python - 添加来自不同数据框的值

Question

df1:
            A    B
0  2002-01-13  3.9
1  2002-01-13  1.9
2  2002-01-14  8.0
3  2002-01-14  9.0

我想创建一个新列 df1["C"]通过B值，每个 A组。

输出应该是:

            A    B     C
0  2002-01-13  3.9   2.9
1  2002-01-13  1.9   2.9
2  2002-01-14  8.0   8.5
3  2002-01-14  9.0   8.5

现在我想分配 C每个 A 的值组，到另一个df2 .

df2:
            A        D
0  2002-01-13   Joseph
1  2002-01-13     Emma
2  2002-01-13  Michael
3  2002-01-14     Anna
4  2002-01-14   Yvonne
5  2002-01-14  Anthony

输出应该是:

            A        D     E
0  2002-01-13   Joseph   2.9
1  2002-01-13     Emma   2.9
2  2002-01-13  Michael   2.9
3  2002-01-14     Anna   8.5
4  2002-01-14   Yvonne   8.5
5  2002-01-14  Anthony   8.5

我试过:

df1["C"] = df1.groupby("A")["B"].mean()

Answer 1

您不必向 df1 添加列，您可以直接将 groupby df1 中的值映射到 df2。

df2['E'] = df2['A'].map(df1.groupby('A').B.mean())


    A           D       E
0   2002-01-13  Joseph  2.9
1   2002-01-13  Emma    2.9
2   2002-01-13  Michael 2.9
3   2002-01-14  Anna    8.5
4   2002-01-14  Yvonne  8.5
5   2002-01-14  Anthony 8.5