如何向 pandas dataframe 添加一列，该列在一个范围内具有最高值但将其应用于每一行？

Question

I have the following code:

import pandas as pd
import numpy as np

df = pd.DataFrame([['red', 1], ['red', 13], ['red', 1], ['blue', 1], ['red', 112], ['blue', 10]])

df.columns = ["colour","rank"]

# df['highest_rank'] = ...

print(df)

"""
  colour  rank  highest_rank
0    red     1     122
1    red    13     122
2    red     1     122
3   blue     1     10
4    red   112     122
5   blue    10     10
"""

Hopefully, the example can show you what I'm trying to do as I'm struggling to describe what I'm wanting - The highest ranking of each colour.

Answer 1

groupby colour and broadcast the highest rank in each group using transform. Code below

df['highest_rank']=df.groupby('colour')['rank'].transform('max')




colour  rank  highest_rank
0    red     1           112
1    red    13           112
2    red     1           112
3   blue     1            10
4    red   112           112
5   blue    10            10