Python Pandas 组中的最大值作为新列 [英] Python Pandas max value in a group as a new column

问题描述

我正在尝试计算一个新列，其中包含多个组中每个组的最大值.我来自 Stata 背景，所以我知道 Stata 代码将是这样的:

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) 

例如:

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

然后我希望它看起来像:

Then I would like it to look like:

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

最终我试图形成一个列，它需要 1/(max-min) *odds 其中 max 和 min 是每个团体.

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

推荐答案

使用 groupby + 转换:

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

这相当于冗长:

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

transform 方法将 groupby 结果与 groupby 索引器对齐，因此不需要显式映射.

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

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