what is the difference between [[False] * 3] * 3 and [[False for i in range(3)] for j in range(3)]?

Question

I am trying to initialize the 2D array with false values. Both instructions mentioned below are producing same results. Please help to know the difference between the two ?

Input

[[False] * 3] * 3 
[[False for i in range(3)] for j in range(3)]

Output

[[False, False, False], [False, False, False], [False, False, False]]
[[False, False, False], [False, False, False], [False, False, False]]

Can you include your code in the post too and not just the title? — Sid, Apr 16 '18 at 08:00
What is the question? They does the same thing, in different way — Chen A., Apr 16 '18 at 08:02
@Vinny No, they don't quite do the same thing. Try `x = [[False]*3]*3` and then `x[0][0] = True`. — Aran-Fey, Apr 16 '18 at 08:03
@Vinny try `my_list[0][1] = True` to see whether they are the same — Maarten Fabré, Apr 16 '18 at 08:03
how do you compare them ? if I write 'a=1st writing', 'b=2nd', then 'a==b' returns True. — Pierre G., Apr 16 '18 at 08:06
@Aran-Fey you're right, it's a small gotcha. The first creates the same list object, the second creates a new list object. — Chen A., Apr 16 '18 at 08:07
@vinothkumar the first one creates one list object, with 3 pointers to it. Changing one item in one list affects all of them (since they are the same object). The second one creates a new list object every iteration - so I guess that's what you need. — Chen A., Apr 16 '18 at 08:08
@Vinny gotcha that's exactly what i want to know thank you :) — vinoth kumar, Apr 16 '18 at 08:11

score 1 · Accepted Answer · answered Apr 16 '18 at 08:04

1

Let' try this:

>>> a = [[False] * 3] * 3
>>> b = [[False for i in range(3)] for j in range(3)]
>>> a
[[False, False, False], [False, False, False], [False, False, False]]
>>> b
[[False, False, False], [False, False, False], [False, False, False]]

Now let's change a single value (do we?):

>>> a[0][0] = True
>>> a
[[True, False, False], [True, False, False], [True, False, False]]

Note that a was changed in three positions.

>>> b[0][0] = True
>>> b
[[True, False, False], [False, False, False], [False, False, False]]

b was not. That's the difference.

answered Apr 16 '18 at 08:04

1

Wow ! Could you explain what is actually happening ? – vinoth kumar Apr 16 '18 at 08:09
Take a look at https://stackoverflow.com/questions/240178/list-of-lists-changes-reflected-across-sublists-unexpectedly – Apr 16 '18 at 08:31
1

I should have searched in that way thanks man ! – vinoth kumar Apr 16 '18 at 08:33

what is the difference between [[False] * 3] * 3 and [[False for i in range(3)] for j in range(3)]?

1 Answers1