[Numpy-discussion] Multiplying a matrix by a vector
Christopher Barker
Chris.Barker@noaa....
Wed Feb 4 16:48:49 CST 2009
Simon Palmer wrote:
> I have a matrix and a vector which has the same number of elements as the
> matrix has rows. I want to multiply each element in a row in the matrix by
> the corresponding element in the vector.
>>> M = np.arange(6).reshape((2,3))
>>> M
array([[0, 1, 2],
[3, 4, 5]])
>>> v = np.array((4,5)).reshape((-1,1)) # make it a column vector
>>> v
array([[4],
[5]])
>>> M * v
array([[ 0, 4, 8],
[15, 20, 25]])
you can also do it with np.newaxis:
>>> v = np.array((4,5))
>>> M * v[:,np.newaxis]
array([[ 0, 4, 8],
[15, 20, 25]])
http://www.scipy.org/EricsBroadcastingDoc
If you are really working with a matrix, rather than a 2-d array, you
may want to look at the np.matrix object.
-Chris
--
Christopher Barker, Ph.D.
Oceanographer
Emergency Response Division
NOAA/NOS/OR&R (206) 526-6959 voice
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Seattle, WA 98115 (206) 526-6317 main reception
Chris.Barker@noaa.gov
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