On Tue, Nov 3, 2009 at 4:20 PM, Robert Berman <bermanrl@[...].com>  wrote:

> 
>    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.
> 
>  For whatever glimmers of clarity anyone can offer. I thank you.
> 

Tuples are immutable types. Thus it is not possible to change one of the
values of a tuple (or even of changing both of them). The only thing you can
do, is create a new tuple, and put that in the same place in the list. In
your example, when you do l1[2][0] = 3, you try to change the tuple l1[2],
which is impossible.

To do what you want to do, you have to create a new array with the same
second but different first value, and put that array in l1[2], that is:

l1[2] = (3, l1[2,1])




-- 
André Engels, andreengels@gmail.com