[Matrix-SIG] Problem with arrays of arrays

[Marc Keller]

Vincenzo Tschinke wrote:

> Hi!
>
> I am doing some basic molecular connectivity operation (graphs). Trying to
> convert a list of bonds (list of edges) into a connectivity table (for each
> vertex [atom], list of connected vertices), by filling an array of arrays, I
> got the right or wrong result depending on how I define the initial arrays.
>
> If I define an array of N emtpy arrays as
>
>         array_of_empties = N * [[]]
>
> I get the *wrong* result. If I use the more exotic definition
>
>         array_of_empties = map ( lambda x : [] , range( N ))
>
> I get the *right* result. Note that after being created, such two arrays
> compare as equal.
>
> Please see the listing below.
>
> Is there anything wrong with the first definition, or do we have a
> Python bug here?
>
> ===========================================================================
>
> Python 1.5.2b1 (#47, Jan 13 1999, 15:14:59)  [CW PPC w/GUSI w/MSL]
>
> >>> ee=[[1,2],[1,3],[1,4]]              # 3 edges : 3 vert. connected to centr. vertex
> >>> ee
> [[1, 2], [1, 3], [1, 4]]
> >>> aa=4*[[]]                           # array of four empty arrays
> >>> AA=map(lambda x : [], range(4))     # same as above
> >>> aa==AA                              # arrays are equal !!!
> 1
> >>> for p in ee:                        # build table of connected verteces
> ...     i=p[0]
> ...     j=p[1]
> ...     aa[i-1].append(j)
> ...     aa[j-1].append(i)
> ...     AA[i-1].append(j)
> ...     AA[j-1].append(i)
> ...
> >>> aa                                  # wrong result !!!
> [[2, 1, 3, 1, 4, 1], [2, 1, 3, 1, 4, 1], [2, 1, 3, 1, 4, 1], [2, 1, 3, 1, 4, 1]]
> >>> AA                                  # correct result
> [[2, 3, 4], [1], [1], [1]]
> >>> aa==AA
> 0
> >>>
>
> =======================
> Vincenzo Tschinke
> Kernmattstr. 24
> CH-4102 Binningen
> Switzerland
> +4161 422 1991 (T+F)
> tschinke@swissonline.ch
> =======================
>
> _______________________________________________
> Matrix-SIG maillist  -  Matrix-SIG@python.org
> http://www.python.org/mailman/listinfo/matrix-sig

Using N * [[]], you are building a list with 4 references to the same list.
Using map ( lambda x : [] , range( N )), each call to lambda produces a new empty list.
Your exemple in an other way:
>>> aa=[[],[],[],[]]
>>> for p in ee:                        # build table of connected verteces
...     i=p[0]
...     j=p[1]
...     aa[i-1].append(j)
...     aa[j-1].append(i)
...
>>> aa
[[2, 3, 4], [1], [1], [1]]
>>>