Creating a reverse order list using while loop - unexpected results

Question

I am a student in an intro-level python class, and my task is to define "reverse(mylist)" using while loop

This is what I have so far:

def reverse(mylist):
    a=0
    b=len(mylist)
    xlist=mylist
    while(a!=b):
        mylist[a]=xlist[(-a)-1]
        a+=1
    return mylist

Let's say the input list is [1,2,3,4,5,6], and using my reverse function I will get the output [6, 5, 4, 4, 5, 6]... And [1,2,3,4,5] will become [5,4,3,4,5]

I am not sure what I am doing wrong here.

Answer 1

Following statement makes both xlist, mylist reference same list object:

xlist = mylist

You need to copy it.

xlist = mylist[:]

BTW, using for loop, you don't need to increment a manually:

for a in range(len(mylist)):
    ....

Answer 2

def reverse(mylist):
  a=0
  b=len(mylist)
  xlist=[]
  while(a!=b):
    mylist[a:a]=xlist[(-a)-1]
    a+=1
  return mylist

list is transfered by referance not by value. you need to create new list. "xlist = mylist" only create a referance.

ps "for in" is more commonly used in python.

for i in range(2, -1, -1):
  xlist.append(mylist[i])

or:

xlist = [mylist[i], for i in range(2, -1, -1) ]