R - Data.table - 在RHS操作中使用变量列名

如何在RHS上使用变量列名:= operations？例如,给定此data.table"dt",我想创建两个新列"first_y"和"first_z",其中包含对"x"值的给定列的第一次观察.

dt <- data.table(x = c("one","one","two","two","three"), 
                 y = c("a", "b", "c", "d", "e"), 
                 z = c(1, 2, 3, 4, 5))

dt
       x y z
1:   one a 1
2:   one b 2
3:   two c 3
4:   two d 4
5: three e 5

如果没有变量列名,您可以这样做.

dt[, c("first_y", "first_z") := .(first(y), first(z)), by = x]

dt
       x y z first_y first_z
1:   one a 1       a       1
2:   one b 2       a       1
3:   two c 3       c       3
4:   two d 4       c       3
5: three e 5       e       5

但是,如果"y"和"z"列名称动态存储在变量中,我该怎么做呢？

cols <- c("y", "z")

# This doesn't work
dt[, (paste0("first_", cols)) := .(first(cols)), by = x]

# Nor does this
q <- quote(first(as.name(cols[1])))
p <- quote(first(as.name(cols[2])))
dt[, (paste0("first_", cols)) := .(eval(q), eval(p)), by = x]

我已经尝试了很多其他的quote()和eval()以及as.name()的组合而没有成功.该操作的LHS似乎按预期工作,并在许多地方记录,但我找不到有关在RHS上使用变量列名称的任何信息.提前致谢.

我不熟悉这个first功能(虽然它看起来像是Hadley所定义的).

dt[, paste0("first_", cols) := lapply(.SD, head, n = 1L), 
   by = x, .SDcols = cols]
#       x y z first_y first_z
#1:   one a 1       a       1
#2:   one b 2       a       1
#3:   two c 3       c       3
#4:   two d 4       c       3
#5: three e 5       e       5