Slice a list of lists?

Mark Lawrence breamoreboy at yahoo.co.uk
Thu Sep 9 18:13:22 CEST 2010


On 08/09/2010 21:23, Jonno wrote:
> On Wed, Sep 8, 2010 at 3:18 PM, Jonno<jonnojohnson at gmail.com>  wrote:
>> 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?
>>
> Seems like the simplest way would be:
> [row[0] for row in a][::2]

The fastest way to find out is probably typing at the interactive 
prompt.  Just jump in at the deep end and see what happens.  Then wait 
until someone tells you to use the timeit module.

Cheers.

Mark Lawrence.




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