| a01 | a02 |
-----------------
a12 | 1.  | 3.  |
a11 | 2.  | 4.  |

>>> a = np.array([1., 2.])
>>> b = np.array([3., 4.])
>>> c = np.array([])
>>> c[a][b] = 1.
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
IndexError: arrays used as indices must be of integer (or boolean) type