Python: Print a list in reverse using recursion

Question

I'm trying to figure out how to print out a list in reverse order using recursion. This is how I reversed a list:

def reverse_list(xs):
    if xs == []:
         return xs
    else:
        new_list = (print_reverse (xs[1:])) + [xs[0]]
        return new_list

and this is how I printed a list in order:

def print_in_order(l, i = 0):
    if i < len(l):
        print (l[i])
        print_in_order(l, i + 1)

I tried just printing out from the new list inside the function like this:

def print_reverse(xs):
    if xs == []:
        return xs
    else:
        new_list = (print_reverse (xs[1:])) + [xs[0]]
    for number in new_list: 
        print (number)

But it raises an error. How do you combine the two processes?

Why don't just modify print_in_order? start from len(l) - 1 and check if i >= 0? — Dmitrii Bychenko
– Dmitrii Bychenko, Commented Oct 2, 2014 at 6:37
print_in_order starts from i = 0 i.e. from the beginning of the string, print_in_reverse should start from i = len(l) - 1 - from the last character of the string. Side note: Do not use l when it's easy to mix it with 1 — Dmitrii Bychenko
– Dmitrii Bychenko, Commented Oct 2, 2014 at 6:46
I tried adding i = len(l)-1 to the 2nd parameter but it raises an error. — Newbie
– Newbie, Commented Oct 2, 2014 at 6:47
why don't use any byway e.g. i = None, and then, in the function's body check if i = None assign it to len(l) - 1? — Dmitrii Bychenko
– Dmitrii Bychenko, Commented Oct 2, 2014 at 6:48

John La Rooy · Accepted Answer · 2014-10-02 07:37:51Z

2

You just need to swap two lines:

def print_in_reverse(lst, i=0):
    if i < len(lst):
        print_in_reverse(lst, i + 1)  # Do the recursive call before you
        print (lst[i])              # print instead of after

edited Oct 2, 2014 at 7:37

answered Oct 2, 2014 at 7:04

John La Rooy

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Comments

Carlo Beccarini · Accepted Answer · 2014-10-02 07:10:46Z

1

One of the simplest way to do this, it's using slicing in this way:

>>> example = [1, 2, 3, 4, 5]
>>> example[::-1]
[5, 4, 3, 2, 1]

..and if you want to print them, just do this:

>>> for number in example:
...     print(number)
...     
1
2
3
4
5

EDITED:

>>> example = [1, 2, 3, 4, 5]

>>> def recursive(list, counter=0):
...     if len(list) > counter:
...         print(list[counter])
...         recursive(list, counter+1)
...     return
... 

>>> recursive(example[::-1])
5
4
3
2
1

edited Oct 2, 2014 at 7:10

answered Oct 2, 2014 at 6:50

Carlo Beccarini

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3 Comments

Newbie Over a year ago

It's a homework exercise that requires you to use recursion and put it in a defined function form.

Newbie Over a year ago

How can I modify my print_in_order function to print in reversed order?

Carlo Beccarini Over a year ago

You can simply pass to the print_in_order method the sliced reversed list, and iterate on it recursively to print it out.

Dmitrii Bychenko · Accepted Answer · 2014-10-02 07:03:34Z

0

Something like that (print_in_order adaptation):

# print l in reverse order, starting from i
# note, that we're allowed not to specify i
def print_in_reverse(l, i = None):
    # if i is not specified    
    if (i is None):
        i = len(l) - 1 

    # we're printing in reverse: 
    # we have to check for beginning of the string instead of ending
    # that's why check for 0 
    if (i >= 0):
        print (l[i])
        print_in_reverse(l, i - 1) # reverse direction: i - 1 instead of i + 1

answered Oct 2, 2014 at 7:03

Dmitrii Bychenko

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Comments

Majlik · Accepted Answer · 2014-10-02 07:03:26Z

0

If you want just print it in reverse order and you don't want to change original list you can just modify print_in_order() like this:

#second parameter is length of list
def print_in_reversed(l, i): 
    if i > 0:
        print (l[i-1])
        print_in_order(l, i - 1)

And you will call it with this example

example = [1,2,3,4,5]    
print_in_order(example, len(example))

answered Oct 2, 2014 at 7:03

Majlik

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