How to initialise a 2D array in Python?

Question

I've been given the pseudo-code:

    for i= 1 to 3
        for j = 1 to 3
            board [i] [j] = 0
        next j
    next i

How would I create this in python?

(The idea is to create a 3 by 3 array with all of the elements set to 0 using a for loop).

score 18 · Accepted Answer · answered Jun 03 '14 at 19:15

18

If you really want to use for-loops:

>>> board = []
>>> for i in range(3):
...     board.append([])
...     for j in range(3):
...         board[i].append(0)
... 
>>> board
[[0, 0, 0], [0, 0, 0], [0, 0, 0]]

But Python makes this easier for you:

>>> board = [[0]*3 for _ in range(3)]
>>> board
[[0, 0, 0], [0, 0, 0], [0, 0, 0]]

answered Jun 03 '14 at 19:15

arshajii

127,459
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I can do this using this as well: `nums = [[0]*4]*4` and it works fine. Is it right method? – mehulmpt Mar 27 '15 at 11:04
8

@MehulMohan No, because each sublist will really be the same object. Try doing `nums[0][0] = 1` and see what `nums` becomes. – arshajii Mar 27 '15 at 12:31
1

Woah @arshaji can you give a detailed reason why did that happen? – mehulmpt Mar 27 '15 at 14:11

Padraic Cunningham · Answer 2 · 2014-06-03T19:59:45.780

arr=[[0,0,0] for i in range(3)] # create a list with 3 sublists containing [0,0,0]
arr
Out[1]: [[0, 0, 0], [0, 0, 0], [0, 0, 0]]

If you want an list with 5 sublists containing 4 0's:

In [29]: arr=[[0,0,0,0] for i in range(5)]

In [30]: arr
Out[30]: 
[[0, 0, 0, 0],
 [0, 0, 0, 0],
 [0, 0, 0, 0],
 [0, 0, 0, 0],
 [0, 0, 0, 0]]

The range specifies how many sublists you want, ranges start at 0, so ranges 4 is 0,1,2,3,4. gives you five [0,0,0,0]

Using the list comprehension is the same as:

arr=[]
for i in range(5):
    arr.append([0,0,0,0])

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

Could you explain your answer? – bjb568 Jun 03 '14 at 19:52 — bjb568, Jun 03 '14 at 19:52

Chris Mukherjee · Answer 3 · 2014-06-03T19:21:52.877

2

If you want something closer to your pseudocode:

board = []
for i in range(3):
    board.append([])
    for j in range(3):
        board[i].append(0)

edited Jun 03 '14 at 19:21

answered Jun 03 '14 at 19:16

Chris Mukherjee

841
9
25

score 0 · Answer 4 · answered Jun 03 '14 at 19:16

0

numpy has something for this:

numpy.zeros((3,3))

answered Jun 03 '14 at 19:16

Ben

2,065
1
13
19

score 0 · Answer 5 · answered Jun 03 '14 at 19:31

You can use the style of pseudocode given or simply just use a python one liner

chess_board = [[x]*3 for _ in range(y)] --> list comprehension

or you can use the plain loop style of other languages like java. I prefer the one liner as it looks much nicer and cleaner.

How to initialise a 2D array in Python?

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