大熊猫通过比较两个数据框创建一个新列

Question

I have two dataframes, df1 and df2.

df1:

ID     Label
1      a
2      b
5      c

df2:

ID
1
2
3

I want to create a new column "label" in df2 by comparing the two dataframes. If the ids match, label in df2 should equal to label in df1. If the id does not occur in df1, I want 0. So the final df2 look like this:

ID     Label
1      a
2      b
3      0

Any help will be appreciated. Thanks!

Answer 1

You can use map and then replace NaN s with fillna :

df2['Label'] = df2['ID'].map(df1.set_index('ID')['Label']).fillna(0)
print (df2)
   ID Label
0   1     a
1   2     b
2   3     0