[Numpy-discussion] How to Keep An Array Two Dimensional

Warren Weckesser warren.weckesser@gmail....
Sun Nov 25 20:35:06 CST 2012


On Sun, Nov 25, 2012 at 8:24 PM, Tom Bennett <tom.bennett@mail.zyzhu.net>wrote:

> Hi,
>
> I am trying to extract n columns from an 2D array and then operate on the
> extracted columns. Below is the code:
>
> A is an MxN 2D array.
>
> u = A[:,:n] #extract the first n columns from A
>
> B = np.dot(u, u.T) #take outer product.
>
> This code works when n>1. However, when n=1, u becomes an 1D array instead
> of an Mx1 2D array and the code breaks down.
>
> I wonder if there is any way to keep u=A[:,:n] an Mxn array no matter what
> value n takes. I do not want to use matrix because array is more convenient
> in other places.
>
>
Tom,

Your example works for me:

In [1]: np.__version__
Out[1]: '1.6.2'

In [2]: A = arange(15).reshape(3,5)

In [3]: A
Out[3]:
array([[ 0,  1,  2,  3,  4],
       [ 5,  6,  7,  8,  9],
       [10, 11, 12, 13, 14]])

In [4]: u = A[:,:1]

In [5]: u
Out[5]:
array([[ 0],
       [ 5],
       [10]])

In [6]: B = np.dot(u, u.T)

In [7]: B
Out[7]:
array([[  0,   0,   0],
       [  0,  25,  50],
       [  0,  50, 100]])



Warren



> Thanks,
> Tom
>
>
>
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