r - data.table 按列名操作

Question

假设我有一个 data.table

a <- data.table(id=c(1,1,2,2,3),a=21:25,b=11:15,key="id")

我可以添加这样的新列:

a[, sa := sum(a), by="id"]
a[, sb := sum(b), by="id"]
> a
   id  a  b sa sb
1:  1 21 11 43 23
2:  1 22 12 43 23
3:  2 23 13 47 27
4:  2 24 14 47 27
5:  3 25 15 25 15

但是，假设我有列名:

for (n in c("a","b")) {
  s <- paste0("s",n)
  a[, s := sum(n), by="id", with=FALSE] # ERROR: invalid 'type' (character) of argument
}

我该怎么办？

Answer 1

你也可以这样做:

a <- data.table(id=c(1,1,2,2,3),a=21:25,b=11:15,key="id")

a[, c("sa", "sb") := lapply(.SD, sum), by = id]

或者更一般地:

cols.to.sum = c("a", "b")
a[, paste0("s", cols.to.sum) := lapply(.SD, sum), by = id, .SDcols = cols.to.sum]