BFTM BFTM - 2 months ago 49
Python Question

Numpy: Divide each row by a vector element

Suppose I have a numpy array:

data = np.array([[1,1,1],[2,2,2],[3,3,3]])


and I have a corresponding "vector:"

vector = np.array([1,2,3])


How do I operate on
data
along each row to either subtract or divide so the result is:

sub_result = [[0,0,0], [0,0,0], [0,0,0]]
div_result = [[1,1,1], [1,1,1], [1,1,1]]


Long story short: How do I perform an operation on each row of a 2D array with a 1D array of scalars that correspond to each row?

Answer

Here you go. You just need to use None (or alternatively np.newaxis) combined with broadcasting:

In [6]: data - vector[:,None]
Out[6]:
array([[0, 0, 0],
       [0, 0, 0],
       [0, 0, 0]])

In [7]: data / vector[:,None]
Out[7]:
array([[1, 1, 1],
       [1, 1, 1],
       [1, 1, 1]])
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