iterating in reverse
glc at well.com
Sat Jan 18 17:25:26 CET 2003
On 16 Jan 2003 15:46:32 -0800, jbperez808 at yahoo.com (Jonathan P.) wrote:
>First of all, many thanks to all the people in the list
>who have taken the time to respond to my earlier
>questions with excellent answers. Now on to the
>problem at hand...
>A list's contents are ordered such that:
>>>> for i in x:
> print i
>will always print x's contents in the
>same sequence. Is there an elegant and
>efficient way to iterate through x in
>reverse without having to create a reversed
>copy of it (i.e. y=x[:]; y.reverse())?
How about something like this (in 2.3):
def seqiter(seq, *args):
'''seqiter(seq, [[start], stop, [step]]) -> seq item iterator'''
if not args:
xr = xrange(len(seq))
slc = slice(*args)
xr = xrange(*slc.indices(len(seq)))
for index in xr:
for i in seqiter(x, None, None, -1):
The above avoids constructing a new list, but it might not be too efficient (I'd
guess that for most lists, it would be better to iterate over the slice,
e.g. x[::-1]). Perhaps the list object should grow a new method taking
([start], stop, [step]) parameters and returning an iterator which produces the
items in the desired order.
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