python 如何将一个 pandas 数据帧中的值计数转置到第二个数据帧中的多列？

Question

I have 2 data frames df1 and df2.

import pandas as pd

df1 = pd.DataFrame({
    'id':['1','1','1','2','2','2', '3', '4','4', '5', '6', '7'],
    'group':['A','A','B', 'A', 'A', 'C', 'A', 'A', 'B', 'B', 'A', 'C']
})

df2 = pd.DataFrame({
'id':['1','2','3','4','5','6','7']
})

I want to add 3 columns to df2 named group_A, group_B, and group_C, where each counts the number of repetitions of each group in df1 according to the id column. so the result of df2 should be likes this:

Answer 1

Use crosstab with DataFrame.join , type of both id has to by same, like here strings:

print (pd.crosstab(df1['id'], df1['group']).add_prefix('group_'))
group  group_A  group_B  group_C
id                              
1            2        1        0
2            2        0        1
3            1        0        0
4            1        1        0
5            0        1        0
6            1        0        0
7            0        0        1
    
df = df2.join(pd.crosstab(df1['id'], df1['group']).add_prefix('group_'), on='id')
print (df)
  id  group_A  group_B  group_C
0  1        2        1        0
1  2        2        0        1
2  3        1        0        0
3  4        1        1        0
4  5        0        1        0
5  6        1        0        0
6  7        0        0        1

Solution without join is possible, if same id s in both DataFrame s:

print (pd.crosstab(df1['id'], df1['group']).add_prefix('group_').reset_index().rename_axis(None, axis=1))
  id  group_A  group_B  group_C
0  1        2        1        0
1  2        2        0        1
2  3        1        0        0
3  4        1        1        0
4  5        0        1        0
5  6        1        0        0
6  7        0        0        1

Answer 2

One option is to get the counts for df2, before joining to df1:

counts = df1.value_counts().unstack(fill_value=0).add_prefix('group_')
df2.join(counts, on='id')

  id  group_A  group_B  group_C
0  1        2        1        0
1  2        2        0        1
2  3        1        0        0
3  4        1        1        0
4  5        0        1        0
5  6        1        0        0
6  7        0        0        1

Another option is with get_dummies , combined with groupby :

counts = pd.get_dummies(df1, columns = ['group']).groupby('id').sum()

df2.join(counts, on='id')

  id  group_A  group_B  group_C
0  1        2        1        0
1  2        2        0        1
2  3        1        0        0
3  4        1        1        0
4  5        0        1        0
5  6        1        0        0
6  7        0        0        1

Answer 3

Another option is groupby on ['id', 'group'] , apply size and unstack .

out = (df1.groupby(['id','group']).size().unstack(fill_value=0)
       .add_prefix('group_').reset_index().rename_axis([None], axis=1)
       .merge(df2, on='id'))

Output:

  id  group_A  group_B  group_C
0  1        2        1        0
1  2        2        0        1
2  3        1        0        0
3  4        1        1        0
4  5        0        1        0
5  6        1        0        0
6  7        0        0        1