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 data is 50000 x 10)

a = numpy.asarray([[2,'a'],[3,'b'],[4,'c'],[5,'d']])
filter = ['a','c']

I need to find all rows in a with a[:, 1] in filter. Expected result:

[[2,'a'],[4,'c']]

My current code is this:

numpy.asarray([x for x in a if x[1] in filter ])

It works okay but I have read somewhere that it is not efficient. What is the proper numpy method for this?

Edit:

Thanks for all the correct answers! Unfortunately I can only mark one as accepted answer. I am surprised that numpy.in1d is not turned up in google searchs for numpy filter 2d array.

Answer 1

You can use a bool index array that you can produce using np.in1d.

You can index a np.ndarray along any axis you want using for example an array of bools indicating whether an element should be included. Since you want to index along axis=0, meaning you want to choose from the outest index, you need to have 1D np.array whose length is the number of rows. Each of its elements will indicate whether the row should be included.

A fast way to get this is to use np.in1d on the second column of a. You get all elements of that column by a[:, 1]. Now you have a 1D np.array whose elements should be checked against your filter. Thats what np.in1d is for.

So the complete code would look like:

import numpy as np

a = np.asarray([[2,'a'],[3,'b'],[4,'c'],[5,'d']])
filter = np.asarray(['a','c'])
a[np.in1d(a[:, 1], filter)]

or in a longer form:

import numpy as np

a = np.asarray([[2,'a'],[3,'b'],[4,'c'],[5,'d']])
filter = np.asarray(['a','c'])
mask = np.in1d(a[:, 1], filter)
a[mask]