Python - Nested lists: One line code

Question

A=???????????
print(A)
A[3][0]=5
print(A)

What can you put in the ?'s to make it output:

[[1], [1], [1], [1], [1], [1], [1], [1], [1], [1]]

[[1], [5], [1], [5], [1], [5], [1], [5], [1], [5]]

Hint: you can answer this question with an answer exactly as long as the number of ?'s. I tried the following.

A=[[1] for i in range(11)]

But this only gives me the first output. How can I make it so that I get the output given when A[3][0]=5?

But here you only change the third element? Not all odd ones... — Willem Van Onsem, Feb 24 '17 at 15:23
you want to change all odd ones by changing only one element? — Jean-François Fabre, Feb 24 '17 at 15:23
Possible duplicate of [Making a flat list out of list of lists in Python](http://stackoverflow.com/questions/952914/making-a-flat-list-out-of-list-of-lists-in-python) — Abdelsalam Shahlol, Feb 24 '17 at 15:24
Possible duplicate of [How to create a 2d list with all same values but can alter multiple elements within? (python)](http://stackoverflow.com/questions/42432230/how-to-create-a-2d-list-with-all-same-values-but-can-alter-multiple-elements-wit) — Stefan Pochmann, Feb 24 '17 at 15:31

score 2 · Answer 1 · answered Feb 24 '17 at 15:26

If you construct your list as

A = [[1],[1]]*5

then it contains 5 references to two lists. It's like saying:

X = [1]
Y = [1]
A = [X, Y, X, Y, X, Y, X, Y, X, Y]

Initially this looks like:

[[1], [1], [1], [1], [1], [1], [1], [1], [1], [1]]

Then altering

A[3][0]=5

means you're altering the list that lies in every alternate position in A, giving you

[[1], [5], [1], [5], [1], [5], [1], [5], [1], [5]]

1 Answers1