self-join with Pandas

Question

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 ap

Answer 1

join with 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

Answer 2

Try this:

In [69]: d.join(d.set_index('i'), rsuffix='_y')
Out[69]:
  some_col  i some_col_y
0        A  2        NaN
1        B  1          B
1        B  1          C
2        C  1          A

or:

In [64]: pd.merge(d[['some_col']], d, left_index=True, right_on='i', suffixes=['_y','']).sort_index()
Out[64]:
  some_col_y some_col  i
0          C        A  2
1          B        B  1
2          B        C  1

Answer 3

Instead of using merge you can also use indexing and assignment:

>>> d['new_col'] = d['some_col'][d['i']].values
>>> d
  some_col  i new_col
0        A  2       C
1        B  1       B
2        C  1       B