python: How to Zig-Zag a Toolpath Array

zigzag.py

"""Demonstration of how to 'zig-zag' an m x n x 3
python array (m = passes, n = points per pass).
Includes some array creation/mod tricks

>>> import numpy as np
>>> a = np.random.randn(4,3,3)
>>> a
array([[[-0.74865489,  0.533361  ,  0.51997179],
        [-0.72109915, -0.74988902, -2.61720425],
        [-0.72792437,  2.43027423,  0.84132144]],

       [[-1.82027469, -0.2597997 , -2.01250823],
        [ 0.37021219, -0.89890801, -0.75788127],
        [-0.77095731, -1.01914784, -0.50027295]],

       [[-0.96289404, -0.48019978,  0.18274626],
        [-0.49190084, -0.14846301, -0.41655213],
        [ 1.99714785,  0.17308239, -0.36529016]],

       [[-1.65683619, -0.25826898, -0.51135952],
        [ 0.83005551,  1.26310339, -1.79057156],
        [-0.62770164, -1.2752175 , -0.25720492]]])

>>> a[:,:,2] = np.c_[[[0, 1, 2]] * 4]
>>> a
array([[[-0.74865489,  0.533361  ,  0.        ],
        [-0.72109915, -0.74988902,  1.        ],
        [-0.72792437,  2.43027423,  2.        ]],

       [[-1.82027469, -0.2597997 ,  0.        ],
        [ 0.37021219, -0.89890801,  1.        ],
        [-0.77095731, -1.01914784,  2.        ]],

       [[-0.96289404, -0.48019978,  0.        ],
        [-0.49190084, -0.14846301,  1.        ],
        [ 1.99714785,  0.17308239,  2.        ]],

       [[-1.65683619, -0.25826898,  0.        ],
        [ 0.83005551,  1.26310339,  1.        ],
        [-0.62770164, -1.2752175 ,  2.        ]]])

>>> a[1::2] = a[1::2, -1::-1]
>>> a
array([[[-0.74865489,  0.533361  ,  0.        ],
        [-0.72109915, -0.74988902,  1.        ],
        [-0.72792437,  2.43027423,  2.        ]],

       [[-0.77095731, -1.01914784,  2.        ],
        [ 0.37021219, -0.89890801,  1.        ],
        [-1.82027469, -0.2597997 ,  0.        ]],

       [[-0.96289404, -0.48019978,  0.        ],
        [-0.49190084, -0.14846301,  1.        ],
        [ 1.99714785,  0.17308239,  2.        ]],

       [[-0.62770164, -1.2752175 ,  2.        ],
        [ 0.83005551,  1.26310339,  1.        ],
        [-1.65683619, -0.25826898,  0.        ]]])
"""

Captured output from an IPython Session