How To Check If A Value In One Dataframe Is Present In Keys In The Other Dataframe

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

Solution 1:

Use numpy.where with isin:

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

Solution 2:

You need :

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

  Letters Boolean0       a       x
1       b       x
2       c     NaN