Return a dataframe from a function and store it in

Question

This is my first week working with R and there is one thing about function I cannot seems to manage.

df <- data.frame(a = c(1:10),
             b = c("a", "a", "b", "c", "c", "b", "a", "c", "c", "b"))

testF = function(select) {
dum = subset(df, b == select)
}

lapply(unique(df$b), testF)

This function now just prints the the data sets on screen. But I would like to store the results as separate data frames in my workspace. In this example this would give three data frames; a, b and c.

Thank for the help.

Answer 1

A solution using the list2env function described in the comment above, assuming you wish to use the subset method regardless of potential issues inside a function.

df <- data.frame(a = c(1:10),
             b = c("a", "a", "b", "c", "c", "b", "a", "c", "c", "b"))

testF = function(select) {
    dum = subset(df, b == select)
    dum                                # you need to return the data frame resulting from the subset back out of the function
}

my.list = lapply(unique(df$b), testF)
names(my.list) = unique(df$b)          # set the names of the list elements to the subsets they represent (a,b,c)
list2env(my.list,envir = .GlobalEnv)   # copy the data frames from the list to the Global Environment

If you had a simple example like the one you portray you could access the elements of the list one-by-one as follows and assign each to a variable.

a = my.list[[1]]
b = my.list[[2]]
c = my.list[[3]]

Finally, you could define the function inline such and make use of the (awesome) data.table package, thereby avoiding the use of subset:

library(data.table)
dt <- data.table(a = c(1:10),
             b = c("a", "a", "b", "c", "c", "b", "a", "c", "c", "b"))   
my.list = lapply(unique(dt$b), function(select) { dt[b == eval(select)]})

Hope this helps.

Answer 2

Your function needs a return value. See help("function") for details.

However, for your specific case you can simply use split:

split(df, df$b)

$a
  a b
1 1 a
2 2 a
7 7 a

$b
    a b
3   3 b
6   6 b
10 10 b

$c
  a b
4 4 c
5 5 c
8 8 c
9 9 c

Answer 3

Roland has the correct solution for the specific problem: more than a split() is not needed. Just to make sure: split() returns a list. To get separate data frames in you workspace, you do:

list2env(split(df,df$b),.GlobalEnv)

Or, using assign:

tmp <- split(df,df$b)
for(i in names(tmp)) assign(i,tmp[[i]])

---

A word on subset

This said, some more detail on why your function is plain wrong. First of all, in ?subset you read:

Warning

This is a convenience function intended for use interactively. For programming it is better to use the standard subsetting functions like [, and in particular the non-standard evaluation of argument subset can have unanticipated consequences.

Translates to: Never ever in your life use subset() within a function again.

---

A word on returning values from a function

Next to that, a function always returns a result:

• if a return() statement is used, it returns whatever is given as an argument to return().
• otherwise it returns the result of the last line.

In your case, the last line contains an assignment. Now an assignment also returns a value, but you don't see it. It's returned invisibly. You can see it by wrapping it in parentheses, for example:

> x <- 10
> (x <- 20)
[1] 20

This is absolutely unnecessary. It's the reason why your function works when used in lapply() (lapply catches invisible output), but won't give you any (visible) output when used at the command line. You can capture it though :

> testF("b")
> x <- testF("b")
> x
    a b
3   3 b
6   6 b
10 10 b

The assignment in your function doesn't make sense: either you return dum explicitly, or you just drop the assignment alltogether

---

Correcting your function

So, given this is just an example and the real problem wouldn't be solved by simply using split() your function would be :

testF <- function(select) {
    dum <- df[df$b=select,]
    return(dum)
}

or simply:

testF <- function(select){
    df[df$b=select,]
}