making a new modified version of a list without modifying the original list in python

Question

i need to modify the contents of my og list into a different list w/out actually changing my og list.

def createList(numbers):
  my_List= [0] * numbers 
  for q in range(0, len(my_List)):
      myList[q]= randint (1, 21)
      q=q+1
  return my_List

def modifyList(newList):
  for i in range(0, len(newList)):
    if i % 2 == 0:
      newList[i]= newList[i] / 2
    else:
      newList[i]= newList[i] * 2
  return newList

def main():
  my_List= createList(10)
  print my_List
  newList= modifyList(my_List)
  print my_List
  print newList

Answer 1

You need to make a copy of the list that is inputted to the modifyList function . This copy isn't done with myList[:] as you are not working with myList here! You are working with a different variable called newList which you need to make a copy of.

You need to remember that a function works with a variable that is passed into it but under the name it has been assigned in the function definition. So here, even though you only call the function with modifyList(myList) , inside the function, you are always working with newList so trying to do anything with myList here will throw an error saying its undefined.

def modifyList(newList):
  newList = newList[:]
  for j in range(0, len(newList)):
    if j % 2 == 0:
      newList[j]= newList[j] / 2
    else:
      newList[j]= newList[j] * 2
  return newList

Answer 2

Here's an alternate way, with list comprehensions. In Python, you usually don't have to create a list with placeholders and put the elements one by one:

>>> from random import randint
>>> my_list = [randint(1, 20) for _ in range(10)]
>>> my_list
[1, 20, 2, 4, 8, 12, 16, 7, 4, 14]
>>> [x * 2 if i % 2 else x / 2 for i, x in enumerate(my_list)]
[0.5, 40, 1.0, 8, 4.0, 24, 8.0, 14, 2.0, 28]

If you want to modify the original list in place, you could use numpy and advanced slicing:

>>> import numpy as np
>>> a = np.array([11, 13, 21, 12, 18, 2, 21, 1, 5, 9])
>>> a[::2] = a[::2] / 2
>>> a[1::2] = a[1::2] * 2
>>> a
array([ 5, 26, 10, 24,  9,  4, 10,  2,  2, 18])