Multi-dimensional list initialization

Mon Nov 5 04:55:45 EST 2012

Le lundi 5 novembre 2012 07:28:00 UTC+1, Demian Brecht a écrit :
> So, here I was thinking "oh, this is a nice, easy way to initialize a 4D matrix" (running 2.7.3, non-core libs not allowed):
>>>> m = [[None] * 4] * 4
>>>> The way to get what I was after was:
>>>> m = [[None] * 4, [None] * 4, [None] * 4, [None * 4]] 
>>>> (Obviously, I could have just hardcoded the initialization, but I'm too lazy to type all that out ;))
>>>> The behaviour I encountered seems a little contradictory to me. [None] * 4 creates four distinct elements in a single array while [[None] * 4] * 4 creates one distinct array of four distinct elements, with three references to it:
>>>> >>> a = [None] * 4
>> >>> a[0] = 'a'
>> >>> a
>> ['a', None, None, None]
>>>> >>> m = [[None] * 4] * 4
>> >>> m[0][0] = 'm'
>> >>> m
>> [['m', None, None, None], ['m', None, None, None], ['m', None, None, None], ['m', None, None, None]]
>>>> Is this expected behaviour and if so, why? In my mind either result makes sense, but the inconsistency is what throws me off.
>>>> Demian Brecht
>> @demianbrecht
>> http://demianbrecht.github.com

----------
You probably mean a two-dimensional matrix not a 4D matrix.
>>> def DefMatrix(nrow, ncol, val):
... return [[val] * ncol for i in range(nrow)]
... 
>>> aa = DefMatrix(2, 3, 1.0)
>>> aa
>>> aa = DefMatrix(2, 3, 1.0)
>>> aa
[[1.0, 1.0, 1.0], [1.0, 1.0, 1.0]]
>>> aa[0][0] = 3.14
>>> aa[1][2] = 2.718
>>> aa
[[3.14, 1.0, 1.0], [1.0, 1.0, 2.718]]
>>>>>> bb = DefMatrix(2, 3, None)
>>> bb
[[None, None, None], [None, None, None]]
>>> bb[0][0] = 3.14
>>> bb[1][2] = 2.718
>>> bb
[[3.14, None, None], [None, None, 2.718]]
jmf