Slicing [N::-1]
in [Python]
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From: Mensanator on 5 Mar 2010 13:52 On Mar 5, 12:28 pm, Steven D'Aprano <st...(a)REMOVE-THIS- cybersource.com.au> wrote: > On Fri, 05 Mar 2010 18:12:05 +0000, Arnaud Delobelle wrote: > >>>> l = range(10) > >>>> l > > [0, 1, 2, 3, 4, 5, 6, 7, 8, 9] > >>>> l[7::-1] > > [7, 6, 5, 4, 3, 2, 1, 0] > >>>> [l[i] for i in range(7, -1, -1)] > > [7, 6, 5, 4, 3, 2, 1, 0] > > Where does the first -1 come from? Slices are supposed to have default > values of 0 and len(seq): The only way to get a 0 from a reverse range() is to have a bound of -1. > > >>> l[7::1] > [7, 8, 9] > >>> [l[i] for i in range(7, len(l), 1)] > [7, 8, 9] > >>> [l[i] for i in range(7, len(l), -1)] > > [] > > I don't believe the actual behaviour is documented anywhere. Well, it's implied. If the stopping bound in a reverse range() is greater than the starting bound, you get an empty return. > > -- > Steven
From: Robert Kern on 5 Mar 2010 14:10 On 2010-03-05 12:28 PM, Steven D'Aprano wrote: > On Fri, 05 Mar 2010 18:12:05 +0000, Arnaud Delobelle wrote: > >>>>> l = range(10) >>>>> l >> [0, 1, 2, 3, 4, 5, 6, 7, 8, 9] >>>>> l[7::-1] >> [7, 6, 5, 4, 3, 2, 1, 0] >>>>> [l[i] for i in range(7, -1, -1)] >> [7, 6, 5, 4, 3, 2, 1, 0] > > Where does the first -1 come from? Slices are supposed to have default > values of 0 and len(seq): Rather, they have 0 and len(seq), respectively, when the step is positive, and len(seq)-1 and -1 when the step is negative. >>>> l[7::1] > [7, 8, 9] >>>> [l[i] for i in range(7, len(l), 1)] > [7, 8, 9] >>>> [l[i] for i in range(7, len(l), -1)] > [] > > > I don't believe the actual behaviour is documented anywhere. True, I don't think it is. -- Robert Kern "I have come to believe that the whole world is an enigma, a harmless enigma that is made terrible by our own mad attempt to interpret it as though it had an underlying truth." -- Umberto Eco
From: Gary Herron on 5 Mar 2010 16:42 Mensanator wrote: > On Mar 5, 12:28 pm, Steven D'Aprano <st...(a)REMOVE-THIS- > cybersource.com.au> wrote: > >> On Fri, 05 Mar 2010 18:12:05 +0000, Arnaud Delobelle wrote: >> >>>>>> l = range(10) >>>>>> l >>>>>> >>> [0, 1, 2, 3, 4, 5, 6, 7, 8, 9] >>> >>>>>> l[7::-1] >>>>>> >>> [7, 6, 5, 4, 3, 2, 1, 0] >>> >>>>>> [l[i] for i in range(7, -1, -1)] >>>>>> >>> [7, 6, 5, 4, 3, 2, 1, 0] >>> >> Where does the first -1 come from? Slices are supposed to have default >> values of 0 and len(seq): >> > > The only way to get a 0 from a reverse range() is to have a bound of > -1. > Not quite. An empty second bound goes all the way to the zero index: >>> range(9)[2::-1] [2, 1, 0] Gary Herron > >>>>> l[7::1] >>>>> >> [7, 8, 9] >> >>>>> [l[i] for i in range(7, len(l), 1)] >>>>> >> [7, 8, 9] >> >>>>> [l[i] for i in range(7, len(l), -1)] >>>>> >> [] >> >> I don't believe the actual behaviour is documented anywhere. >> > > Well, it's implied. If the stopping bound in a reverse range() > is greater than the starting bound, you get an empty return. > > >> -- >> Steven >> > >
From: Terry Reedy on 5 Mar 2010 16:46 On 3/5/2010 2:10 PM, Robert Kern wrote: > Rather, they have 0 and len(seq), respectively, when the step is > positive, and len(seq)-1 and -1 when the step is negative. >> I don't believe the actual behaviour is documented anywhere. > > True, I don't think it is. There are at least two open issues. http://bugs.python.org/issue1446619 http://bugs.python.org/issue7460
From: Robert Kern on 5 Mar 2010 17:12 On 2010-03-05 13:10 PM, Robert Kern wrote: > On 2010-03-05 12:28 PM, Steven D'Aprano wrote: >> On Fri, 05 Mar 2010 18:12:05 +0000, Arnaud Delobelle wrote: >> >>>>>> l = range(10) >>>>>> l >>> [0, 1, 2, 3, 4, 5, 6, 7, 8, 9] >>>>>> l[7::-1] >>> [7, 6, 5, 4, 3, 2, 1, 0] >>>>>> [l[i] for i in range(7, -1, -1)] >>> [7, 6, 5, 4, 3, 2, 1, 0] >> >> Where does the first -1 come from? Slices are supposed to have default >> values of 0 and len(seq): > > Rather, they have 0 and len(seq), respectively, when the step is > positive, and len(seq)-1 and -1 when the step is negative. Well, not entirely true. [N:-1:-1] obviously doesn't work for this. Rather, leaving the second argument in the slice empty means "go to the end if step > 0 or go to the beginning if step < 0". There is no explicit translation of the latter because there is no numerical index for the element before the first element. -- Robert Kern "I have come to believe that the whole world is an enigma, a harmless enigma that is made terrible by our own mad attempt to interpret it as though it had an underlying truth." -- Umberto Eco
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