Numpy Slice Array Column Based on Row Condition

Question

Given a 2xn array how do I slice the columns into a new array given a condition on the first row?

x = np.array([[1,2,3],[10,20,30]])

In:
Out[20]: 
array([[ 1,  2,  3],
       [10, 20, 30]])

Say I want all the column where row 0 is <= 2.

In:  x[x[:]<=2]
Out: array([1, 2])

This didn't get me the 10 and 20 along with the 1 and 2. I'm expecting

array([[ 1,  2],
       [10, 20]])

What is the right condition to put inside x[] to get my slice?

Answer 1

You can boolean slice the 2nd axis using:

x[:,x[0]<=2]
# returns:
array([[ 1,  2],
       [10, 20]])