Returning from a Function

In this lesson, we will learn how to return a value from a function in R.

We'll cover the following

The return() Function

Let’s take an example: finding the maximum number between two numbers.

maxNumber <- function(myNumber1, myNumber2) 
{
  if(myNumber1 > myNumber2)
  {
    print(myNumber1)
  } else
  {
    print(myNumber2)
  }
}

Here, we are printing the maximum number. Now, suppose instead of printing the maximum number our program needs to perform some task with the maximum number. Therefore, instead of printing, we want the function maxNumber to give us the number, i.e., return the maximum number so that we can use it.

This is where the function return() comes in handy.

Syntax of return() Function

return(expression)

The value returned from a function can be any valid object.

We can simply return the maximum number and the program calling the function can receive it.

Press + to interact
maxNumber <- function(myNumber1, myNumber2) 
{
  if(myNumber1 > myNumber2)
  {
    return(myNumber1)
  } else
  {
    return(myNumber2)
  }
}
# Driver Code
m <- 2
n <- 5
max <- maxNumber(m, n) # calling the maxNumber() function and 
                      # getting the returned value in the variable max
# Now we can perform any task with this value for example
cat("The maximum value between ", m, " and ", n, " is: ", max)

In the above code snippet, the function maxNumber() uses return() to return the larger of the two numbers. The calling code, or driver code, receives this value and stores it in the max variable.

Returning a Value Without the return() Function

However, the interesting thing about the R language is; we need not specifically call the return() function. If there are no explicit returns from a function, the value of the last evaluated expression is returned automatically in R language.

Let’s try this out:

Press + to interact
maxNumber <- function(myNumber1, myNumber2) 
{
  if(myNumber1 > myNumber2)
  {
    myNumber1 # No return function; but this is the last expression
              # if conditional statement is satisfied
  } else
  {
    myNumber2 # No return function; but this is the last expression
              # if conditional statement is not satisfied
  }
}
# Driver Code
m <- 2
n <- 5
max <- maxNumber(m, n) # calling the maxNumber() function and 
                      # getting the returned value in the variable max
cat("The maximum value between ", m, " and ", n, " is: ", max)

In the above code snippet, Line 5 is the last expression to be evaluated if the conditional statement myNumber1 > myNumber2 is satisfied. Therefore, this expression myNumber1 is simply returned. Similarly, Line 9 is the last line to be executed if the conditional statement myNumber1 > myNumber2 is NOT satisfied. Therefore, this expression myNumber2 is simply returned.

Processing Lists and Vectors

Let’s begin by modifying our example: Instead of two arguments, our function will now take 11 argument which is a vector containing nn numbers and finds the maximum number amongst all of them.

Now, since we have a vector, and we are going to iterate over each of its elements, we can use a loop.

Press + to interact
maxNumber <- function(myVector) 
{
  max <- -Inf # Here -Inf is a reserved word for negative infinity (the minimum number possible)
            # we need to set this to perform the comparison with the first element
  
  # looping over the entire vector
  for(v in myVector)
  {
    if(max < v) # compared the current element with max variable
    {
      max = v # if a local maximum is found
               # i.e current value is maximum uptil now, update the max variable
    }
  }
  
  max # returning the max variable. This is the last expression being evlauted in the function.
}
# Driver Code
inputs <- c(2, 5, 4, 10)
output <- maxNumber(inputs) # calling the maxNumber() function and 
                      # getting the returned value in the variable max
print(paste("The maximum value in the vector is: ", output), quote = FALSE)

In the above code, we used a reserved word:

-Inf

Reserved words are the set of words that have special meaning and purpose. These keywords cannot be used as identifiers.

Here, -Inf is used so that initially the variable max has the least value possible. Now, when the loop from Line 7 will start, the conditional statement max < v will evaluate to TRUE for the first element in any vector, unless, of course, if the first element is -Inf itself. This value will be stored in max.

From the second iteration onwards, the max variable is compared with the current element v. If the conditional statement max < v is satisfied, variable max is updated, otherwise, there is no change to max.

Returning Multiple Values in R

How about we introduce another modification. Now, we want our function to return the maximum as well as the minimum value.

For this, we can store both the maximum and minimum value in a list and then return it. Let’s see how this is implemented:

Press + to interact
maxMinNumber <- function(myVector) 
{
  max <- -Inf # initially the max number is set to -Inf
            # we need to set this to perform the comparison with the first element
  
  min <- Inf # Here Inf is a reserved word for infinity (the maximum number possible)
              # we need to set this to perform the comparison with the first element
  # looping over the entire vector
  for(v in myVector)
  {
    if(max < v) # check the current element with the max variable
    {
      max = v # if a local maximum is found
               # i.e current value is maximum uptil now, update the max variable
    }
    if(min > v) # check the current element with the min variable
    {
      min = v # if a local minimum is found
               # i.e current value is minimum uptil now, update the min variable
    }
  }
  
  list(max, min) # returning both the max and min in the form of a list. This is the last expression being evlauted in the function.
}
# Driver Code
input <- c(2, 5, 4, 10)
output <- maxMinNumber(input) # calling the maxMinNumber() function and 
    
# Here the variable output is a list where first element is the maximum value 
# and the second element is the minimum value
print(paste("The maximum value in the vector is: ", output[1]), quote = FALSE)
print(paste("The minimum value in the vector is: ", output[2]), quote = FALSE)

In this code on Line 24 we return a list and therefore when we receive the output, we have to treat it like a list.

The variable output in this code snippet is a list with the first element the maximum value and the second element the minimum value.