Initializing matrix python3

Question

I don't know whether this is a bug, or I got a wrong semantic meaning of the * token in arrays:

>>> arr = [None] * 5    # Initialize array of 5 'None' items
>>> arr
[None, None, None, None, None]
>>> arr[2] = "banana"
>>> arr
[None, None, 'banana', None, None]
>>> # right?
... 
>>> mx = [ [None] * 3 ] * 2     # initialize a 3x2 matrix with 'None' items
>>> mx
[[None, None, None], [None, None, None]]
>>> # so far, so good, but then:
... 
>>> mx[0][0] = "banana"
>>> mx
[['banana', None, None], ['banana', None, None]]
>>> # Huh?

Is this a bug, or did I got the wrong semantic meaning of the __mult__ token?

Ben · Accepted Answer · 2013-09-22 16:21:39Z

1

You're copying the same reference to the list multiple times. Do it like this:

matrix = [[None]*3 for i in range(2)]

answered Sep 22, 2013 at 16:21

Ben

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3 Comments

user2804578 Over a year ago

So, then what's exactly the meaning of the * token? If it does copy the reference of an object, then I should expect that, on the first part, when passing arr[2] = "banana", the list should be ['banana', 'banana', 'banana', 'banana', 'banana'] and not that one above.

user2804578 Over a year ago

Oh, just bumped to the answer elsewhere on StackOverflow However, I find it quite odd that the * token doesn't have the same meaning on list as for other elementary data types

Ben Over a year ago

It actually does have the same meaning. It's just that for immutable types, doing things like "copying" always make a new object that's assigned the same value, whereas mutable objects just have their reference copied. Here's some more good reading on what it means for a type to be immutable. stackoverflow.com/questions/8056130/…

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