I have 2 dataframes and I want to add a new column to one of my dataframes from another dataframe '''
(df1)
Id height ...
12 190
13 180
34 173
. . .
(df2)
Id amount ...
12 234
15 256
13 248
. . .
''' how can I add a column "amount" to df1 that contains the value amount of df2 for each Id of df1. the size of df1 and df2 are not equal
You can merge the two DataFrames on the Id column and then assign the resulting amount column to your first df:
import pandas as pd
df1 = pd.DataFrame(
{"Id": [12, 13, 34], "height": [1, 2, 3]}
)
df2 = pd.DataFrame(
{"Id": [12, 15, 13], "amount": [4, 5, 6]}
)
df1["amount"] = df1.merge(df2, on="Id")["amount"]
Result:
>>> print(df1)
Id height amount
0 12 1 4.0
1 13 2 6.0
2 34 3 NaN
Id Id is index column then you can use update method to do that
df1 = pd.DataFrame([[12, 190.0], [13, 180.0], [34, 173.0]], columns=('Id', 'height')).set_index("Id")
df2 = pd.DataFrame([[12, 234.0], [15, 256.0], [13, 248.0]], columns=('Id', 'amount')).set_index("Id")
df1["amount"] = np.nan
df1.update(df2, overwrite=True)
print(df1)
Result
height amount
Id
12 190.0 234.0
13 180.0 248.0
34 173.0 NaN
df.join(df1.set_index('Id'), on='Id')