4.8.2 Array functions

There are a number of functions acting on arrays that do not act componentwise. Examples of such functions are max, min, and sum. These functions may operate on the entire matrix, row-wise, or column-wise. When no argument is provided, they act on the entire matrix.

Suppose:

The function sum acting on that matrix returns a scalar:

sum(A) # 36

The command has an optional parameter, axis. It allows us to choose along which axis to perform the operation. For instance, if the axis is , it means that the sum should be computed along the first axis. The sum along axis  of an array of shape  will be a vector of length .

Suppose we compute the sum of A along the axis :

sum(A, axis=0) # array([ 6, 8, 10, 12])

This amounts to computing the sum on the columns:

The result is a vector:

Now suppose we compute the sum along axis 1:

A.sum(axis=1) # array([10, 26])

This amounts to computing...