I am using the map function of the purrr package in R which gives as output a list. Now I would like the output to be a named list based on the input. An example is given below.
input <- c("a", "b", "c")
output <- purrr::map(input, function(x) {paste0("test-", x)})
From this I would like to access elements of the list using:
output$a
Or
output$b
c("a", "b", "c") %>%
purrr::set_names() %>%
purrr::map(~paste0('test-', .))
We just need to name the list
names(output) <- input
and then extract the elements based on the name
output$a
#[1] "test-a"
If this needs to be done using tidyverse
library(tidyverse)
output <- map(input, ~paste0('test-', .)) %>%
setNames(input)
purrr imports set_names() from rlang for this reason. Not that there is any significant difference between base::setNames()...
x %>% set_names() %>% map(fn) which is the recommended solution to this problem.Update
Now in 2020 the answer form @mihagazvoda describes the correct approach: simply set_names before applying map
c("a", "b", "c") %>%
purrr::set_names() %>%
purrr::map(~paste0('test-', .))
Outdated answer
The accepted solution works, but suffers from a repeated argument (input) which may cause errors and interrupts the flow when using piping with %>%.
An alternative solution would be to use a bit more power of the %>% operator
1:5 %>% { set_names(map(., ~ .x + 3), .) } %>% print # ... or something else
This takes the argument from the pipe but still lacks some beauty. An alternative could be a small helper method such as
map_named = function(x, ...) map(x, ...) %>% set_names(x)
1:5 %>% map_named(~ .x + 1)
This already looks more pretty and elegant. And would be my preferred solution.
Finally, we could even overwrite purrr::map in case the argument is a character or integer vector and produce a named list in such a case.
map = function(x, ...){
if (is.integer(x) | is.character(x)) {
purrr::map(x, ...) %>% set_names(x)
}else {
purrr::map(x, ...)
}
}
1 : 5 %>% map(~ .x + 1)
However, the optimal solution would be if purrr would implement such behaviour out of the box.
purrr maintainers do not like it because it creates different semantics depending on the input. I'd imagine that they prefer some more explicit approach, like with a parameter, but this would support the usecase as well. Also, it would break backward compatibility in quite a few cases I guess. But sure, why not asking, see github.com/tidyverse/purrr/issues/691
base::Map:output2 <- Map(function(x) {paste0("test-", x)}, input).