R-如何根据列和列名称重塑数据帧？

import pandas as pd

# 创建示例数据框
df = pd.DataFrame({'date': ['2022-01-01', '2022-01-02', '2022-01-03'],
                   'category': ['A', 'B', 'A'],
                   'value': [1, 2, 3]})

# 使用pivot函数重塑数据框
pivot_df = df.pivot(index='date', columns='category', values='value')

print(pivot_df)

category      A    B
date                
2022-01-01  1.0  NaN
2022-01-02  NaN  2.0
2022-01-03  3.0  NaN

import pandas as pd

# 创建示例数据框
df = pd.DataFrame({'date': ['2022-01-01', '2022-01-02', '2022-01-03'],
                   'A': [1, None, 3],
                   'B': [None, 2, None]})

# 使用melt函数重塑数据框
melt_df = df.melt(id_vars='date', var_name='category', value_name='value')

print(melt_df)

         date category  value
0  2022-01-01        A    1.0
1  2022-01-02        A    NaN
2  2022-01-03        A    3.0
3  2022-01-01        B    NaN
4  2022-01-02        B    2.0
5  2022-01-03        B    NaN