Pandas Groupby First - 从原始数据帧中提取索引 [英] Pandas Groupby First - Extract Index from Original Dataframe

问题描述

我有一个非常简单的问题.我想获取一个数据框，对某些列执行 groupby，并提取每个组中第一行的索引(在原始数据框中).我该怎么做?

I have a very simple problem. I'd like to take a data frame, perform a groupby on some columns, and extract the index (in the original data frame) of the first row in each group. How do I do this?

我尝试过使用 as_index、group_keys、reset_index()，但似乎没有任何效果.

I've tried playing with as_index, group_keys, reset_index() and nothing seems to work.

推荐答案

你需要first函数:

x = pd.DataFrame([{'name': 'b1', 'group': 'a'},
                  {'name': 'b2', 'group': 'a'},
                  {'name': 'b3', 'group': 'a'},
                  {'name': 'b4', 'group': 'b'},
                  {'name': 'b5', 'group': 'b'},
                  {'name': 'b6', 'group': 'a'},
                  {'name': 'b7', 'group': 'c'},
                  {'name': 'b8', 'group': 'c'},])
x = x.reset_index() # add the indices as a column
xc = x.groupby('group').first()
print(xc)


       index    name
group               
a          0  b1
b          3  b4
c          6  b7

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