了解Numpy中的语法X [Y == c]

Question

请不要理解这个语法的工作原理：current_x = X [Y == c]。

我打印出来IPython的一些示例与代码（我用熊猫这里PD）：

In [120]: pd.DataFrame(X) 
Out[120]: 
     0   1   2 3 4 5 6 7 8 9 ... 475 \ 
0 0.000000 0.000000 0.000000 0.0 0.0 0.0 0.0 0.0 0.0 0.0 ... 0.0 
1 0.070588 0.000000 0.000000 0.0 0.0 0.0 0.0 0.0 0.0 0.0 ... 0.0 
2 0.000000 0.000000 0.000000 0.0 0.0 0.0 0.0 0.0 0.0 0.0 ... 0.0 
3 0.000000 0.000000 0.000000 0.0 0.0 0.0 0.0 0.0 0.0 0.0 ... 0.0 
4 0.000000 0.000000 0.000000 0.0 0.0 0.0 0.0 0.0 0.0 0.0 ... 0.0 
5 0.000000 0.000000 0.000000 0.0 0.0 0.0 0.0 0.0 0.0 0.0 ... 0.0 
6 0.000000 0.000000 0.000000 0.0 0.0 0.0 0.0 0.0 0.0 0.0 ... 0.0 
7 0.596078 0.596078 0.062745 0.0 0.0 0.0 0.0 0.0 0.0 0.0 ... 0.0 
8 0.000000 0.000000 0.000000 0.0 0.0 0.0 0.0 0.0 0.0 0.0 ... 0.0 
9 0.000000 0.000000 0.000000 0.0 0.0 0.0 0.0 0.0 0.0 0.0 ... 0.0 

    476 477 478 479 480 481 482 483 484 
0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 
1 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 
2 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 
3 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 
4 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 
5 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 
6 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 
7 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 
8 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 
9 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 

[10 rows x 485 columns] 

In [121]: pd.DataFrame(Y) 
Out[121]: 
    0 
0 1 
1 7 
2 9 
3 4 
4 5 
5 1 
6 1 
7 6 
8 1 
9 1 

In [122]: pd.DataFrame(X[Y == 5]) 
Out[122]: 
    0 1 2 3 4 5 6 7 8 9 ... 475 476 477 478 \ 
0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 ... 0.0 0.0 0.0 0.0 

    479 480 481 482 483 484 
0 0.0 0.0 0.0 0.0 0.0 0.0 

[1 rows x 485 columns] 

我也不怎么X [Y == 5]已经有这个结果。 请HEP-LP

Answer 1

# this returns a list of True or False depending if Y[i] == 5 for all i in Y 
Y==5 => [False, False, False, False, True, False, False, False, False, False] 

# this is known as boolean indexing ... 
X[ [False, False, False, False, True, False, False, False, False, False] ] => your output