Mutating Reverse Rlist
Write a function mutating reverse that takes in an Rlist and mutates it by reversing the elements in the rlist. The function should not return anything and the original input rlist should be mutated. Hint: You might want to make one, possibly two helper functions. If you’re stuck, check out mutating reverse list for the same sort of function, except over regular lists instead of rlists.
def mutating_reverse(rlist):
"""
>>> r = Rlist(1, Rlist(2, Rlist(3)))
>>> mutating_reverse(r)
>>> r
Rlist(3, Rlist(2, Rlist(1)))
"""
"***YOUR CODE HERE***"
def mutating_reverse(rlist):
"""
>>> r = Rlist(1, Rlist(2, Rlist(3)))
>>> mutating_reverse(r)
>>> r
Rlist(3, Rlist(2, Rlist(1)))
"""
if rlist is Rlist.empty:
return
if rlist.rest is not Rlist.empty:
item = pop(rlist)
mutating_reverse(rlist)
insert_beginning(item, rlist)
def insert_beginning(item, rlist):
"""
>>> r = Rlist(1, Rlist(2, Rlist(3)))
>>> insert_beginning(6, r)
>>> r
Rlist(6, Rlist(1, Rlist(2, Rlist(3))))
"""
if rlist is not Rlist.empty:
rlist.rest = Rlist(rlist.first, rlist.rest)
rlist.first = item
def pop(rlist):
"""Removes the last item from the rlist and returns it.
>>> r = Rlist(1, Rlist(2, Rlist(3)))
>>> pop(r)
3
>>> r
Rlist(1, Rlist(2))
"""
if rlist.rest.rest is Rlist.empty:
item = rlist.rest.first
rlist.rest = Rlist.empty
return item
return pop(rlist.rest)
That’s a lot of code. Let’s break this problem down. Our general approach to the mutating_reverse function is to pop off the last item of the rlist (which mutates the rlist), reverse the rest of the rlist. Then insert the item that we popped off into the beginning of the reversed rlist.
To do this, we write up two helper functions, insert_beginning and pop which work with rlists to do exactly that. For insert_beginning, we’re going to take in an item, and an rlist and mutate the input rlist by adding item to the beginning of it.
For pop, we’re going to iterate over the rlist until we reach the second to last item. From there, we’re going to grab the item that we want to return, and then mutate the rlist such that it no longer contains that item. Then we are going to return that item.
I don't claim to be perfect so if you find an error on this page, please send me an email preferably with a link to this page so that I know what I need to fix!