Easiest to show an example for my question. I want to set the column name to something other than N in the code below:

dt <- data.table(
  ID = c("b","b","b","a","a","c"),
  x = 1:6,
  y = 7:12,
  z = 13:18
)

cols = c("x", "y")
dt[, c(lapply(.SD, mean),.N), .SDcols=cols, keyby = c("ID")]
# returns

   ID   x    y N
1:  a 4.5 10.5 2
2:  b 2.0  8.0 3
3:  c 6.0 12.0 1

I would like to set the name of the N column in the original call (my actual use case will involve more columns and functions) so that the output is

   ID   x    y Members
1:  a 4.5 10.5 2
2:  b 2.0  8.0 3
3:  c 6.0 12.0 1

I could of course rename N in a separate command after, but I am curious if I can do it as part of the groupby line.

# attempt 1 which still returns column named N

dt[, c(lapply(.SD, mean), Members = .N), .SDcols=cols, keyby = c("ID")]
>
   ID   x    y N
1:  a 4.5 10.5 2
2:  b 2.0  8.0 3
3:  c 6.0 12.0 1

# attempt 2 errors
dt[, (c(cols, "Members")) = c(lapply(.SD, mean), .N), .SDcols=cols, keyby = c("ID")]
Error in parse(text = .DATABRICKS_CURRENT_TEMP_CMD__) : 
  <text>:10:27: unexpected '='
9: cols = c("x", "y")
10: dt[, (c(cols, "Members")) =
                              ^
Error in parse(text = .DATABRICKS_CURRENT_TEMP_CMD__): <text>:10:27: unexpected '='
Error in parse(text = .DATABRICKS_CURRENT_TEMP_CMD__): <text>:10:27: unexpected '='
9: cols = c("x", "y")
10: dt[, (c(cols, "Members")) =

Versions: data.table 1.14.8 and R 4.3.1