How to put multiple return from a function in one or more columns using `tidyverse` packages

Question

Here is my problem: I have a function that take 1 double and return several doubles and I would like to directly get them on different columns of the tibble. I tried several methods with mutate() and/or map().

I do not want to call the method n times and take each time a different element of the list returned by the function.

Here is a generic problem of what I am trying to do:

library(tidyverse)

## a random function that return more than 1 element
f <- function(x){
  return(list(x/2,x**2))
}

## a tibble with a column on which I apply the function
tib <- tibble( x = rep(100:120)) 

tib%>%
  mutate(y = f(x))

## error: 
#Error : Problem with `mutate()` input `y`.
#x Input `y` can't be recycled to size 21.
#ℹ Input `y` is `f(x)`.
#ℹ Input `y` must be size 21 or 1, not 2.

## what I want to avoid:
tib %>%
  mutate(y = f(x)[[1]], z = f(x)[[2]])

I have been struggling with this issue for some times now. Apologies if it has been answered already I searched an answer on several forums but got nothing.

Thank you in advance !

Robert Wilson · Accepted Answer · 2020-07-21 16:22:28Z

2

You can solve this using nested data frames (or list columns depending on your preferred terminology).

The code below is a generic example of how to do this using mutate and map, which will create the nested data frame, and then unnest:

f <- function(x){
  tibble(y = x* 2, z = x*3)
}

tib <- tibble(x = rep(100:120)) %>% 
  mutate(data = map(x, f)) %>% 
  unnest()
tib

answered Jul 21, 2020 at 16:22

Robert Wilson

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2 Comments

Tom Over a year ago

thank you ! I still have to work on how map() works then cause it look like a simple application of the purrr package.

Robert Wilson Over a year ago

If this the kind of problem you are coming across often then working with the purrr's map functions and nested data frames is definitely something worth learning

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