reverse - Reversing a list slice in python

6
votes

I am trying to reverse slice of a list in python but it returns an empty list. But when I try with whole list, it works fine. Am I missing anything here?

l=[1,2,3,4,5,6,7,8]
l[::-1]=[8, 7, 6, 5, 4, 3, 2, 1]     <<< This worked fine.

l[2:5]=[3, 4, 5]
l[2:5:-1]=[]       <<< Expecting [5,4,3] here.

Any clues?

2
pythonreverseslice
l[2:5][::-1] Doing it in different square brackets, it works. - user5077528
you would need l[4:1:-1] using a negative step. -1 is not reversing it is stepping - Padraic Cunningham
@PadraicCunningham why doesn't this work though as l[2:5:2] does work I'm intrigued - EdChum
@EdChum, you are starting at 2 and stepping -1 trying to get to 5, it makes no sense to step from 2 to 5 going backwards - Padraic Cunningham
Because you're trying to count backwards from 2 to 5. - TigerhawkT3

2 Answers

8
votes

The syntax is always [start:end:step] so if you go backwards your start needs to be greater than the end. Also remember that it includes start and excludes end, so you need to subtract 1 after you swap start and end.

l[5:2:-1]= [6, 5, 4]
l[4:1:-1]= [5, 4, 3]
0
votes

Slice notation is [start:stop:step]. This means "begin at start, then increase by step until you get to end." It's similar to this construct:

counter = 0
stop = 10
step = 1
while counter < stop:
    print(counter)
    counter += step

This will produce, as expected, 0 through 9.

Now imagine if we tried it with the values you're using:

counter = 2
stop = 5
step = -1
while counter < stop:
    print(counter)
    counter += step

If we actually executed this, it would print 2, then add -1 to that to get 1, then print 1 and add -1 to that to get 0, then it would be -1, -2, and so on, forever. You would have to manually halt execution to stop the infinite loop.

If you leave start or stop empty, as in [:4] or [::-1], this indicates the beginning or end of the sequence, as determined by the step. Python will go forwards with a positive step and backwards with a negative step (trying to use a step of 0 produces an error).

>>> l[2::]
[3, 4, 5, 6, 7, 8]
>>> l[2::-1]
[3, 2, 1]
>>> l[:2:]
[1, 2]
>>> l[:2:-1]
[8, 7, 6, 5, 4]

If you specify a start, end, and step that couldn't work (an empty step defaults to 1), Python will simply return an empty sequence.