[Tutor] Using lists as table-like structure

[Danny Yoo]

On Wed, 16 Nov 2005, Bernard Lebel wrote:

> Let say I have a list of lists. Each individual lists have a bunch of
> elements. Now I would like to either get or set the first element of
> each individual list. I could do a loop and/or list comprehension, but I
> was wondering if it was possible with something like:
>
> aList = [ [1,1,1], [2,2,2,], [3,3,3] ]
> aList[:][0] = 10


Hi Bernard,

I think I see what you're trying to do; you're trying to clear the first
column of each row in your matrix.  Unfortunately, this is not done so
easily in standard Python.  However, if you use the third-party Numeric
Python (numarray) package, you can use its array type to do what you want.


> If I print aList[:], I get the list with the nested sublists.
>
> >>> aList[:]
> [[1, 1, 1], [2, 2, 2], [3, 3, 3]]

Yes, sounds good so far.


> But as soon as I introduce the [0], in an attempt to access the first
> element of each sublist, I get the first sublist in its entirety:
>
> >>> aList[:][0]
> [1, 1, 1]


Let's do a quick substitution model thing here.  You mentioned earlier
that:

> >>> aList[:]
> [[1, 1, 1], [2, 2, 2], [3, 3, 3]]

So if we just plug that value into aList[:][0]:

     aList[:][0]  ==>  [[1, 1, 1,], [2, 2, 2], [3, 3, 3]] [0]

then we see that we're just asking for the first element of aList[:],
which is [1, 1, 1].



> I would have hoped to get something like [1, 2, 3]

Take a look into Numeric Python: it'll give you the row/column slicing
operations that you're expecting.  As a concrete example:

######
>>> import numarray
>>> a = numarray.array([[1, 2, 3],
...                     [4, 5, 6],
...                     [7, 8, 9]])
>>> a[:, 0]
array([1, 4, 7])
>>> a[:, 1]
array([2, 5, 8])
>>> a[:, 2]
array([3, 6, 9])
######


Best of wishes!