Initialise array of empty arrays

Question

This is my code:

a = [[]] * 10
a[0].append(1)
print a # Outputs [[1], [1], [1], [1], [1], [1], [1], [1], [1], [1]]

How can I get a to output

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

?

http://docs.python.org/faq/programming.html#how-do-i-create-a-multidimensional-list — Praveen Gollakota, Feb 19 '12 at 20:17
[Initialise a list to a specific length in Python](http://stackoverflow.com/questions/983699/initialise-a-list-to-a-specific-length-in-python) — Paolo Moretti, Feb 19 '12 at 20:20
@PaoloMoretti: That's not the same question, although there are countless duplicates on SO. — Wooble, Feb 19 '12 at 20:39
@Wooble No, it's not, but the answer from Alex Martelli also answers this question. — Paolo Moretti, Feb 19 '12 at 20:45

hochl · Accepted Answer · 2012-04-13T23:36:25.010

Try

a=[[] for i in xrange(10)]

In your code you're adding the same list 10 times. The following output should clarify this:

>>> a=[[]] * 5
>>> for i in a: print id(i)
... 
155302636
155302636
155302636
155302636
155302636
>>> a=[[] for i in xrange(5)]
>>> for i in a: print id(i)
... 
155302668
155302732
155302924
155303020
155303052

As you can see, in the first example a contains 5 times a reference to the same array object, in the second example it contains references to 5 different array objects.

score 3 · Answer 2 · answered Feb 19 '12 at 20:17

3

Your current code creates an array containing the same array ten times.

Use [[] for i in xrange(10)] so you actually create separate arrays.

answered Feb 19 '12 at 20:17

ThiefMaster

310,957
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score 1 · Answer 3 · answered Feb 19 '12 at 20:17

1

Try this:

>>> a = [[] for i in range(10)]
>>> a[0].append(1)
>>> a
[[1], [], [], [], [], [], [], [], [], []]

answered Feb 19 '12 at 20:17

grifaton

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Initialise array of empty arrays

3 Answers3