How could I remove the rows of an array if one of the elements of the row does not satisfy a condition?

Question

I would like to remove the rows of an array when the elements of third column of the array are less than specific amount. For example:

a=np.array([[2331.13,1         


        

Answer 1

I think the most straight forward way to remove rows or columns is to use np.delete

>>> a = np.arange(16).reshape(4,4)
>>> a
array([[ 0,  1,  2,  3],
       [ 4,  5,  6,  7],
       [ 8,  9, 10, 11],
       [12, 13, 14, 15]])
>>> a[:,2]
array([ 2,  6, 10, 14])
>>> to_remove = a[:,2] < 7
>>> to_remove
array([ True,  True, False, False], dtype=bool)
>>> new_a = np.delete(a, to_remove, axis=0)
>>> new_a
array([[ 8,  9, 10, 11],
       [12, 13, 14, 15]])

In one line:

new_a = np.delete(a, a[:,2] < 7, axis=0)

Answer 2

You can do:

a[a[:,2]>=15.0, :]

Note the inversion of a[:,2]<15.0 to a[:,2]>=15.0, so that you're describing rows that you want to keep rather than remove.

If inverting your condition isn't as simple as that, you could also use ~:

a[~(a[:,2]<15.0), :]