In [11]: a = np.array([2, 13, 4, 11, 7, 20, 25, 9])

    In [12]: a[1]
    Out[12]: 13

    In [13]: a[1:4]
    Out[13]: array([13,  4, 11])

    In [14]: a > 10
    Out[14]: array([False,  True, False,  True, False,  True,  True, False])

    In [15]: a[a > 10]
    Out[15]: array([13, 11, 20, 25])