从数据框架，有一个简单的方法来同时聚合(即求和)多个变量？
以下是一些示例数据：
days = 365*2
date = 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)
我想同时聚合来自df2数据框架的x1和x2变量按年和月。以下代码聚合x1变量，但是是否也可以同时聚合x2变量？
### aggregate variables by year month df2=aggregate(x1~year+month,data=df1,sum,na.rm=TRUE) head(df2)
任何建议将不胜感激。
这个year()函数在哪里？
原文链接：https://www.f2er.com/javaschema/282913.html您还可以使用reshape2包执行此任务：
require(reshape2)
df_melt <- melt(df1,id = c("date","year","month"))
dcast(df_melt,year + month ~ variable,sum)
# year month x1 x2
1 2000 1 -80.83405 -224.9540159
2 2000 2 -223.76331 -288.2418017
3 2000 3 -188.83930 -481.5601913
4 2000 4 -197.47797 -473.7137420
5 2000 5 -259.07928 -372.4563522
