How to calculate the sum of all columns of a 2D numpy array (efficiently)

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Check out the documentation for numpy.sum, paying particular attention to the axis parameter. To sum over columns: 

>>> import numpy as np
>>> a = np.arange(12).reshape(4,3)
>>> a.sum(axis=0)
array([18, 22, 26])

Or, to sum over rows:

>>> a.sum(axis=1)
array([ 3, 12, 21, 30])

Other aggregate functions, like numpy.mean, numpy.cumsum and numpy.std, e.g., also take the axis parameter.

From the Tentative Numpy Tutorial:

Many unary operations, such as computing the sum of all the elements
in the array, are implemented as methods of the ndarray class. By
default, these operations apply to the array as though it were a list
of numbers, regardless of its shape. However, by specifying the axis
parameter you can apply an operation along the specified axis of an
array:

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