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

Sebastian Haase haase@msg.ucsf....
Fri Mar 23 00:02:36 CDT 2007


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.

-Sebastian Haase


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