Reverse Part of a List

Question

Take list foo

foo = [5, 6, 7, 8, 9, 10, 11, 12]

How can I reverse only the elements of indices x to y within the list?
For example:

x = 1
y = 5
# reverse foo[x:y]
foo = [5, 9, 8, 7, 6, 10, 11, 12]

Slice it out, reverse it, and concatenate everything back together. — Jonathon Reinhart, Mar 29 '15 at 14:41

vaultah · Answer 1 · 2015-04-04T12:08:17.610

4

Python allows you to assign to slices

Slicings may be used as expressions or as targets in assignment or del statements

thus it's possible to do everything on one line:

foo[x:y] = foo[y-1:x-1:-1]

Note thatfoo[y-1:x-1:-1] has the same meaning as foo[x:y][::-1].

edited Apr 04 '15 at 12:08

answered Mar 29 '15 at 14:42

vaultah

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it doesn't seems to work when x is pointing the index 0 – Alessandro Teruzzi Apr 09 '21 at 08:25

JuniorCompressor · Answer 2 · 2015-03-29T14:49:09.723

2

It's as simple as:

foo[x:y] = foo[y - 1:x - 1:-1]

For example:

>>> foo = [5, 6, 7, 8, 9, 10, 11, 12]
>>> foo[1:5] = foo[4:0:-1]
>>> foo
[5, 9, 8, 7, 6, 10, 11, 12]

edited Mar 29 '15 at 14:49

answered Mar 29 '15 at 14:44

JuniorCompressor

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can I get that in terms of x and y? – chambln Mar 29 '15 at 14:48

score 1 · Answer 3 · answered Mar 29 '15 at 14:50

1

def reverse(l, x, y):
    l[x:y+1] = l[y:x-1:-1]

foo = [5, 6, 7, 8, 9, 10, 11, 12]
x = 1
y = 4

reverse(foo, x, y)
print(foo) # [5, 9, 8, 7, 6, 10, 11, 12]

answered Mar 29 '15 at 14:50

maahl

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Reverse Part of a List

3 Answers3