分组的data.table摘要，用于多列

Question

dt = data.table(a = rep(x = c(1, 2, 3), 3), y = as.numeric(1:9), z = as.numeric(2:10))

我知道这可以按组汇总一列的值：

dt = data.table(a = rep(x = c(1, 2, 3), 3), y = as.numeric(1:9), z = as.numeric(2:10))

但是，如果我尝试输入多列：

dt[, .(c('y', 'z') = list(mean(y) mean(z)), by = a]

我得到了错误

  

错误：“ dt [，。（c（'y'，'z'）=”

中出现意外的'='

我在寻找什么：

## data.table equivalent to
dt %>% group_by(a) %>% summarise(y = mean(y), z = mean(z))
#  a     y     z
#  <dbl> <dbl> <dbl>
#1     1     4     5
#2     2     5     6
#3     3     6     7

Answer 1

dt <- dt[, .(y = mean(y), z = mean(z)), by=.(a)]