问题
I'm trying to fill a column of a dataframe from another dataframe based on conditions. Let's say my first dataframe is df1 and the second is named df2.
df1 is described as bellow :
+------+------+
| Col1 | Col2 |
+------+------+
| A | 1 |
| B | 2 |
| C | 3 |
| A | 1 |
+------+------+
And :
df2 is described as bellow :
+------+------+
| Col1 | Col2 |
+------+------+
| A | NaN |
| B | NaN |
| D | NaN |
+------+------+
Each distinct value of Col1 has her an id number (In Col2), so what I want is to fill the NaN values in df2.Col2 where df2.Col1==df1.Col1 .
So that my second dataframe will look like :
df2 :
+------+------+
| Col1 | Col2 |
+------+------+
| A | 1 |
| B | 2 |
| D | NaN |
+------+------+
I'm using Python 2.7
回答1:
Use drop_duplicates with set_index and combine_first:
df = df2.set_index('Col1').combine_first(df1.drop_duplicates().set_index('Col1')).reset_index()
If need check dupes only in id column:
df = df2.set_index('Col1').combine_first(df1.drop_duplicates().set_index('Col1')).reset_index()
回答2:
Here is a solution with the filter df1.Col1 == df2.Col1
df2['Col2'] = df1[df1.Col1 == df2.Col1]['Col2']
It is even better to use loc (but less clear from my point of view)
df2['Col2'] = df1.loc[df1.Col1 == df2.Col2, 'Col2']
来源：https://stackoverflow.com/questions/51097981/fill-column-of-a-dataframe-from-another-dataframe