Question: I have a function in my real-world problem that returns a list. Is there any way to use this with the dplyr mutate()? This toy example doesn't work -:

```
it = data.table(c("a","a","b","b","c"),c(1,2,3,4,5), c(2,3,4,2,2))

myfun = function(arg1,arg2) {

temp1 = arg1 + arg2
temp2 = arg1 - arg2
list(temp1,temp2)

}

myfun(1,2)

it%.%mutate(new = myfun(V2,V3))

```

I see that it is cycling through the output of the function in the first "column" of the *new* variable, but do not understand why.

Thanks!

Answer: 
The idiomatic way to do this using `data.table` would be to use the `:=` (assignment by reference) operator. Here's an illustration:

```
it[, c(paste0("V", 4:5)) := myfun(V2, V3)]

```

---

If you really want a list, why not:

```
as.list(it[, myfun(V2, V3)])

```

Alternatively, maybe this is what you want, but why don't you just use the `data.table` functionality:

```
it[, c(.SD, myfun(V2, V3))]
#    V1 V2 V3 V4 V5
# 1:  a  1  2  3 -1
# 2:  a  2  3  5 -1
# 3:  b  3  4  7 -1
# 4:  b  4  2  6  2
# 5:  c  5  2  7  3    

```

Note that if `myfun` were to name it's output, then the names would show up in the final result columns:

```
#    V1 V2 V3 new.1 new.2
# 1:  a  1  2     3    -1
# 2:  a  2  3     5    -1
# 3:  b  3  4     7    -1
# 4:  b  4  2     6     2
# 5:  c  5  2     7     3    

```
The mutate() function is designed to add new columns to the existing data frame. A data frame is a list of vectors of the same length. Thus, you cant add a list as a new column, because a list is not a vector.

You can rewrite your function as two functions, each of which return a vector. Then apply each of these separately using mutate() and it should work.