python - Pandas 遍历行并找到列名

Question

我有两个数据框:

df = pd.DataFrame({'America':["Ohio","Utah","New York"],
                   'Italy':["Rome","Milan","Venice"],
                   'Germany':["Berlin","Munich","Jena"]});


df2 = pd.DataFrame({'Cities':["Rome", "New York", "Munich"],
                   'Country':["na","na","na"]})

我想迭代 df2“城市”列以在我的 (df) 上找到城市并将城市的国家(df 列名称)附加到 df2 国家列

Answer 1

使用melt与 map按字典:

df1 = df.melt()
print (df1)
  variable     value
0  America      Ohio
1  America      Utah
2  America  New York
3    Italy      Rome
4    Italy     Milan
5    Italy    Venice
6  Germany    Berlin
7  Germany    Munich
8  Germany      Jena

df2['Country'] = df2['Cities'].map(dict(zip(df1['value'], df1['variable'])))
#alternative, thanks @Sandeep Kadapa 
#df2['Country'] = df2['Cities'].map(df1.set_index('value')['variable'])
print (df2)
     Cities  Country
0      Rome    Italy
1  New York  America
2    Munich  Germany