Renaming multiple columns in R data.table - r

Question

This is related to this question from Henrik
Assign multiple columns using := in data.table, by group
But what if I want to create a new data.table with given column names instead of assigning new columns to an existing one?
f <- function(x){list(head(x,2),tail(x,2))}
dt <- data.table(group=sample(c('a','b'),10,replace = TRUE),val=1:10)
> dt
group val
1: b 1
2: b 2
3: a 3
4: b 4
5: a 5
6: b 6
7: a 7
8: a 8
9: b 9
10: b 10
I want to get a new data.table with predefined column names by calling the function f:
dt[,c('head','tail')=f(val),by=group]
I wish to get this:
group head tail
1: a 1 8
2: a 3 10
3: b 2 6
4: b 5 9
But it gives me an error. What I can do is create the table then change the column names, but that seems cumbersome:
> dt2 <- dt[,f(val),by=group]
> dt2
group V1 V2
1: a 1 8
2: a 3 10
3: b 2 6
4: b 5 9
> colnames(dt2)[-1] <- c('head','tail')
> dt2
group head tail
1: a 1 8
2: a 3 10
3: b 2 6
4: b 5 9
Is it something I can do with one call?

Answer 1

From running your code as-is, this is the error I get:
dt[,c('head','tail')=f(val),by=group]
# Error: unexpected '=' in "dt2[,c('head','tail')="
The problem is using = instead of := for assignment.
On to your problem of wanting a new data.table:
dt2 <- dt[, setNames(f(val), c('head', 'tail')), by = group]