Python numpy filter two-dimensional array by condition

Question

Python newbie here, I have read Filter rows of a numpy array? and the doc but still can\'t figure out how to code it the python way.

Example array I have: (the real

Answer 1

A somewhat elaborate pure numpy vectorized solution:

>>> import numpy
>>> a = numpy.asarray([[2,'a'],[3,'b'],[4,'c'],[5,'d']])
>>> filter = numpy.array(['a','c'])
>>> a[(a[:,1,None] == filter[None,:]).any(axis=1)]
array([['2', 'a'],
       ['4', 'c']], 
      dtype='|S21')

None in the index creates a singleton dimension, therefore we can compare the column of a and the row of filter, and then reduce the resulting boolean array

>>> a[:,1,None] == filter[None,:]
array([[ True, False],
       [False, False],
       [False,  True],
       [False, False]], dtype=bool)

over the second dimension with any.