Getting only those values that fulfill a condition in a numpy array

Question

There must a be a (very) quick and efficient way to get only elements from a numpy array, or even more interestingly from a slice of it. Suppose I have a numpy array:

<

Answer 1

How about:

In [19]: b = a[s]

In [20]: b[b > 0]
Out[20]: array([2, 3, 4])