python - Compare two pandas dataframe with different size

Question

I have one massive pandas dataframe with this structure:

df1:
    A   B
0   0  12
1   0  15
2   0  17
3   0  18
4   1  45
5   1  78
6   1  96
7   1  32
8   2  45
9   2  78
10  2  44
11  2  10

And a second one, smaller like this:

df2
   G   H
0  0  15
1  1  45
2  2  31

I want to add a column to my first dataframe following this rule: column df1.C = df2.H when df1.A == df2.G

I manage to do it with for loops, but the database is massive and the code run really slowly so I am looking for a Pandas-way or numpy to do it.

Many thanks,

Boris

Answer 1

You probably want to use a merge:

df=df1.merge(df2,left_on="A",right_on="G")

will give you a dataframe with 3 columns, but the third one's name will be H

df.columns=["A","B","C"]

will then give you the column names you want