大 pandas 根据联接将列从一个数据框添加到另一个数据框 [英] Pandas add column from one dataframe to another based on a join

问题描述

假设我有2个数据帧.我想基于列查找将数据框1的列添加到数据框2.如果无法进行联接，则需要在额外的列中使用某个常数(以便可以对此进行过滤).

Assume I have 2 dataframes. I want to add a column of dataframe 1 to dataframe 2 based on a column lookup. If the join is not possible, I want in the extra column a certain constant (so I can filter for that).

图形方式:

代码:

import pandas as pd
import numpy as np

data = np.array([['','Col1','Col2'],
                ['Row1','2','TWO'],
                ['Row2','1','ONE']]
            )

data2 = np.array([['','Col3','Col4'],
                ['Row1','1','T1'],
                ['Row2','2','T2'],
                ['Row3','3','T3']]
            )

df = pd.DataFrame(data=data[1:,1:],
                  index=data[1:,0],
                  columns=data[0,1:])

df2 = pd.DataFrame(data=data2[1:,1:],
                  index=data2[1:,0],
                  columns=data2[0,1:])

result_df = df2 + join Col2 based on df2.Col3 = df.Col1. Add certain string constant if join fails. 

print(df)
print(df2)
print(result_df)

推荐答案

使用 join 或

---

df2['Col2'] = df2['Col3'].map(df.set_index('Col1')['Col2'])
print (df2)
     Col3 Col4 Col2
Row1    1   T1  ONE
Row2    2   T2  TWO
Row3    3   T3  NaN

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