How do I add multiple versions of a list (meaning I'm updating the list) to a list, without having all the lists within the list be exactly the same?

Question

I'm having trouble getting the output I want. I want to create a list and have the first element of a new list be that initial list. Then I want to modify the first list, and have the new modified version of that list be the second element of the new list. Right now, this outputs [[2,3,4,5,5],[2,3,4,5,5]]. How would I modify it so that the output is [[1,2,3,4,5], [2,3,4,5,5]]?

noChange = [1,2,3,4,5]
lotsOfLists = []

lotsOfLists += [noChange]

for i in range(4):
    noChange[i] += 1
lotsOfLists += [noChange] 

print(lotsOfLists) # current output: [[2, 3, 4, 5, 5], [2, 3, 4, 5, 5]]
# WANT: [[1,2,3,4,5], [2,3,4,5,5]]

`listOfLists` contains many references to the same list (i.e. `noChange`). You have to make new list instances if you want them to be different. — zvone, Jul 26 '18 at 20:23

score 0 · Answer 1 · answered Jul 26 '18 at 20:25

0

Whenever you use the line

lotsOfLists += [noChange]

change it to

lotsOfLists += [noChange[:]]

Adding the [:] will make a copy of the list and not change it later

answered Jul 26 '18 at 20:25

Buzz

1,877
21
25

score 0 · Answer 2 · answered Jul 26 '18 at 20:25

Lists are mutable, that is causing the trouble. Use slices for copies.

noChange = [1,2,3,4,5]
lotsOfLists = []

lotsOfLists += [noChange[:]]

for i in range(4):
    noChange[i] += 1
lotsOfLists += [noChange[:]] 

print(lotsOfLists) #output [[1, 2, 3, 4, 5], [2, 3, 4, 5, 5]]

score 0 · Accepted Answer · answered Jul 26 '18 at 20:26

Lists are accessed by reference, so you need .copy() to create a new one.

noChange = [1,2,3,4,5]
lotsOfLists = []

lotsOfLists += [noChange.copy()]

for i in range(4):
    noChange[i] += 1
lotsOfLists += [noChange]

print(lotsOfLists) # output: [[1,2,3,4,5], [2,3,4,5,5]]

As others say, you can also use the slicing operator [:] to achieve the same result. IMHO, the semantics is different. With the slicing operator, you tell a reader that you want to slice something. With the copy() operation, you tell a reader, that you want to copy something.

How do I add multiple versions of a list (meaning I'm updating the list) to a list, without having all the lists within the list be exactly the same?

3 Answers3