根据另一列填写空白单元格

Question

I want to match/map the missing value in a dataframe based on another column. For example,

         City         State              Country
      Chicago            IL        United States
       Boston            MA        United States
    San Diego            
  Los Angeles            CA        United States
San Francisco
   Sacramento     
    Vancouver            BC               Canada

So, if I want to fill in the empty cells of the provinces and countries of those three cities same as Los Angeles. What should I do?

Below is my code but I'm completely stuck in it.

CA_cities = ['San Diego', 'Los Angeles', 'San Francisco', 'Sacramento']
df.loc[df['City'] == CA_cities, 'State' = 'CA' and 'Country' = 'United States']

Any help will be greatly appreciated.

Answer 1

You can use groupby with mask created by isin , then replace NaN s by back and forward filling:

CA_cities = ['San Diego', 'Los Angeles', 'San Francisco', 'Sacramento']

df = df.groupby(df['City'].isin(CA_cities)).apply(lambda x: x.ffill().bfill())
print (df)
            City State        Country
0        Chicago    IL  United States
1         Boston    MA  United States
2      San Diego    CA  United States
3    Los Angeles    CA  United States
4  San Francisco    CA  United States
5     Sacramento    CA  United States
6      Vancouver    BC         Canada

More general solution is create groups of cities, eg in dictionaries, swap keys wih values and map column:

print (df)
            City State        Country
0        Chicago    IL  United States
1       Chicago1   NaN            NaN
2         Boston    MA  United States
3      San Diego   NaN            NaN
4    Los Angeles    CA  United States
5  San Francisco   NaN            NaN
6     Sacramento   NaN            NaN
7      Vancouver    BC         Canada

cities = {'CA': ['San Diego', 'Los Angeles', 'San Francisco', 'Sacramento'], 
          'IL':['Chicago','Chicago1']}
d = {k: oldk for oldk, oldv in cities.items() for k in oldv}

df = df.groupby(df['City'].map(d).fillna(df['City'])).apply(lambda x: x.ffill().bfill())
#slowier alternative
#df = df.groupby(df['City'].replace(d)).apply(lambda x: x.ffill().bfill())
print (df)
            City State        Country
0        Chicago    IL  United States
1       Chicago1    IL  United States
2         Boston    MA  United States
3      San Diego    CA  United States
4    Los Angeles    CA  United States
5  San Francisco    CA  United States
6     Sacramento    CA  United States
7      Vancouver    BC         Canada

Detail :

print (df['City'].map(d).fillna(df['City']))
0           IL
1           IL
2       Boston
3           CA
4           CA
5           CA
6           CA
7    Vancouver
Name: City, dtype: object

print (d)
{'San Diego': 'CA', 'Los Angeles': 'CA', 'San Francisco': 'CA', 
 'Sacramento': 'CA', 'Chicago': 'IL', 'Chicago1': 'IL'}

Answer 2

Or just split it , and using fillna .

CA_cities = ['SanDiego', 'LosAngeles', 'SanFrancisco', 'Sacramento']
s=df.loc[df.City.isin(CA_cities),:]
t=df.loc[~df.City.isin(CA_cities),:]
pd.concat([s.fillna({'State':'CA','Country':'UnitedStates'}),t])
Out[1023]: 
           City State       Country
2      SanDiego    CA  UnitedStates
3    LosAngeles    CA  UnitedStates
4  SanFrancisco    CA  UnitedStates
5    Sacramento    CA  UnitedStates
0       Chicago    IL  UnitedStates
1        Boston    MA  UnitedStates
6     Vancouver    BC        Canada