映射到2列 pandas [英] Map on 2 columns pandas

问题描述

我有一个看起来像这样的数据框:

I have a dataframe that looks like:

        Col1           Col2
0        A              PY
1        B              PA
2        C              PB
3        B              PB

和本系列:

                            Value
   Col1       Col2     
    A          PY             20
    B          PB             30

如果该系列具有一个索引(仅适用于Col1)，则我想进行映射:

I would like to do a mapping, if the series has one index(ex Col1 only) its pretty straightforward with:

  df['Value'] = df['Col1'].map(s)

如何基于2个索引/列进行映射?

How can I do mapping based on 2 indexes/columns ?

谢谢！

推荐答案

您可以将它们合并到公共列(具有相同名称的列)中:

you can merge them on the common columns (columns with the same names):

In [28]: d1
Out[28]:
  Col1 Col2
0    A   PY
1    B   PA
2    C   PB
3    B   PB

In [29]: d2
Out[29]:
           Value
Col1 Col2
A    PY       20
B    PB       30

In [30]: d1.merge(d2.reset_index())
Out[30]:
  Col1 Col2  Value
0    A   PY     20
1    B   PB     30

或使用左外部联接:

In [33]: d1.merge(d2.reset_index(), how='left')
Out[33]:
  Col1 Col2  Value
0    A   PY   20.0
1    B   PA    NaN
2    C   PB    NaN
3    B   PB   30.0

相同的解决方案也适用于d2系列:

same solution will work also for d2 as Series:

In [31]: s
Out[31]:
Col1  Col2
A     PY      20
B     PB      30
Name: Value, dtype: int64

In [32]: d1.merge(s.reset_index())
Out[32]:
  Col1 Col2  Value
0    A   PY     20
1    B   PB     30

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