Range of values (maximum  minimum) along an axis.
The name of the function comes from the acronym for ‘peak to peak’.
Parameters:  a : array_like
axis : int, optional
out : array_like


Returns:  ptp : ndarray

Examples
>>> x = np.arange(4).reshape((2,2))
>>> x
array([[0, 1],
[2, 3]])
>>> np.ptp(x, axis=0)
array([2, 2])
>>> np.ptp(x, axis=1)
array([1, 1])