Python 将索引上的数据帧与熊猫合并
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Merging dataframes on index with pandas
提问by km1234
I have two dataframes and each one has two index columns. I would like to merge them. For example, the first dataframe is the following:
我有两个数据框，每个数据框都有两个索引列。我想合并它们。例如，第一个数据帧如下：
V1
A 1/1/2012 12
2/1/2012 14
B 1/1/2012 15
2/1/2012 8
C 1/1/2012 17
2/1/2012 9
The second dataframe is the following:
第二个数据框如下：
V2
A 1/1/2012 15
3/1/2012 21
B 1/1/2012 24
2/1/2012 9
D 1/1/2012 7
2/1/2012 16
and as result I would like to get the following:
结果我想得到以下内容：
V1 V2
A 1/1/2012 12 15
2/1/2012 14 N/A
3/1/2012 N/A 21
B 1/1/2012 15 24
2/1/2012 8 9
C 1/1/2012 7 N/A
2/1/2012 16 N/A
D 1/1/2012 N/A 7
2/1/2012 N/A 16
I have tried a few versions using the pd.mergeand .joinmethods, but nothing seems to work. Do you have any suggestions?
我已经使用pd.merge和.join方法尝试了几个版本，但似乎没有任何效果。你有什么建议吗？
回答by Alexander
You should be able to use join, which joins on the index as default. Given your desired result, you must use outeras the join type.
您应该能够使用join, 默认情况下连接索引。给定您想要的结果，您必须使用outer作为连接类型。
>>> df1.join(df2, how='outer')
V1 V2
A 1/1/2012 12 15
2/1/2012 14 NaN
3/1/2012 NaN 21
B 1/1/2012 15 24
2/1/2012 8 9
C 1/1/2012 17 NaN
2/1/2012 9 NaN
D 1/1/2012 NaN 7
2/1/2012 NaN 16
Signature: _.join(other, on=None, how='left', lsuffix='', rsuffix='', sort=False) Docstring: Join columns with other DataFrame either on index or on a key column. Efficiently Join multiple DataFrame objects by index at once by passing a list.
签名：_.join(other, on=None, how='left', lsuffix='', rsuffix='', sort=False) Docstring：在索引或键列上将列与其他 DataFrame 连接。通过传递一个列表，一次通过索引有效地连接多个 DataFrame 对象。
回答by root
You can do this with merge:
你可以这样做merge：
df_merged = df1.merge(df2, how='outer', left_index=True, right_index=True)
The keyword argument how='outer'keeps all indices from both frames, filling in missing indices with NaN. The left_indexand right_indexkeyword arguments have the merge be done on the indices. If you get all NaNin a column after doing a merge, another troubleshooting step is to verify that your indices have the same dtypes.
关键字参数how='outer'保留两个帧中的所有索引，用NaN. 在left_index和right_index关键字参数有合并是对指数进行。如果NaN在合并后将所有内容都放在一列中，另一个故障排除步骤是验证您的索引是否具有相同的dtypes.
The mergecode above produces the following output for me:
merge上面的代码为我生成以下输出：
V1 V2
A 2012-01-01 12.0 15.0
2012-02-01 14.0 NaN
2012-03-01 NaN 21.0
B 2012-01-01 15.0 24.0
2012-02-01 8.0 9.0
C 2012-01-01 17.0 NaN
2012-02-01 9.0 NaN
D 2012-01-01 NaN 7.0
2012-02-01 NaN 16.0
