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Array-building starts at 4:14. For those of us who watch first and then go back to go through the steps.
Yes that's correct, the first four minutes go over some crucial notes about what's going on under the hood, with the computer's memory allocation in NumPy, and what its benefits are. Here are the notes of @craigsdennis:
## Differences between lists and NumPy Arrays * An array's size is immutable. You cannot append, insert or remove elements, like you can with a list. * All of an array's elements must be of the same [data type](https://docs.scipy.org/doc/numpy-1.14.0/user/basics.types.html). * A NumPy array behaves in a Pythonic fashion. You can `len(my_array)` just like you would assume.