[Tutor] Being beaten up by a tuple that's an integer thats a tuple that may be an unknown 'thing'.

[Mark Tolonen]

"Robert Berman" <bermanrl at cfl.rr.com> wrote in message 
news:1257261606.29483.23.camel at bermanrl-desktop...
>
> In [69]: l1=[(0,0)] * 4
>
> In [70]: l1
> Out[70]: [(0, 0), (0, 0), (0, 0), (0, 0)]
>
> In [71]: l1[2][0]
> Out[71]: 0
>
> In [72]: l1[2][0] = 3
> ---------------------------------------------------------------------------
> TypeError                                 Traceback (most recent call
> last)
>
> /home/bermanrl/<ipython console> in <module>()
>
> TypeError: 'tuple' object does not support item assignment
>
> First question, is the error referring to the assignment (3) or the
> index [2][0]. I think it is the index but if that is the case why does
> l1[2][0] produce the value assigned to that location and not the same
> error message.
>
> Second question, I do know that l1[2] = 3,1 will work. Does this mean I
> must know the value of both items in l1[2] before I change either value.
> I guess the correct question is how do I change or set the value of
> l1[0][1] when I specifically mean the second item of an element of a 2D
> array?
>
> I have read numerous explanations of this problem thanks to Google; but
> no real explanation of setting of one element of the pair without
> setting the second element of the pair as well.

Tuples are read-only, so you can't change just one element of a tuple:

    >>> x=0,0
    >>> x
    (0, 0)
    >>> x[0]=1
    Traceback (most recent call last):
      File "<interactive input>", line 1, in <module>
    TypeError: 'tuple' object does not support item assignment

You can, however, replace the whole thing, as you found:

    >>> x=1,1
    >>> x
    (1, 1)

To do what you want, you a list of lists, not a list of tuples, but there is 
a gotcha.  This syntax:

    >>> L=[[0,0]]*4
    >>> L
    [[0, 0], [0, 0], [0, 0], [0, 0]]

Produces a list of the *same* list object, so modifying one modifies all:

    >>> L[2][0]=1
    >>> L
    [[1, 0], [1, 0], [1, 0], [1, 0]]

Use a list comprehension to create lists of lists, where each list is a 
*new* list:

    >>> L = [[0,0] for i in range(4)]
    >>> L
    [[0, 0], [0, 0], [0, 0], [0, 0]]
    >>> L[2][0] = 1
    >>> L
    [[0, 0], [0, 0], [1, 0], [0, 0]]

-Mark