理解python和numpy语法来屏蔽和过滤数组

Question

Please help me understand how the lines below are working.

1. How does a pair of parentheses create an array and then individual elements go through logical condition check and create a new array?

2. How does condition in square brackets filter elements create another sub-array?

    import numpy as np my_vector=np.array([-17, 4, 0, 2, 21, 37, 105]) array([-17, 4, 0, 2, 21, 37, 105]) zero_mod_7_mask= 0 == (my_vector % 7) #question 1 array([False, False, True, False, True, False, True]) my_subarray = my_vector[zero_mod_7_mask] # question 2 array([ 0, 21, 105])

Answer 1

1. The parentheses do nothing but ensure that what's inside of them is done first

2. The array just keeps the True elements

Answer 2

In a ipython interactive session:

In [230]: my_vector=np.array([-17, 4, 0, 2, 21, 37, 105])

The modulus calculation:

In [231]: my_vector%7                                                           
Out[231]: array([4, 4, 0, 2, 0, 2, 0])

identifying elements of that which are 0:

In [232]: 0 == my_vector%7                                                      
Out[232]: array([False, False,  True, False,  True, False,  True])

Indexes of those True elements:

In [233]: np.nonzero(_)                                                         
Out[233]: (array([2, 4, 6]),)

Selecting those elements from my_vector :

In [234]: my_vector[_]                                                          
Out[234]: array([  0,  21, 105])

Doing the same with the boolean array:

In [235]: my_vector[Out[232]]                                                   
Out[235]: array([  0,  21, 105])