Python Pandas max value in a group as a new column

Question

I am trying to calculate a new column which contains maximum values for each of several groups. I'm coming from a Stata background so I know the Stata code would be something like this:

by group, sort: egen max = max(odds)  

For example:

data = {'group' : ['A', 'A', 'B','B'],     'odds' : [85, 75, 60, 65]} 

Then I would like it to look like:

    group    odds    max      A        85      85      A        75      85      B        60      65      B        65      65 

Eventually I am trying to form a column that takes 1/(max-min) * odds where max and min are for each group.

Answer 1

Use groupby + transform:

df['max'] = df.groupby('group')['odds'].transform('max') 

This is equivalent to the verbose:

maxima = df.groupby('group')['odds'].max() df['max'] = df['group'].map(maxima) 

The transform method aligns the groupby result to the groupby indexer, so no explicit mapping is required.