Using Dplyr In A Function To Create New Dataframes

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?

jennybryan · Accepted Answer · 2015-11-20 06:32:37Z

5

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

answered Nov 20, 2015 at 6:32

jennybryan

2,6562 gold badges21 silver badges33 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

thelatemail Over a year ago

I can't help but feel this goes against everything I've been taught in R, in terms of grouping similar data in structures like lists. And if the point of dplyr is to simplify things, mashing it together with assign and environment manipulation seems like overkill. gears <- by(mtcars, mtcars$cyl, FUN=function(x) data.frame(table(x$gear)) ) and then accessing like gears[["4"]] seems so much less error-prone.

jennybryan Over a year ago

Yes that would normally be my thinking too. But maybe there are circumstances where you actually need these data frames as separate objects?

thelatemail Over a year ago

I honestly can't think of an occasion when that would be necessary. If you have the data.frames of cyl4 cyl6 cyl18 floating about in the GlobalEnv then you need to loop over a pasted together vector of "cyl" and i in c(4,6,18) and use get() to retrieve them, when you could just do gears[[i]]

Collectives™ on Stack Overflow

Using Dplyr In A Function To Create New Dataframes

I'm Trying to create new dataframes from dplyr 0.4.3 functions using R 3.2.2.

1 Answer 1

3 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

I'm Trying to create new dataframes from dplyr 0.4.3 functions using R 3.2.2.

1 Answer 1

3 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related