Mastering pandas - Second Edition

Array masking

NumPy arrays can be used as filters on the larger original array. This process of using arrays as filters is called array masking. For example, see the following snippet:

    In [146]: np.random.seed(10)
              ar=np.random.random_integers(0,25,10); ar
    Out[146]: array([ 9,  4, 15,  0, 17, 25, 16, 17,  8,  9])
    In [147]: evenMask=(ar % 2==0); evenMask
    Out[147]: array([False,  True, False,  True, False, False,  True, False,  True, False], dtype=bool)
    In [148]: evenNums=ar[evenMask]; evenNums
    Out[148]: array([ 4,  0, 16,  8])

In the following example, we randomly generate an array of 10 integers between 0 and 25. Then, we create a boolean mask array that is used to filter out only the even numbers. This masking feature can be very useful, say, for ...