can't modify copy of array without changing original array

Question

I tried modifying the array "newTab" but without use tab.copy() but it always modifies the original array.

tab = [[1]*2]*3
newTab = [None] * len(tab)
for i in range(0, len(tab)):
    newTab[i] = tab[i]

newTab[0][0] = 2
print(tab)
[[2, 1], [2, 1], [2, 1]]
print(newTab)
[[2, 1], [2, 1], [2, 1]]

I also tried using something like this : a = b[:] but it doesn't work. Somehow the original array is always a reference to the new one. I just started learning python and we can only use the basics for our homework. So i'm not allowed to use things like deepcopy() Any help would be appreciated!

tab and newTab are both pointing to the same objects. Besides copying the list, what are you actually trying to accomplish? — Paul H
– Paul H, Commented Nov 12, 2022 at 0:12
Does this answer your question? List of lists changes reflected across sublists unexpectedly — Swifty
– Swifty, Commented Nov 12, 2022 at 0:13

shawnjb · Accepted Answer · 2022-11-12 00:24:22Z

1

You could use the copy library.

import copy

tab = [[1] * 2] * 3
newTab = [None] * len(tab)
for i in range(len(tab)):
    newTab[i] = copy.deepcopy(tab[i])
    newTab[i][0] = 2

print(tab)
print(newTab)

answered Nov 12, 2022 at 0:24

shawnjb

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3 Comments

Patrick Kelvin Over a year ago

what exactly does import copy do? I've seen people use it multiple times but i don't know what it does

shawnjb Over a year ago

Importing is like using modules in Lua with require("module"), or using a namespace in java like import java.io.InputStream for example. They just call for third party dependencies that the project needs to use.

Patrick Kelvin Over a year ago

Okay thanks. Unfortunately, i can't use that either...

KEcoding · Accepted Answer · 2022-11-12 00:58:36Z

0

tab = [[1]*2]*3
tab2 = []
for t in tab:
    tab2.append(t.copy())
#check that it worked - none of the values from tab2 should be the same as tab:
for t in tab:
    print(id(t))
for t in tab2:
    print(id(t))

as for why this happens: technically, things like lists and dictionaries are pointers in python. meaning, you aren't handling the list itself, you are holding an address to where your info is stored. Thus, when you just assign a new variable to your list (that's inside another list), you're saying "I want to point to this address" , not "I want to put that same thing in another memory address"

answered Nov 12, 2022 at 0:58

KEcoding

513 bronze badges

1 Comment

KEcoding Over a year ago

I'm glad I could help - I had to learn C to finally understand this so I'm glad I could spare you the confusion.

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