Using Dplyr In A Function To Create New Dataframes - r

Question

I'm Trying to create new dataframes from dplyr 0.4.3 functions using R 3.2.2.
What I want to do is create some new dataframes using dplyr::filter to separate out data from one ginormous dataframe into a bunch of smaller dataframes.
For my reproducible base case bog simple example, I used this:
filter(mtcars, cyl == 4)
I know I need to assign that to a dataframe of its own, so I started with:
paste("Cylinders:", x, sep = "") <- filter(mtcars, cyl == 4))
That didn't work -- it gave me the error found here: Assignment Expands to Non-Language Object
From there, I found this: Create A Variable Name with Paste in R
(also, big ups to the authors of the above)
And that led me to this, which works:
assign(paste("gears_cars_cylinders", 4, sep = "_"), filter(mtcars, cyl == 4)) %>%
group_by(gear) %>%
summarise(number_of_cars = n())
and by "works," I mean I get a dataframe named gears_cars_cylinders_4 with all the goodies from
filter(mtcars, cyl == 4) %>%
group_by(gear) %>%
summarise(number_of_cars = n())
But ultimately, I think I need to wrap this whole thing in a function and be able to feed it the cylinder numbers from mtcars$cyl. I'm thinking something like plyr::ldply(mtcars$cyl, function_name)?
In my real-life data, I have about 70 different classes I need to split out into separate dataframes to drop into DT::datatable tabs in Shiny, which is a whole nuther mess. Anyway.
When I try this:
function_name <- function(x){
assign(paste("gears_cars_cylinders", x, sep = "_"), filter(mtcars, cyl == x)) %>%
group_by(gear) %>%
summarise(number_of_cars = n())
}
and then function_name(6),
I get the output of the dataframe to the screen, but not a dataframe with the name.
Am I looking right over the answer here?

Answer 1

You need to assign the new data frames into the environment from which you're calling function_name(). Try something like this:
library(dplyr)
foo <- function(x) {
assign(paste("gears_cars_cylinders", x, sep = "_"),
envir = parent.frame(),
value = mtcars %>%
filter(cyl == x) %>%
count(gear))
}
for(cyl in sort(unique(mtcars$cyl))) foo(cyl)
ls()
#> [1] "cyl" "foo"
#> [3] "gears_cars_cylinders_4" "gears_cars_cylinders_6"
#> [5] "gears_cars_cylinders_8"
gears_cars_cylinders_4
#> Source: local data frame [3 x 2]
#>
#> gear n
#> (dbl) (int)
#> 1 3 1
#> 2 4 8
#> 3 5 2