Slice a list of lists?

Jonno jonnojohnson at gmail.com
Wed Sep 8 22:06:15 CEST 2010


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.



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