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

[Eric Firing]

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]])


> 
> -Sebastian Haase
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