匯總/匯總每組的多個變量（例如總和，平均值）從數據幀，是否有聚集（一個簡單的方法sum，mean，max同時等c）中多個變量？以下是一些示例數據：library(lubridate)days = 365*2date = seq(as.Date("2000-01-01"), length = days, by = "day")year = year(date)month = month(date)x1 = cumsum(rnorm(days, 0.05)) x2 = cumsum(rnorm(days, 0.05))df1 = data.frame(date, year, month, x1, x2)我想同時按年和月匯總數據框中的變量x1和x2變量df2。以下代碼聚合x1變量，但是是否也可以同時聚合x2變量？### aggregate variables by year monthdf2=aggregate(x1 ~ year+month, data=df1, sum, na.rm=TRUE)head(df2)任何建議將不勝感激。