Python how to print a list of lists backwards? - list

Question

How can I print a list of lists backwards in Python 3.x?
I have created an adjacency list by writing a list of lists as following:
adjazenzListe = [[1], #0
[1, 3], #1
[1], #2
[4, 5],#3
[3], #4
[3, 6, 7],#5
[5], #6
[8, 10],#7
[9, 10], #8
[8], #9
[7, 8, 11],#10
[10, 12, 13], #11
[11], #12
[14, 15],#13
[13], #14
[13]]#15
Now I want to print that but not in order 0, 1, 2, 3... but backwards (15, 14, 13...).
My current way of printing works via for-loop:
n = len(adjazenzListe)
for k in range(0,n):
print(k, ": ", adjazenzListe[k])
Now I looked a bit through Google and found stuff like using index [::-1] or the built-in reversed function. But these don't work with list as indices.

Answer 1

You must not have been using the indices correctly.
Try the following:
adjazenzListe = [[1], #0
[1, 3], #1
[1], #2
[4, 5],#3
[3], #4
[3, 6, 7],#5
[5], #6
[8, 10],#7
[9, 10], #8
[8], #9
[7, 8, 11],#10
[10, 12, 13], #11
[11], #12
[14, 15],#13
[13], #14
[13]]#15
for i, k in enumerate(adjazenzListe[::-1]):
print(len(adjazenzListe) - i - 1, k)
With reverse:
adjazenzListe = [[1], #0
[1, 3], #1
[1], #2
[4, 5],#3
[3], #4
[3, 6, 7],#5
[5], #6
[8, 10],#7
[9, 10], #8
[8], #9
[7, 8, 11],#10
[10, 12, 13], #11
[11], #12
[14, 15],#13
[13], #14
[13]]#15
adjazenzListe.reverse()
for i, k in enumerate(adjazenzListe):
print(len(adjazenzListe) - i - 1, k)

Answer 2

>>> n = len(adjazenzListe)-1
>>> for k in range(n,-1,-1):
... print(k, ": ", adjazenzListe[k])
Martijn Pieters was spot on, as usual

Answer 3

Hmmm I am not sure why you had problems using the alternative techniques stated. Though there have already been great answers of using range() and list slicing, I am just going to showcase another technique for future visitors to this page:
Approach 1: The reversed() function
adjazenzListe = [[1], #0
[1, 3], #1
[1], #2
[4, 5],#3
[3], #4
[3, 6, 7],#5
[5], #6
[8, 10],#7
[9, 10], #8
[8], #9
[7, 8, 11],#10
[10, 12, 13], #11
[11], #12
[14, 15],#13
[13], #14
[13]]#15
for row in reversed(adjazenzListe):
print(row)
reversed() will simply reverse the order of a sequential object or structure when evaluated against a loop variable. Please note that this function is best used in loop statements, with range() or conditional statements if used by itself. If you wish to use it for variable assignment, you will need to accompany it with a formatting function such as list() to see your results if output. See example below:
adjazenzListe = [[1], #0
[1, 3], #1
[1], #2
[4, 5],#3
[3], #4
[3, 6, 7],#5
[5], #6
[8, 10],#7
[9, 10], #8
[8], #9
[7, 8, 11],#10
[10, 12, 13], #11
[11], #12
[14, 15],#13
[13], #14
[13]]#15
reversed_list = list(reversed(adjazenzListe))
for row in reversed_list:
print(row)
As previously stated: It can also be used with range():
adjazenzListe = [[1], #0
[1, 3], #1
[1], #2
[4, 5],#3
[3], #4
[3, 6, 7],#5
[5], #6
[8, 10],#7
[9, 10], #8
[8], #9
[7, 8, 11],#10
[10, 12, 13], #11
[11], #12
[14, 15],#13
[13], #14
[13]]#15
for i in reversed(range(len(adjazenzListe))):
print(adjazenzListe[i])
Here is a reference for further learning: https://www.geeksforgeeks.org/python-reversed-function/