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).
Graphically:
Code:
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)
Use join
or map
:
df = df2.join(df.set_index('Col1'), on='Col3')
print (df)
Col3 Col4 Col2
Row1 1 T1 ONE
Row2 2 T2 TWO
Row3 3 T3 NaN
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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