[Numpy-discussion] concatenating 1-D arrays to 2D
Bill Baxter
wbaxter@gmail....
Fri Mar 23 01:53:00 CDT 2007
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
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