En Sun, 18 Oct 2009 03:02:27 -0200, StarWing <weasley...@sina.com>
escribió:
On 10月18日, 上午9时09分, Raymond Hettinger <pyt...@rcn.com> wrote:
[StarWing]
> > > sometimes I want to iterate a part of a sequence. but don't want
to
> > > copy it. i.e.
. . .
> I had checked it for serval times. maybe it's my inattention :-(. but
> what i could find the nearest thing is itertools.islice. but it can't
> process negative index -- that's supported by slice. so I need
> something, bind object with slice, and return a iter. I can find
> anything like it...:-(
If it really is a sequence (with len and getitem), you can write your
own indexing iterator:
def myslice(seq, start, stop, step):
'Allow forward or backwards iteration over a subslice'
for i in range(start, stop, step):
yield seq[i]
Raymond
Thank you. but it can't support negative index :-(
Terry Reedy is right. since a range (or xrange or slice etc.) don't
have a length, so it can't support a negative index. so the best way
to do it is that binding a range with a object. and return a iter.
I think, why standard library didn't have anything like that, that
will be very useful. maybe we should have a builtin functoin
itertools.bslice (stands for bind slice)...
A variation of the code above would do, using a slice() object:
py> def myslice(seq, start, stop, step):
... 'Allow forward or backwards iteration over a subslice'
... for i in xrange(*slice(start, stop, step).indices(len(seq))):
... yield seq[i]
...
py> import string
py> lst = list(string.lowercase)
py> list(myslice(lst, 5, 18, 2)) # as in lst[5:18:2]
['f', 'h', 'j', 'l', 'n', 'p', 'r']
py> list(myslice(lst, -5, None, None)) # as in lst[-5:]
['v', 'w', 'x', 'y', 'z']
py>py> list(myslice(lst, None, None, -1)) # as in lst[::-1]
['z', 'y', 'x', 'w', 'v', 'u', 't', 's', ..., 'a']
Slice objects are not-so-well known, and are documented here:
http://docs.python.org/reference/datamodel.html#types
--
Gabriel Genellina
--
http://mail.python.org/mailman/listinfo/python-list
