how to replace column value with range in pandas dataframe

Question

I have data frame called 'df' and I want to replace values within a range of columns in a data frame with a corresponding value in another column.

1. 6 <= age < 11 then 1

   11 <= age < 16 then 2

   16 <= age < 21 then 3

   21 <= age then 4

           age
   86508   12.0
   86509   6.0
   86510   7.0
   86511   8.0
   86512   10.0
   86513   15.0
   86514   15.0
   86515   16.0
   86516   20.0
   86517   23.0
   86518   23.0
   86519   7.0
   86520   18.0
   

Results are

            age    stage
    86508   12.0    2
    86509   6.0     1    
    86510   7.0     1
    86511   8.0     1
    86512   10.0    1
    86513   15.0    2
    86514   15.0    2
    86515   16.0    2
    86516   20.0    3
    86517   23.0    4
    86518   23.0    4
    86519   7.0     1
    86520   18.0    3

Thanks.

Answer 1

Use pd.cut():

In [37]: df['stage'] = pd.cut(df.age, bins=[0,11,16,21,300], labels=[1,2,3,4])

In [38]: df
Out[38]:
        age stage
86508  12.0     2
86509   6.0     1
86510   7.0     1
86511   8.0     1
86512  10.0     1
86513  15.0     2
86514  15.0     2
86515  16.0     2
86516  20.0     3
86517  23.0     4
86518  23.0     4
86519   7.0     1
86520  18.0     3

or more generic solution provided by @ayhan:

In [39]: df['stage'] = pd.cut(df.age, bins=[0, 11, 16, 21, np.inf], labels=False, right=True) + 1

In [40]: df
Out[40]:
        age  stage
86508  12.0      2
86509   6.0      1
86510   7.0      1
86511   8.0      1
86512  10.0      1
86513  15.0      2
86514  15.0      2
86515  16.0      2
86516  20.0      3
86517  23.0      4
86518  23.0      4
86519   7.0      1
86520  18.0      3