将前一天的值累计到当前日期

import pandas as pd

# 创建示例数据
data = {
    'date': ['2023-01-01', '2023-01-02', '2023-01-03', '2023-01-04'],
    'value': [10, 20, 30, 40]
}
df = pd.DataFrame(data)
df['date'] = pd.to_datetime(df['date'])

# 计算累计值
df['cumulative_value'] = df['value'].cumsum()

print(df)

# 生成完整的日期序列
full_dates = pd.date_range(start=df['date'].min(), end=df['date'].max())

# 合并原始数据与完整日期序列
df_full = pd.DataFrame({'date': full_dates}).merge(df, on='date', how='left')

# 前向填充缺失值
df_full['value'].fillna(method='ffill', inplace=True)

# 计算累计值
df_full['cumulative_value'] = df_full['value'].cumsum()

print(df_full)

# 按月累计
monthly_cumulative = df_full.set_index('date').resample('M')['value'].cumsum().reset_index()

print(monthly_cumulative)