Returning Multiple Objects in an R Function

Returning multiple objects in an R function

Unlike many other languages, R functions don't return multiple objects in the strict sense. The most general way to handle this is to return a list object. So if you have an integer foo and a vector of strings bar in your function, you could create a list that combines these items:

foo <- 12
bar <- c("a", "b", "e")
newList <- list("integer" = foo, "names" = bar)

Then return this list.

After calling your function, you can then access each of these with newList$integer or newList$names.

Other object types might work better for various purposes, but the list object is a good way to get started.

Return multiple values from a function

In R you can only return one object. You could put the three values in a list or a vector and then return that list or vector.

How to assign from a function which returns more than one value?

(1) list[...]<- I had posted this over a decade ago on r-help. Since then it has been added to the gsubfn package. It does not require a special operator but does require that the left hand side be written using list[...] like this:

library(gsubfn)  # need 0.7-0 or later
list[a, b] <- functionReturningTwoValues()

If you only need the first or second component these all work too:

list[a] <- functionReturningTwoValues()
list[a, ] <- functionReturningTwoValues()
list[, b] <- functionReturningTwoValues()

(Of course, if you only needed one value then functionReturningTwoValues()[[1]] or functionReturningTwoValues()[[2]] would be sufficient.)

See the cited r-help thread for more examples.

(2) with If the intent is merely to combine the multiple values subsequently and the return values are named then a simple alternative is to use with :

myfun <- function() list(a = 1, b = 2)

list[a, b] <- myfun()
a + b

# same
with(myfun(), a + b)

(3) attach Another alternative is attach:

attach(myfun())
a + b

ADDED: with and attach

How to return multiple objects from R script executed in Python

Q.1: How do I pull the various elements from the result?

Ans.1: After you run your R script:

test = ro.r(your_R_script)

You can use this code to print out all the names and values in the test object.

# iterate on names and values
# be careful output is v long
for n,v in test.items():
    print(n)
    print(v)

To list all available names, run this code:

test.names

The output:

StrVector with 12 elements.
'coeffici... 'residuals' 'effects' 'rank' ... 'xlevels' 'call'  'terms' 'model'

To print values of the 'residuals', run this:

test[test.names.index('residuals')]

Q.2: What happens to df?

Ans.2: It is still available in R environment until you delete it. You can run simple R code to check:

ro.r('''
        # View dataframe
        df
        ''')

Q.3: Is there a much much better way to do this?

Ans.3: (No answer.)

How to return multiple values from a function in R for each iteration of a loop? Simple life history simulation

You could return a dataframe from getMC function. Also instead of relying on vectors in global environment (walkpop, cathpop etc) pass them into the function.

getMC <- function(data, ref_data){
   walkpop = ref_data$walkpop*data$walksurv
   cathpop = ref_data$cathpop*data$cathsurv
   abovepop = ref_data$abovepop*data$abovesurv
   walkpop_pre_trans = walkpop
   walkpop = walkpop - walkpop_pre_trans*data$walktrans_out_C - 
                       walkpop_pre_trans*data$walktrans_out_A
   return(data.frame(walkpop, cathpop, abovepop))
}


walkpop = 10000
abovepop = 1000
cathpop = 100
ref_data <- data.frame(walkpop, abovepop, cathpop)

totallist = vector('list', 2)
for (i in 1:2) {
  ref_data <- getMC(data, ref_data)
  totallist[[i]] <- ref_data
}

Checking the output.

lapply(totallist, head)
#[[1]]
#   walkpop cathpop abovepop
#1 8905.669      80 833.4328
#2 9327.844      80 833.4328
#3 8905.669      80 970.8581
#4 9327.844      80 970.8581
#5 8696.702      80 833.4328
#6 9108.971      80 833.4328

#[[2]]
#   walkpop cathpop abovepop
#1 7931.094      64 694.6102
#2 8700.868      64 694.6102
#3 7931.094      64 942.5655
#4 8700.868      64 942.5655
#5 7563.262      64 694.6102
#6 8297.335      64 694.6102

R return multiple data.tables from function

return(list(train = data.table(train), test =data.table(test)))
train=tt_list$train

here:Returning multiple objects in an R function
and here:
How to assign from a function which returns more than one value?

How to handle multiple outputs in an R function?

You cannot return multiple times in R function. After encountering the first return it doesn't go any further.

Return lists instead.

mycustomfunction <- function (input1,input2,input3) {
  output1 = input1 + input2
  output2 = input3
  
  return(list(output1 = output1, output2 = output2))
}

result <- mycustomfunction(10,35,3)

result
#$output1
#[1] 45

#$output2
#[1] 3

You can access individual values using $ operator.

result$output1
#[1] 45

result$output2
#[1] 3

Reuse multiple objects created in a function

I'd use a simple return() with a list() or c():

test1 <- function(x){
  y <- x + 1
  z <- x * 2 # if you don't use return these two won't leave test1

  return(list(y=y, z=z)) # equivalent without return() is list(y=y, z=z)
#or: list(y=y, z=z)
}

test2 <- function(...){
  ret1 <- test1(x)
  print(ret1$y * ret1$z)
 #or: return(ret1$y * ret1$z)
}

Note that the use of return() is not necessary, since the last object not assigned in a function is returned automatically.

The use of return() may help with readability though.

A read about return().

How to return list of multiple objects from a function in R?

After some trial and error I found a simple solution - I'll post here in case there is interest:

f <- (function(a,b) 
    {
      trainy <- c(log(a$y))
      trainx <- data.matrix(a)
      testx <- data.matrix(b)
      list_i <<- list(trainy, testx)
    })

fit <- mapply(f,train_subset,test_subset)

where the object fit is a 2x3 matrix containing the trainy and testx output.

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