用基于其中一列的另一个数据框替换数据框中的值

Question

I have two dataframes, one that contains some information and a shorter dataframe that contains some amendments I want to make to the first. I have tried something similar to that written below.

for x in df['var']:
    if x in df2['var']:
         df['var2'] == df2['var2']

For example:

---> df
        num_legs  num_wings  num_specimen_seen
falcon         2          2                 10
dog            4          0                  2
spider         8          0                  1
fish           0          0                  8

---> df1
        num_legs  num_wings  num_specimen_seen
dog            4          0                  5
spider         8          0                  9

---> output
        num_legs  num_wings  num_specimen_seen
falcon         2          2                 10
dog            4          0                  5
spider         8          0                  9
fish           0          0                  8

Answer 1

Have you tried df.update()

df.update(df1)



        num_legs  num_wings  num_specimen_seen
falcon       2.0        2.0               10.0
dog          4.0        0.0                5.0
spider       8.0        0.0                9.0
fish         0.0        0.0                8.0

Alternative

import numpy as np
df[df.index.isin(df1.index)]=np.nan
df1.combine_first(df)

How it works

#Find shared index between the two dataframes which are in df .

df[df.index.isin(df1.index)]

#Make them NaN


df[df.index.isin(df1.index)]=np.nan

#Use [.combine_first][1] to fill them

df1.combine_first(df)