j in xrange(i):
>             yield seq[j], el

Maybe because I was not smart enough to invent that :-)

-------------------

Mark Dickinson:

> 3) xpairs(seq)
>     >>> list(xpairs(range(5)))
>     [(0, 1), (0, 2), (0, 3), (0, 4), (1, 2), (1, 3), (1, 4), (2, 3),
> (2, 4), (3, 4)]

Doesn't itertools.combinations already do this for you?
>>> list(itertools.combinations(range(5), 2))

I have written most of those functions when I have used Python 2.4,
before itertools.combinations. But you are right, xpairs is now almost
useless, just one line long :-) Combinatorics is so fun :-)

Thank you very much to both.

Bye,
bearophile



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