Velocity Reviews > circular iteration

# circular iteration

Flavio codeco coelho
Guest
Posts: n/a

 01-21-2005
hi,

is there a faster way to build a circular iterator in python that by doing this:

c=['r','g','b','c','m','y','k']

for i in range(30):
print c[i%len(c)]

thanks,

Flávio

Duncan Booth
Guest
Posts: n/a

 01-21-2005
Flavio codeco coelho wrote:

> hi,
>
> is there a faster way to build a circular iterator in python that by
> doing this:
>
> c=['r','g','b','c','m','y','k']
>
> for i in range(30):
> print c[i%len(c)]
>
> thanks,
>
> Flávio
>

>>> import itertools
>>> c=['r','g','b','c','m','y','k']
>>> circ = itertools.cycle(c)
>>> for i in range(30):

print circ.next(),

r g b c m y k r g b c m y k r g b c m y k r g b c m y k r g
>>>

Simon Brunning
Guest
Posts: n/a

 01-21-2005
On 21 Jan 2005 08:31:02 -0800, Flavio codeco coelho <(E-Mail Removed)> wrote:
> hi,
>
> is there a faster way to build a circular iterator in python that by doing this:
>
> c=['r','g','b','c','m','y','k']
>
> for i in range(30):
> print c[i%len(c)]

I don''t know if it's faster, but:

>>> import itertools
>>> c=['r','g','b','c','m','y','k']
>>> for i in itertools.islice(itertools.cycle(c), 30):

.... print i

--
Cheers,
Simon B,
http://www.velocityreviews.com/forums/(E-Mail Removed),
http://www.brunningonline.net/simon/blog/

Fredrik Lundh
Guest
Posts: n/a

 01-21-2005
"Flavio codeco coelho" wrote:

> is there a faster way to build a circular iterator in python that by doing this:
>
> c=['r','g','b','c','m','y','k']
>
> for i in range(30):
> print c[i%len(c)]

have you benchmarked this, and found it lacking, or are you just trying
to optimize prematurely?

</F>

Alex Martelli
Guest
Posts: n/a

 01-22-2005
Simon Brunning <(E-Mail Removed)> wrote:
...
> > is there a faster way to build a circular iterator in python that by

doing this:
> >
> > c=['r','g','b','c','m','y','k']
> >
> > for i in range(30):
> > print c[i%len(c)]

>
> I don''t know if it's faster, but:
>
> >>> import itertools
> >>> c=['r','g','b','c','m','y','k']
> >>> for i in itertools.islice(itertools.cycle(c), 30):

> ... print i

Whenever you're using itertools, the smart money's on "yes, it's
faster".

E.g., on a slow, old iBook...:

kallisti:~ alex\$ python -mtimeit -s'c="rgbcmyk"' 'for i in range(30):
c[i%len(c)]'
10000 loops, best of 3: 47 usec per loop

kallisti:~ alex\$ python -mtimeit -s'c="rgbcmyk"; import itertools as it'
'for i in it.islice(it.cycle(c),30): i'
10000 loops, best of 3: 26.4 usec per loop

Of course, if you do add back the print statements they'll take orders
of magnitude more time than the cyclic access, so /F's point on
premature optimization may well be appropriate. But, if you're doing
something VERY speedy with each item you access, maybe roughly halving
the overhead for the cyclic access itself MIGHT be measurable (maybe
not; it IS but a few microseconds, after all).

I like itertools' approach because it's higher-abstraction and more
direct. Its blazing speed is just a trick to sell it to conservative
curmudgeons who don't see abstraction as an intrinsic good -- some of
those are swayed by microseconds

Alex

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