How to Find the Max Value by Group in Pandas

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Often you may be interested in finding the max value by group in a pandas DataFrame.

Fortunately this is easy to do using the groupby() and max() functions with the following syntax:

df.groupby('column_name').max()

This tutorial explains several examples of how to use this function in practice using the following pandas DataFrame:

import pandas as pd

#create pandas DataFrame
df = pd.DataFrame({'team': ['A', 'A', 'B', 'B', 'B', 'C', 'C'],
                   'points':[24, 23, 27, 11, 14, 8, 13],
                   'rebounds': [11, 8, 7, 6, 6, 5, 12]})

#display DataFrame
print(df)

  team  points  rebounds
0    A      24        11
1    A      23         8
2    B      27         7
3    B      11         6
4    B      14         6
5    C       8         5
6    C      13        12

Example 1: Max Value of Multiple Columns Grouped by One Variable

The following code shows how to find the max value of multiple columns, grouped by one variable in a DataFrame:

#find max values of points and rebounds, grouped by team
df.groupby('team').max().reset_index()


        team	points	rebounds
0	A	24	11
1	B	27	7
2	C	13	12

From the output we can see that:

• Team A has a max points value of 24 and a max rebounds value of 11.
• Team B has a max points value of 27 and a max rebounds value of 7.
• Team C has a max points value of 13 and a max rebounds value of 12.

Note that we used the reset_index() function to ensure that the index matches the index in the original DataFrame.

Example 2: Max Value of a Single Column Grouped by One Variable

The following code shows how to find the max value of just one column, grouped on a single variable:

#find max value of points, grouped by team
df.groupby('team')['points'].max().reset_index()

        team	points
0	A	24
1	B	27
2	C	13

Example 3: Sort by Max Values

We can also use the sort_values() function to sort the max values.

We can specify ascending=False to sort from largest to smallest:

#find max value by team, sort descending
df.groupby('team')['points'].max().reset_index().sort_values(['points'], ascending=False)


        team	points
1	B	27
0	A	24
2	C	13

Or we can specify ascending=True to sort from smallest to largest:

#find max value by team, sort ascending 
df.groupby('team')['points'].max().reset_index().sort_values(['points'], ascending=True) 

        team	points
2	C	13
0	A	24
1	B	27

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