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1.5.4 Filtering Series Data

            Next, let us consider another example of series comprising numbers that denote marks of students in an examination:
             >>> import pandas as pd
             >>> marks = [90, 70, 95, 45, 100, 65, 78, 29]
             >>> students = pd.Series(marks, index = [101, 102 , 103, 106, 107, 108, 201, 301])
             >>> print(students)
            output:
                 101     90
                 102     70
                 103     95
                 106     45
                 107    100
                 108     65
                 201     78
                 301     29
                 dtype: int64
            Suppose, we need to retrieve the roll numbers of students who have achieved a distinction (i.e. scored at least 75
            marks). Pandas provides a simple mechanism to create a series of Boolean values having the same index as the original
            series. In the present example, we need to find roll numbers (indexes) of students who have scored marks>=75. So, we
            construct a series of Boolean values as follows: for each roll number (index), the corresponding value is set as True if
            marks>=75 and False otherwise, as shown below:
             >>> boolIndex = (students >= 75)
             >>> print(boolIndex)
            output:
                 101     True
                 102    False
                 103     True
                 106    False
                 107     True
                 108    False
                 201     True
                 301    False
                 dtype: bool
            Next, we retrieve the sub-series comprising only those values for which the index corresponds to the value True, as
            shown below:
             >>> print(students[boolIndex])
            output:
                 101     90
                 103     95
                 107    100
                 201     78
                 dtype: int64
            Note that only those rows for which the boolean condition holds True are retrieved.



             C T  07     1.  Write Python code to retrieve ids of those employees earning more than 50000 salary.
                         2.  Use the output obtained in step 1 to retrieve their names.









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