Question

For Python: I'm trying to replicate a list of lists. This function takes in a list and is supposed to return a duplicate. However, it just gives me a duplicate reference to the list, rather than a list independent of the first one.

def getListCopy(list):
dupeList = []
for x in list:
dupeList.append(x)
return dupeList

Can anyone explain why this returns a list reference instead of a duplicate for the list values?

Answer 1

you have a list of objects a = [object1, object2] - here your objects are lists
b = a[:] will create a new list, containing the same objects (lists)
the simplest way around this is to use deepcopy -
from copy import deepcopy
dupList = deepcopy(a)
it copies copies of the objects in a
( google deepcopy in python for better explanations...)

Answer 2

Most languages pass primitives ( int, float, bool, char, maybe string ) by value. Objects and collections are passed by reference because they can be large and take a while to copy and really suck up memory.

Answer 3

Right, I understand that objects are passed by reference. I guess my question is, why would something like

def getListCopy(list):
dupeList = []
for x,y in list:
dupeList += [[x,y]]
return dupeList

work for a list of lists rather than my first post? Here, my objects are still lists and my new list contains all the same objects of my old list. This function returns a duplicate list independent of the first one. That is to say, if dupeList=getListCopy(list), list != dupeList. In my first post, dupeList = list. Anything I changed in list, I changed in dupeList. Why is this?

Answer 4

a = [[1,2],[2,3],[3,4]]
a is a list of references to lists
a[0] references the list [1,2], that is, a[0] is the address which contains the list [1,2]
so in your original question, dupList was copying the references to the lists, not the lists
themselves.
what you can do is:
d=[]
for x in a:
...e = []
...for y in e:
......e.apend[y]
...d.append[e]
but maybe i've misunderstood (again).

Answer 5

eeech...for y in x:

Answer 6

Okay there we go I get it. This is the response I was looking for. In my original function, I was just appending the reference to the lists...that's why if I changed anything in list, I was also changing it in dupeList. When you explained it the first time, I thought you were just explaining the concept of references to lists (which I get). I should have sent you the second function the first time, because what I really wanted to know was why append() didn't work (it copied references). So thanks!

I also looked into deepcopy. It's really useful