[Numpy-discussion] concatenating 1-D arrays to 2D

Sebastian Haase seb.haase@gmx....
Fri Mar 23 10:24:40 CDT 2007


On 3/22/07, Bill Baxter <wbaxter@gmail.com> wrote:
> On 3/23/07, Eric Firing <efiring@hawaii.edu> wrote:
> > Sebastian Haase wrote:
> > > On 3/22/07, Stefan van der Walt <stefan@sun.ac.za> wrote:
> > >> On Thu, Mar 22, 2007 at 08:13:22PM -0400, Brian Blais wrote:
> > >>> Hello,
> > >>>
> > >>> I'd like to concatenate a couple of 1D arrays to make it a 2D array, with two columns
> > >>> (one for each of the original 1D arrays).  I thought this would work:
> > >>>
> > >>>
> > >>> In [47]:a=arange(0,10,1)
> > >>>
> > >>> In [48]:a
> > >>> Out[48]:array([0, 1, 2, 3, 4, 5, 6, 7, 8, 9])
> > >>>
> > >>> In [49]:b=arange(-10,0,1)
> > >>>
> > >>> In [51]:b
> > >>> Out[51]:array([-10,  -9,  -8,  -7,  -6,  -5,  -4,  -3,  -2,  -1])
> > >>>
> > >>> In [54]:concatenate((a,b))
> > >>> Out[54]:
> > >>> array([  0,   1,   2,   3,   4,   5,   6,   7,   8,   9, -10,  -9,  -8,
> > >>>          -7,  -6,  -5,  -4,  -3,  -2,  -1])
> > >>>
> > >>> In [55]:concatenate((a,b),axis=1)
> > >>> Out[55]:
> > >>> array([  0,   1,   2,   3,   4,   5,   6,   7,   8,   9, -10,  -9,  -8,
> > >>>          -7,  -6,  -5,  -4,  -3,  -2,  -1])
> > >>>
> > >>>
> > >>> but it never expands the dimensions.  Do I have to do this...
> > >>>
> > >>> In [65]:concatenate((a.reshape(10,1),b.reshape(10,1)),axis=1)
> > >>> Out[65]:
> > >>> array([[  0, -10],
> > >>>         [  1,  -9],
> > >>>         [  2,  -8],
> > >>>         [  3,  -7],
> > >>>         [  4,  -6],
> > >>>         [  5,  -5],
> > >>>         [  6,  -4],
> > >>>         [  7,  -3],
> > >>>         [  8,  -2],
> > >>>         [  9,  -1]])
> > >>>
> > >>>
> > >>> ?
> > >>>
> > >>> I thought there would be an easier way.  Did I overlook something?
> > >> How about
> > >>
> > >> N.vstack((a,b)).T
> > >>
> > > Also mentioned here should be the use of
> > > newaxis.
> > > As in
> > > a[:,newaxis]
> > >
> > > However I never got a "good feel" for how to use it, so I can't
> > > complete the code you would need.
> >
> > n [9]:c = N.concatenate((a[:,N.newaxis], b[:,N.newaxis]), axis=1)
> >
> > In [10]:c
> > Out[10]:
> > array([[  0, -10],
> >         [  1,  -9],
> >         [  2,  -8],
> >         [  3,  -7],
> >         [  4,  -6],
> >         [  5,  -5],
> >         [  6,  -4],
> >         [  7,  -3],
> >         [  8,  -2],
> >         [  9,  -1]])
> >
>
> Then of course, there's r_ and c_:
>
> c = numpy.c_[a,b]
>
> c = numpy.r_[a[None],b[None]].T
>
> --bb
So,
None is the same as newaxis - right?

But what is    a[None]   vs.  a[:,N.newaxis] ?

-Sebastian


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