Populate column in data frame based on a range found in another dataframe

Question

I\'m attempting to populate a column in a data frame based on whether the index value of that record falls within a range defined by two columns in another data frame.

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Answer 1

You can use IntervalIndex (requires v0.20.0).

First construct the index:

df2.index = pd.IntervalIndex.from_arrays(df2['START'], df2['STOP'], closed='both')

df2
Out: 
        START  STOP  CLASS
[2, 3]      2     3      1
[5, 7]      5     7      2
[8, 8]      8     8      3

Now if you index into the second DataFrame it will lookup the value in the intervals. For example,

df2.loc[6]
Out: 
START    5
STOP     7
CLASS    2
Name: [5, 7], dtype: int64

returns the second class. I don't know if it can be used with merge or with merge_asof but as an alternative you can use map:

df1['CLASS'] = df1.index.to_series().map(df2['CLASS'])

Note that I first converted the index to a Series to be able to use the Series.map method. This results in

df1
Out: 
     a  CLASS
0    4    NaN
1   45    NaN
2    7    1.0
3    5    1.0
4   48    NaN
5   44    2.0
6   22    2.0
7   89    2.0
8   45    3.0
9   44    NaN
10  23    NaN