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], [], [], [], [], [], [], [], [], []]
?
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1docs.python.org/faq/…Praveen Gollakota– Praveen Gollakota2012-02-19 20:17:38 +00:00Commented Feb 19, 2012 at 20:17
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1Initialise a list to a specific length in PythonPaolo Moretti– Paolo Moretti2012-02-19 20:20:47 +00:00Commented Feb 19, 2012 at 20:20
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1@PaoloMoretti: That's not the same question, although there are countless duplicates on SO.Wooble– Wooble2012-02-19 20:39:02 +00:00Commented Feb 19, 2012 at 20:39
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@Wooble No, it's not, but the answer from Alex Martelli also answers this question.Paolo Moretti– Paolo Moretti2012-02-19 20:45:49 +00:00Commented Feb 19, 2012 at 20:45
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3 Answers
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.
Comments
Your current code creates an array containing the same array ten times.
Use [[] for i in xrange(10)] so you actually create separate arrays.
Comments
Try this:
>>> a = [[] for i in range(10)]
>>> a[0].append(1)
>>> a
[[1], [], [], [], [], [], [], [], [], []]
