与 pandas 自我加入 [英] self-join with Pandas

问题描述

我想对Pandas数据框执行自联接，以便将某些行附加到原始行。每行都有一个标记 i，指示应在右侧追加哪一行。

I would like to perform a self-join on a Pandas dataframe so that some rows get appended to the original rows. Each row has a marker 'i' indicating which row should get appended to it on the right.

d = pd.DataFrame(['A','B','C'], columns = ['some_col'])
d['i'] = [2,1,1]

In [17]: d
Out[17]: 
  some_col  i
0        A  2
1        B  1
2        C  1

所需的输出：

  some_col  i some_col_y
0        A  2          C
1        B  1          B
2        C  1          B

也就是说，第2行附加到第0行，第1行附加到第1行，第1行附加到第2行（如i所示）。

That is, row 2 gets appended to row 0, row 1 to row 1, row 1 to row 2 (as indicated by i).

我的想法是

pd.merge(d, d, left_index = True, right_on = 'i', how = 'left')

但它会产生其他结果。

But it produces something else altogether. How to do it correctly?

推荐答案

加入与 on ='i'

d.join(d.drop('i', 1), on='i', rsuffix='_y')

  some_col  i some_col_y
0        A  2          C
1        B  1          B
2        C  1          B

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