기본 코드

import pandas as pd
import numpy as np

student_list = [{'name': 'John', 'major': "Computer Science", 'sex': "male"},
                {'name': 'Nate', 'major': "Computer Science", 'sex': "male"},
                {'name': 'Abraham', 'major': "Physics", 'sex': "male"},
                {'name': 'Brian', 'major': "Psychology", 'sex': "male"},
                {'name': 'Janny', 'major': "Economics", 'sex': "female"},
                {'name': 'Yuna', 'major': "Economics", 'sex': "female"},
                {'name': 'Jeniffer', 'major': "Computer Science", 'sex': "female"},
                {'name': 'Edward', 'major': "Computer Science", 'sex': "male"},
                {'name': 'Zara', 'major': "Psychology", 'sex': "female"},
                {'name': 'Wendy', 'major': "Economics", 'sex': "female"},
                {'name': 'Sera', 'major': "Psychology", 'sex': "female"}
         ]

df = pd.DataFrame(student_list, columns = ['name', 'major', 'sex'])

• groupby

groupby_major = df.groupby('major')
groupby_major.groups
{'Computer Science': [0, 1, 6, 7], 'Economics': [4, 5, 9], 'Physics': [2], 'Psychology': [3, 8, 10]}

for name, group in groupby_major:
    print(name + ": " + str(len(group)))
    print(group)
    print()

    
Computer Science: 4
       name             major     sex
0      John  Computer Science    male
1      Nate  Computer Science    male
6  Jeniffer  Computer Science  female
7    Edward  Computer Science    male

Economics: 3
    name      major     sex
4  Janny  Economics  female
5   Yuna  Economics  female
9  Wendy  Economics  female

Physics: 1
      name    major   sex
2  Abraham  Physics  male

Psychology: 3
     name       major     sex
3   Brian  Psychology    male
8    Zara  Psychology  female
10   Sera  Psychology  female

그룹 객체를 데이터프레임으로 생성할 수 있다.

df_major_cnt = pd.DataFrame( {'count' : groupby_major.size()} )
df_major_cnt
                  count
major                  
Computer Science      4
Economics             3
Physics               1
Psychology            3

위 출력에서 major, count를 보면 이상하게 나온다.

major를 column 값으로 넣기 위해서 reset_index()하면 된다.

df_major_cnt = pd.DataFrame( {'count' : groupby_major.size()} ).reset_index()
df_major_cnt
              major  count
0  Computer Science      4
1         Economics      3
2           Physics      1
3        Psychology      3