[FIXED] comparing vector to each element of another vector with numpy

Issue

I have 2 vectors, I would like to compute a matrix where the i'th row is the first vector compared to the i'th element of the second vector. This code does that:

a1 = np.array([1, 2, 3])
a2 = np.array([1, 2, 3])

print(np.array([a1 > e for e in a2]))

where we get:

[[False  True  True]
 [False False  True]
 [False False False]]

I want to have the same behavior but done efficiently with "numpy magic". How can I do this?

Solution

You can broadcast the function by making one of the arrays have one singleton dimension:

In [1]: import numpy as np                                                      

In [2]: a1 = np.array([1, 2, 3])
   ...: a2 = np.array([1, 2, 3])[:,None]                                        

In [3]: a1>a2                                                                   
Out[3]: 
array([[False,  True,  True],
       [False, False,  True],
       [False, False, False]])

Answered By - jhso