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 =          


        

Answer 1

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.

Answer 2

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    

Answer 3

Given the title to this question, I thought I'd post a tidyverse solution that uses dplyr::mutate. Note that myfun needs to output a data.frame to work.

library(tidyverse)

it = data.frame(
  v1 = c("a","a","b","b","c"),
  v2 = c(1,2,3,4,5), 
  v3 = c(2,3,4,2,2))

myfun = function(arg1,arg2) {

  temp1 = arg1 + arg2
  temp2 = arg1 - arg2
  data.frame(temp1, temp2)

}

it %>% 
  nest(data = c(v2, v3)) %>% 
  mutate(out = map(data, ~myfun(.$v2, .$v3))) %>% 
  unnest(cols = c(data, out))
#> # A tibble: 5 x 5
#>   v1       v2    v3 temp1 temp2
#>   <fct> <dbl> <dbl> <dbl> <dbl>
#> 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

^{Created on 2020-02-04 by the reprex package (v0.3.0)}