Trying to understand this Python code

Question

I'm new to python and I don´t understand the last line of this python code. What does it mean?

import np as numpy

    def goat_door(prizedoors, guesses):

        #strategy: generate random answers, and
        #keep updating until they satisfy the rule
        #that they aren't a prizedoor or a guess
        result = np.random.randint(0, 3, prizedoors.size)
        while True:
            bad = (result == prizedoors) | (result == guesses)
            if not bad.any():
                return result
            result[bad] = np.random.randint(0, 3, bad.sum())

prizedoors and guesses are np.random.choice(2,number of simulations)

Result is an array and I don´t know what result[bad] means.

Edit: I've just write import np as numpy

Answer 1

result is a numpy ndarray of length prizedoors.size, where each element is randomly drawn from [0, 3). For example:

>>> result = np.random.randint(0, 3, 5)
>>> result
array([1, 1, 2, 0, 1])

bad is a boolean array which is True wherever result == prizedoors or result == guesses. Probably prizedoors and guesses are boolean arrays too. In any case, bad will wind up looking something like

>>> bad
array([ True,  True,  True, False,  True], dtype=bool)

bad.sum() counts the number of Trues:

>>> bad.sum()
4

result[bad] selects the elements of result where bad == True:

>>> result[bad]
array([1, 1, 2, 1])

and finally, the last line fills the bad values with new random values (not necessarily good values, only new ones):

>>> result[bad] = np.random.randint(0, 3, bad.sum())
>>> result
array([1, 1, 0, 0, 1])