On Wed, Sep 8, 2010 at 4:23 PM, Jonno <jonnojohn...@gmail.com> wrote: > On Wed, Sep 8, 2010 at 3:18 PM, Jonno <jonnojohn...@gmail.com> wrote: >> On Wed, Sep 8, 2010 at 3:06 PM, Jonno <jonnojohn...@gmail.com> wrote: >>> On Wed, Sep 8, 2010 at 2:11 PM, Benjamin Kaplan >>> <benjamin.kap...@case.edu> wrote: >>>> On Wed, Sep 8, 2010 at 2:55 PM, Jonno <jonnojohn...@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]
Yes. You could either do that, or do [row[0] for row in a[::2]]. It doesn't make that much of a difference, unless memory is really tight (doing the ::2 first means your list is smaller to begin with) > -- > http://mail.python.org/mailman/listinfo/python-list > -- http://mail.python.org/mailman/listinfo/python-list
