How to check if a value in one dataframe is present in keys in the other dataframe

问题

I have two dataframes:

df_1:

         Letters Boolean
          a         Nan
          b         Nan
          c         Nan

df_2:

 a     b     d
 2.7   1.2   3.6
 1      2     3

How do I check if df_1['letters'] is present in df_2.keys(). If it is present, I want boolean to take the value 'x': Something like:

     Letters Boolean
      a         x
      b         x
      c         Nan

I tried using this code:

for x in df_1['letters']:
   if x in df_2.keys():
     df_1['Boolean']='x'

回答1:

Use numpy.where with isin:

df1['Boolean'] = np.where(df1['Letters'].isin(df2.columns), 'x', np.nan)

回答2:

You need :

df1['Boolean']=df1.Letters.isin(df2.columns).map({True:'x',False:np.nan})
print(df1)

  Letters Boolean
0       a       x
1       b       x
2       c     NaN

来源：https://stackoverflow.com/questions/54551692/how-to-check-if-a-value-in-one-dataframe-is-present-in-keys-in-the-other-datafra