[英]Pandas: Fill missing dataframe values from other dataframe
I have two dataframes of different size: 我有两个大小不同的数据框:
df1 = pd.DataFrame({'A':[1,2,None,4,None,6,7,8,None,10], 'B':[11,12,13,14,15,16,17,18,19,20]})
df1
A B
0 1.0 11
1 2.0 12
2 NaN 13
3 4.0 14
4 NaN 15
5 6.0 16
6 7.0 17
7 8.0 18
8 NaN 19
9 10.0 20
df2 = pd.DataFrame({'A':[2,3,4,5,6,8], 'B':[12,13,14,15,16,18]})
df2['A'] = df2['A'].astype(float)
df2
A B
0 2.0 12
1 3.0 13
2 4.0 14
3 5.0 15
4 6.0 16
5 8.0 18
I need to fill missing values (and only them) in column A of the first dataframe with values from the second dataframe with common key in the column B. It is equivalent to a SQL query: 我需要用第二个数据帧中的值(在B列中具有公共键)填充第一个数据帧的A列中的缺失值(并且只有它们)。这等效于SQL查询:
UPDATE df1 JOIN df2
ON df1.B = df2.B
SET df1.A = df2.A WHERE df1.A IS NULL;
I tried to use answers to similar questions from this site, but it does not work as I need: 我试图使用此站点上类似问题的答案,但由于我的需要而无法正常工作:
df1.fillna(df2)
A B
0 1.0 11
1 2.0 12
2 4.0 13
3 4.0 14
4 6.0 15
5 6.0 16
6 7.0 17
7 8.0 18
8 NaN 19
9 10.0 20
df1.combine_first(df2)
A B
0 1.0 11
1 2.0 12
2 4.0 13
3 4.0 14
4 6.0 15
5 6.0 16
6 7.0 17
7 8.0 18
8 NaN 19
9 10.0 20
Intended output is: 预期的输出为:
A B
0 1.0 11
1 2.0 12
2 3.0 13
3 4.0 14
4 5.0 15
5 6.0 16
6 7.0 17
7 8.0 18
8 NaN 19
9 10.0 20
How do I get this result? 我如何得到这个结果?
您正确使用combine_first()
,只是两个数据帧必须共享相同的索引,并且索引必须是列B:
df1.set_index('B').combine_first(df2.set_index('B')).reset_index()
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