Mastering pandas - Second Edition

Array slicing

Arrays can be sliced using the syntax ar[startIndex: endIndex: stepValue]:

    In [82]: ar=2*np.arange(6); ar
    Out[82]: array([ 0,  2,  4,  6,  8, 10])
    In [85]: ar[1:5:2]
    Out[85]: array([2, 6])

Note that if we wish to include the endIndex value, we need to go above it, as follows:

    In [86]: ar[1:6:2]
    Out[86]: array([ 2,  6, 10])  

Obtain the first nelements using ar[:n]:

    In [91]: ar[:4]
    Out[91]: array([0, 2, 4, 6])  

The implicit assumption here is that startIndex=0, step=1.

Start at element 4 and select all the elements till the end:

    In [92]: ar[4:]
    Out[92]: array([ 8, 10])  

Slice array with stepValue=3:

    In [94]: ar[::3]
    Out[94]: array([0, 6]) 

To illustrate the scope of indexing in NumPy, let's refer to the following diagram, which ...