Slice a list of lists?

[Jonno]

On Wed, Sep 8, 2010 at 3:06 PM, Jonno <jonnojohnson at gmail.com> wrote:
> On Wed, Sep 8, 2010 at 2:11 PM, Benjamin Kaplan
> <benjamin.kaplan at case.edu> wrote:
>> On Wed, Sep 8, 2010 at 2:55 PM, Jonno <jonnojohnson at gmail.com> wrote:
>>> I know that I can index into a list of lists like this:
>>> a=[[1,2,3],[4,5,6],[7,8,9]]
>>> a[0][2]=3
>>> a[2][0]=7
>>>
>>> but when I try to use fancy indexing to select the first item in each
>>> list I get:
>>> a[0][:]=[1,2,3]
>>> a[:][0]=[1,2,3]
>>>
>>> Why is this and is there a way to select [1,4,7]?
>>> --
>>
>> It's not fancy indexing. It's called taking a slice of the existing
>> list. Look at it this way
>> a[0] means take the first element of a. The first element of a is [1,2,3]
>> a[0][:] means take all the elements in that first element of a. All
>> the elements of [1,2,3] are [1,2,3].
>>
>> a[:] means take all the elements of a. So a[:] is [[1,2,3],[4,5,6],[7,8,9]].
>> a[:][0] means take the first element of all the elements of a. The
>> first element of a[:] is [1,2,3].
>>
>> There is no simple way to get [1,4,7] because it is just a list of
>> lists and not an actual matrix. You have to extract the elements
>> yourself.
>>
>> col = []
>> for row in a:
>>    col.append(row[0])
>>
>>
>> You can do this in one line using a list comprehension:
>> [ row[0] for row in a ]
>>
> Thanks! (to Andreas too). Totally makes sense now.
>

Now if I want to select the first item in every 2nd item of list a
(ie: [1,7]) can I use ::2 anywhere or do I need to create a list of
indices to use in a more complex for loop?