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 ]
