python list comprehension: creating 2d array

Question

Possible Duplicate:
Python 2D list has weird behavor when trying to modify a single value

folks,

I am wondering if the two following statements are the same?

a = [[0]*3]*3
b = [[0]*3 for i in range(3)]

The results look the same. But would one way be better than the other? What is the difference here.

Thanks very much for your help.

nos

There are literally thousands questions covering this exact topic... — JBernardo, Jan 05 '12 at 19:53

score 18 · Accepted Answer · answered Jan 05 '12 at 19:54

18

They're not the same

>>> a[1][2] = 5
>>> a
>>> [[0, 0, 5], [0, 0, 5], [0, 0, 5]]


>>> b[1][2] = 5
>>> b
>>> [[0, 0, 0], [0, 0, 5], [0, 0, 0]]

The first one creates an outer array of pointers to a single inner array while the second actually creates 3 separate arrays.

answered Jan 05 '12 at 19:54

Charles Ma

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1

I was typing the *exact* same thing. +1 to you. – Makoto Jan 05 '12 at 19:56
yup, in the first one, all three arrays are the same object. If you use integers instead for example [1]*5 instead of [[]]*5, a new integer is used in each position – robert king Jan 05 '12 at 20:00
3

these are not python arrays...these are python lists. – joaquin Jan 05 '12 at 20:04

joaquin · Answer 2 · 2012-01-05T20:00:03.540

7

No they are not.
In the first one you have (a list of) 3 identical lists, same reference, in the second you have three different lists

>>> a = [[0]*3]*3
>>> a
[[0, 0, 0], [0, 0, 0], [0, 0, 0]]
>>> a[0][0]=1

>>> a
[[1, 0, 0], [1, 0, 0], [1, 0, 0]]


>>> b = [[0]*3 for i in range(3)]
>>> b
[[0, 0, 0], [0, 0, 0], [0, 0, 0]]
>>> b[0][0] = 1

>>> b
[[1, 0, 0], [0, 0, 0], [0, 0, 0]]

edited Jan 05 '12 at 20:00

answered Jan 05 '12 at 19:52

joaquin

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+1. Try `a[0].append("foo")`. – Fred Foo Jan 05 '12 at 19:53

score 0 · Answer 3 · answered Jan 06 '12 at 03:49

0

It's a classic case of shallow-copy vs deep copy, as explained here in the Python docs :)

answered Jan 06 '12 at 03:49

Ambidextrous

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python list comprehension: creating 2d array

3 Answers3