How to add Column Values from 2nd DataFrame Based on Some Column Values in First dataframe in Pandas

Question

My 1st DataFrame is so:

RowCol  Value_Pre
Key1 234
Key3 235
Key2 235
Key4 237

My Second Dataframe is:

RowCol  Value_Post
Key3 235
Key1 334
Key4 237
Key2 435

How to create a third dataframe like a the one below by combining the two dataframe

RowCol  Value_Pre Value_Post
Key1 	234	334
Key3	235	235
Key2	235	435
Key4	237	237

How to develop this code in Python?

jezrael · Accepted Answer · 2018-10-24 10:25:02Z

1

Use map:

df1['Value_Post'] = df1['RowCol'].map(df2.set_index('RowCol')['Value_Post'])
print (df1)
  RowCol  Value_Pre  Value_Post
0   Key1        234         334
1   Key3        235         235
2   Key2        235         435
3   Key4        237         237

Or merge with left join, especially if necessary append multiple new columns:

df = df1.merge(df2, on='RowCol', how='left')

edited Oct 24, 2018 at 10:25

answered Oct 24, 2018 at 10:20

jezrael

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2 Comments

Adhil Over a year ago

How do you create another column with values depending if Value_Pre is not equal to Value_Post?

jezrael Over a year ago

@Adhil - Do you think df1['new'] = np.where(df1['Value_Pre'].ne(df1['Value_Post']), 10, 20) ?

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