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 somethi

Answer 1

Using the approach from jpp above works, but it also gives a "SettingWithCopyWarning". While this may not be an issue, I believe the code below would remove that warning:

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

Answer 2

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.

Answer 3

df['max'] = df.group_col.map(lambda x: df.groupby('group_col').odds.max()[x])