将Pandas Groupby结果合并回DataFrame
问题描述：
我有一个看起来像这样的DataFrame ...
I have a DataFrame that looks like this...
idn value
0 ID1 25
1 ID1 30
2 ID2 30
3 ID2 50
我想在此框架中添加另一列，即"idn"分组的最大值"
I want to add another column to this frame that is the max 'value' grouped by 'idn'
我想要一个看起来像这样的结果.
I want a result that looks like this.
idn value max_val
0 ID1 25 30
1 ID1 30 30
2 ID2 30 50
3 ID2 50 50
我可以像这样用一个组来提取'value'的最大值...
I can extract the max of 'value' using a group by like so...
df[['idn', 'value']].groupby('idn')['value'].max()
但是，我无法将结果合并回原始DataFrame中.
However, I am unable to merge that result back into the original DataFrame.
获得理想结果的最佳方法是什么?
What is the best way to get the desired result?
谢谢
答
对groupby对象使用transform方法:
Use the transform method on a groupby object:
In [5]: df['maxval'] = df.groupby(by=['idn']).transform('max')
In [6]: df
Out[6]:
idn value maxval
0 ID1 25 30
1 ID1 30 30
2 ID2 30 50
3 ID2 50 50