could someone explain this code( list of list in python)?

Question

what I want to manipulate is the first element of v, the second print is supposed to be

[[3], [], [], [], []].

Code:

v = [[]] * 4

print(v)
v[0].append(3);  #what i want to manipulate is the first element of v 
print(v)

output:

[[], [], [], [], []]
[[3], [3], [3], [3], [3]]

i got it: v = [[] for n in range(4)] – wenson chen Dec 20 '17 at 17:05 — wenson chen, Dec 20 '17 at 17:05

score 0 · Answer 1 · answered Dec 20 '17 at 16:39

0

Python mutable variables (like list) are passed by reference.

v = [[]] * 4

Gives you a list of the same reference, so when you update one element of the list, all other moved with.

v = [[], [], [], []]

Will fix it

answered Dec 20 '17 at 16:39

Arount

Thanks for your quick answer(I am waiting online).
but what if dynamic size like: v = [[]] * N – wenson chen Dec 20 '17 at 16:42

1 Answers1