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? -- http://mail.python.org/mailman/listinfo/python-list
