What does this kind of array mean/do?

Question

I found this declaration in a piece of code involved in a puzzle, could anyone explain what it's doing? I've tried looking myself but I don't really understand.

test = [[0] * 9] * 9

@zhangxaochen are you sure? That guy knew what the `*` was doing, although not fully, and got caught up in the references. The OP here might need a more in-depth and from scratch explanation. — Guy, Mar 02 '14 at 19:58

score 10 · Accepted Answer · answered Mar 02 '14 at 19:49

When you do

[0] * 9

you get a list with nine 0's:

[0, 0, 0, 0, 0, 0, 0, 0, 0]

When you do

[[0] * 9] * 9

you get

[[0, 0, 0, 0, 0, 0, 0, 0, 0], [0, 0, 0, 0, 0, 0, 0, 0, 0], [0, 0, 0, 0, 0, 0, 0, 0, 0], [0, 0, 0, 0, 0, 0, 0, 0, 0], [0, 0, 0, 0, 0, 0, 0, 0, 0], [0, 0, 0, 0, 0, 0, 0, 0, 0], [0, 0, 0, 0, 0, 0, 0, 0, 0], [0, 0, 0, 0, 0, 0, 0, 0, 0], [0, 0, 0, 0, 0, 0, 0, 0, 0]]

In other words, you get [0, 0, 0, 0, 0, 0, 0, 0, 0] 9 times. But you have to be careful because this last, makes a shallow copy of the list. If you modify one element of that list, then it will be "modified" in all the lists (in fact because all those elements are the same list). If you want each list to be a different one, you could make a deep copy.

You can easily see this by using a print statement:

test = [[0] * 9] * 9
test[0][1] = 2
print test
>>> [[0, 2, 0, 0, 0, 0, 0, 0, 0], [0, 2, 0, 0, 0, 0, 0, 0, 0], [0, 2, 0, 0, 0, 0, 0, 0, 0], [0, 2, 0, 0, 0, 0, 0, 0, 0], [0, 2, 0, 0, 0, 0, 0, 0, 0], [0, 2, 0, 0, 0, 0, 0, 0, 0], [0, 2, 0, 0, 0, 0, 0, 0, 0], [0, 2, 0, 0, 0, 0, 0, 0, 0], [0, 2, 0, 0, 0, 0, 0, 0, 0]]

Note: List is a better name for [...] in Python. Read more about lists and tuples.

Carreful though, this is 9 references to the same list. – Bite code Mar 02 '14 at 19:53 — Bite code, Mar 02 '14 at 19:53
@e-satis yes I've already added that clarification. – Christian Tapia Mar 02 '14 at 19:54 — Christian Tapia, Mar 02 '14 at 19:54

score 2 · Answer 2 · edited May 23 '17 at 10:26

Note: That is wrong. Don't use that as boilerplate code.

When you do

print [0] * 9

you get

[0, 0, 0, 0, 0, 0, 0, 0, 0]

The same element in the original array is referenced by all the nine elements in the new array. So, all of them print 0. The same way, when you say

print [[0] * 9] * 9

it creates a list of 9 zeros (think of this as l1) and then another list of 9 elements, all of which refer to the same list (l1). So, when you change one element in the newly created list, change will be reflected in all the lists.

The proper way to do this would be

print [[0 for j in xrange(9)] for i in xrange(9)]

Since, integers are immutables in Python, you can write the same as

print [[0] * 9 for i in xrange(9)]

score 0 · Answer 3 · answered Mar 02 '14 at 19:51

0

The syntax [0] * 9 will generate a list of 9 zeroes. So I believe the full code will generate a list of 9 lists of 9 zeroes.

answered Mar 02 '14 at 19:51

Tyler Brabham

218
2
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What does this kind of array mean/do?

3 Answers3