Get line numbers of lines matching a condition in numpy

Question

Suppose I have a numpy array, for example:

a = array([[1, 2, 3], [4, 5, 6], [7, 8, 9], [3, 2, 1]]) 

I want to check if there is a second element == 2.

I know I can do this:

 >>> a[:,1]==2 array([ True, False, False, True], dtype=bool) 

returns boolean values. My question is: how do I get line numbers in lines where the condition is true? In this example, I would like to return array([0, 3]) , because the 0th and 3rd rows correspond to the condition of the second element == 2.

Answer 1

Use np.where to return indexes:

 In [79]: np.where(a[:,1]==2) Out[79]: (array([0, 3], dtype=int64),)