Return a list in dplyr mutate()

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!

Accepted 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    
```

Return a list in dplyr mutate()

Solution

Problem