How can I reverse a section of a list using a loop in Python?

Question

I'm in need of a little help reversing a section of a list in Python using a loop.

I have a list: mylist = ['a', 'b', 'c', 'd', 'e', 'f']

Also have an index number, this number will tell where to start the reversing. For example, if the reverse-index number is 3, it needs to be something like this: ['d', 'c', 'b', 'a', 'e', 'f']

What I currently have:

def list_section_reverse(list1, reverse_index):
    print("In function reverse()")

    n_list = []

    for item in range( len(list1) ):
        n_list.append( (list1[(reverse_index - 1) - item]) )
        item += 1
    return n_list

mylist = ['a', 'b', 'c', 'd', 'e', 'f']
print( list_section_reverse(mylist, 3) )

Which returns ['c', 'b', 'a', 'f', 'e', 'd']

How can I alter my code, so that it prints out ['d', 'c', 'b', 'a', 'e', 'f']?

Selcuk · Accepted Answer · 2016-05-26 12:56:24Z

2

You can simply use:

def list_section_reverse(list1, reverse_index):
    return list(reversed(list1[:reverse_index+1])) + list1[reverse_index+1:]

Edit: The problem with your existing solution is that you keep reversing after reverse_index. If you have to use a loop, try this:

def list_section_reverse(list1, reverse_index):
    print("In function reverse()")

    n_list = list1[:]

    for i in range(reverse_index + 1):
        n_list[i] = list1[-i-reverse_index]
    return n_list

mylist = ['a', 'b', 'c', 'd', 'e', 'f']
print(list_section_reverse(mylist, 3))

edited May 26, 2016 at 12:56

answered May 26, 2016 at 12:50

Selcuk

60.1k12 gold badges114 silver badges119 bronze badges

Sign up to request clarification or add additional context in comments.

6 Comments

user2572329 Over a year ago

I want to achieve this using a loop, not with built-in functions.

user2572329 Over a year ago

@Selcuk Sorry, I thought mentioning that I only have to use loops in my title would have been enough. Turns out not.

user2572329 Over a year ago

@Selcuk Now, I get an error stating IndexError: list assignment index out of range

Selcuk Over a year ago

@user2572329 It works fine for me, are you sure you are running the same code? Note that initialization of n_list has also changed.

user2572329 Over a year ago

@Selcuk Yep, same code, but different list (same size list). I can't see anything different...

|

Dunes · Accepted Answer · 2016-05-26 13:24:46Z

2

The pythonic solution:

list1[reverse_index::-1] + list1[reverse_index+1:]

Now, that's not using loops like you asked. Well, not explicitly... Instead we can break down the above into its constituent for loops.

def list_section_reverse(list1, reverse_index):
    if reverse_index < 0 or reversed_index >= len(list1):
        raise ValueError("reverse index out of range")
    reversed_part = []
    for i in range(reverse_index, -1, -1): # like for i in [n, n-1, ..., 1, 0]
        reversed_part.append(list1[i]

    normal_part = []
    for i in range(reverse_index + 1, len(list1)):
        normal_part.append(list1[i])

    return reversed_part + normal_part

answered May 26, 2016 at 13:24

Dunes

42.1k7 gold badges86 silver badges107 bronze badges

Comments

Luis · Accepted Answer · 2016-05-26 13:35:26Z

0

Is it allowed to make a copy of the list?

def list_section_reverse(list1, reverse_index):
    print("In function reverse()")

    n_list = [ element for element in list1 ]

    for item in range( reverse_index + 1 ):
        n_list[ item ] = list1[ reverse_index - item ]
        item += 1

    return n_list

mylist = ['a', 'b', 'c', 'd', 'e', 'f']
print(list_section_reverse(mylist, 3))

Outs:

In function reverse()
['d', 'c', 'b', 'a', 'e', 'f']

answered May 26, 2016 at 13:35

Luis

3,4977 gold badges38 silver badges63 bronze badges

2 Comments

user2572329 Over a year ago

This works, and works with all sizes of Lists. Appreciate it mate, thanks.

Luis Over a year ago

Glad it helped! ^_^

hilberts_drinking_problem · Accepted Answer · 2016-05-26 13:46:48Z

0

You can modify the list inplace using a slice.

mylist[:4] = mylist[:4][::-1]

answered May 26, 2016 at 13:46

hilberts_drinking_problem

11.6k3 gold badges25 silver badges55 bronze badges

Comments

RoadRunner · Accepted Answer · 2016-05-26 14:28:27Z

0

You can try this. This makes use of no slicing, and can use either a while loop or for loop.

    def reversed_list(my_list, index):
        result = []
        list_copy = my_list.copy()

        i = 0
        while i < index+1:
            result.append(my_list[i])
            list_copy.remove(my_list[i])
            i+=1

        result.reverse()

        return result + list_copy

Or with a for loop

    def reversed_list(my_list, index):
        result = []
        list_copy = my_list.copy()

        for i in range(len(my_list)):
            if i < index + 1:
                result.append(my_list[i])
                list_copy.remove(my_list[i])

        result.reverse()

        return result + list_copy

edited May 26, 2016 at 14:28

answered May 26, 2016 at 13:15

RoadRunner

26.4k6 gold badges46 silver badges77 bronze badges

4 Comments

RoadRunner Over a year ago

oops sorry, I didnt see the loop part in the description. I will add that

user2572329 Over a year ago

Cheers. Also, if possible, can I accomplish this without slice expressions?

RoadRunner Over a year ago

I updated the answer. Do you still want to remove all sorts of slicing?

RoadRunner Over a year ago

@user2572329, this solution has no slicing, and uses only one loop each.

Collectives™ on Stack Overflow

How can I reverse a section of a list using a loop in Python?

5 Answers 5

6 Comments

Comments

2 Comments

Comments

4 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

5 Answers 5

6 Comments

Comments

2 Comments

Comments

4 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related