>> a = range(10)
>> print a[::-1]
[9, 8, 7, 6, 5, 4, 3, 2, 1, 0]
This slice gives reversed list. How does it work?
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Oleg Razgulyaev
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2I'd argue it's more "pythonic" to use [9, 8, 7, 6, 5, 4, 3, 2, 1, 0].reverse() . In Python the focus should usually be on readability. – SpliFF May 01 '12 at 04:31
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3Or even `reversed(aSeq)` if you want the efficiency of an iterator – jdi May 01 '12 at 04:32
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@SpliFF - they do slightly different things though. – detly May 01 '12 at 04:48
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@SpliFF, `[::-1]` works for strings too though. I think it's a good idiom to know because `''.join(reversed(mystr))` isn't exactly readable either – John La Rooy May 01 '12 at 06:08
3 Answers
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The third argument is a step modifier. In this case you are using a step of -1.
You could also use a step of 2 to print every even index.
>>> a = range(10)
>>> a[::2]
[0, 2, 4, 6, 8]
>>> a[::-2]
[9, 7, 5, 3, 1]
GWW
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Once you grok extended slices, be sure to appreciate the under appreciated slice() function:
>>> for sl in [(1,-1),(0,20,3),(10,),(None,None,-3),(None,None,4)]:
... print range(20)[slice(*sl)]
...
[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18]
[0, 3, 6, 9, 12, 15, 18]
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
[19, 16, 13, 10, 7, 4, 1]
[0, 4, 8, 12, 16]
It is especially useful for fixed length data formats.
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This is the extended slice syntax. See the docs.
For range(x,y,z) x is the start, y is the stop, z is the stride.
This has been covered well in this SO post.
