creating a reverse method for a python list from scratch

Question

I want to create a reverse method for a list. I know there already is such a method built into python but I want to try it from scratch. Here's what I have and it seems to make sense to me but it just returns the list in the same order. My understanding was that lists are mutable and I could just reassign values in the loop.

def reverse(data_list):
    length = len(data_list)
    s = length

    for item in data_list:
        s = s - 1
        data_list[s] = item
    return data_list

Solution 1

You are changing the list that you iterate on it (data_list) because of that it's not working , try like this:

def reverse(data_list):
    length = len(data_list)
    s = length

    new_list = [None]*length

    for item in data_list:
        s = s - 1
        new_list[s] = item
    return new_list

Solution 2

def reverse(data_list):
    return data_list[::-1]

>> reverse([1,2,3,4,5])
[5, 4, 3, 2, 1]

Solution 3

By the time you are half-way through the list, you have swapped all the items; as you continue through the second half, you are swapping them all back to their original locations again.

Instead try

def reverse(lst):
    i = 0            # first item
    j = len(lst)-1   # last item
    while i<j:
        lst[i],lst[j] = lst[j],lst[i]
        i += 1
        j -= 1
    return lst

This can be used in two ways:

a = [1,2,3,4,5]
reverse(a)        # in-place
print a           # -> [5,4,3,2,1]

b = reverse(a[:]) # return the result of reversing a copy of a
print a           # -> [5,4,3,2,1]
print b           # -> [1,2,3,4,5]

Solution 4

an easy way in python (without using the reverse function) is using the [] access operator with negative values such as (print and create a new list in reverse order):

x = [1, 2 ,3, 4, 5]
newx = []
for i in range(1, len(x)+1):
  newx.append(x[-i])
  print x[-i]

the function would be:

def reverse(list):
  newlist = []
  for i in range(1, len(list)+1):
    newlist.append(list[-1])
  return newlist

Solution 5

There are two simple ways to solve the problem : First using a temp variable :

maList = [2,5,67,8,99,34]
halfLen = len(maList) // 2
for index in range(halfLen):
    temp = maList[index]
    maList[index] = maList[len(maList) - 1 - index]
    maList[len(maList) - 1 - index] = temp
print(maList)

Second is using a new list where to store the reversed values :

newList = []
for index,value in enumerate(maList):
    newList.append(maList[len(maList) - 1 - index])
print(newList)

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