计算值与其组中第一个值之间的差值

import pandas as pd

# 示例数据
data = {
    'group': ['A', 'A', 'B', 'B', 'B', 'C'],
    'value': [10, 15, 20, 25, 30, 35]
}

# 创建DataFrame
df = pd.DataFrame(data)

# 按'group'列分组，并找到每组的第一个'value'
first_values = df.groupby('group')['value'].first().reset_index()

# 合并原始数据和每组的第一个值
merged_df = pd.merge(df, first_values, on='group', suffixes=('', '_first'))

# 计算差值
merged_df['difference'] = merged_df['value'] - merged_df['value_first']

print(merged_df[['group', 'value', 'difference']])

  group  value  difference
0     A      10           0
1     A      15           5
2     B      20           0
3     B      25           5
4     B      30          10
5     C      35           0