[Tutor] Matrix

[Hugo González Monteverde]

Brian van den Broek wrote:
> 2) To get around that, and be more efficient with matricies with many 
> empty cells:
> .>>> my_matrix_as_dict = {(1,1):4, (1,2):6, (1,3):8,
>                      (2,1):56, (2,3):12,
>                      (3,1):3, (3,2):3}
> 
> .>>> my_matrix_as_dict[(3,1)]
> 3
> .>>> my_matrix_as_dict[(2,1)]
> 56
> 
> So, you just can use the tuple co-ordinates you've defined in order to 
> access cells. Note also that the list way you'd have to represent empty 
> cells with a standard null value -- None is the usual choice. But this 
> way, you just don't define some tuples as keys, as I didn't define (2,2) 
> as a key. Thus:
> 
> .>>> my_matrix_as_dict[(2,2)]
> 
> Traceback (most recent call last):
>   File "<pyshell#19>", line 1, in -toplevel-
>     my_matrix_as_dict[(2,2)]
> KeyError: (2, 2)
> 
> You can make that more graceful with a try/except block:
> 
> .>>> try:
>     my_matrix_as_dict[(2,2)]
> except KeyError:
>     print "That cell is empty"
>     
> That cell is empty
> .>>>
> 

if you want None to be returned for an empty cell, you can just do:

my_matrix_as_dict.get((2,2), None)

so that  the None object will de returned for all inexistant cells...

Hugo