Pandas: Difference between largest and smallest value within group

Question

Given a data frame that looks like this

GROUP VALUE
  1     5
  2     2
  1     10
  2     20
  1     7

I would like to compute the differe

Answer 1

groupby/agg generally performs best when you take advantage of the built-in aggregators such as 'max' and 'min'. So to obtain the difference, first compute the max and min and then subtract:

import pandas as pd
df = pd.DataFrame({'GROUP': [1, 2, 1, 2, 1], 'VALUE': [5, 2, 10, 20, 7]})
result = df.groupby('GROUP')['VALUE'].agg(['max','min'])
result['diff'] = result['max']-result['min']
print(result[['diff']])

yields

       diff
GROUP      
1         5
2        18