[Numpy-discussion] dot() function question

Chris Barker chris.barker@noaa....
Thu Jun 28 13:14:31 CDT 2012


On Wed, Jun 27, 2012 at 2:38 PM,  <x.piter@gmail.com> wrote:
> How how can I perform matrix multiplication of two vectors?
> (in matlab I do it like a*a')

np.outer is a bit cleaner, I suppose, but you can exactly the same
thing you do with matlab if a is a column (single column 2-d array):

In [40]: a = np.arange(4).reshape((-1,1))

In [41]: a
Out[41]:
array([[0],
       [1],
       [2],
       [3]])

In [42]: np.dot(a,a.T)
Out[42]:
array([[0, 0, 0, 0],
       [0, 1, 2, 3],
       [0, 2, 4, 6],
       [0, 3, 6, 9]])

or, of course, 2 arrays to begin with:

In [13]: a = np.arange(4).reshape((4,1))

In [14]: b = np.arange(4).reshape((1,4))

In [15]: np.dot(a,b)
Out[15]:
array([[0, 0, 0, 0],
       [0, 1, 2, 3],
       [0, 2, 4, 6],
       [0, 3, 6, 9]])


-Chris


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Christopher Barker, Ph.D.
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