Nested Functions

We are going to learn all about nested functions in this lesson.

We'll cover the following

What are Nested Functions?

A function is a set of statements that are executed together to achieve a specific goal or task.

In such a situation, we might have to use another function in the current function. This is called nested functions. We will be learning about user-defined functions being used in another user-defined function.

Syntax of a Nested Function

functionName1 <- function(argument1, argument2, ..., argumentN) 
{
  # Statements
}

functionName2 <- function(argument1, argument2, ..., argumentN) 
{
  # Statements
  functionName1(argument1, argument2, ..., argumentN)
}

Let’s take an example of finding the maximum and minimum number in a vector using multiple functions.

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maxNumber <- function(myVector) 
{
  max <- -Inf 
  for(v in myVector)
  {
    if(max < v) 
    {
      max = v
    }
  }
  max 
}
minNumber <- function(myVector) 
{
  min <- Inf
  
  for(v in myVector)
  {
    if(min > v) 
    {
      min = v
    }
  }
  min
}
  
maxMinNumber <- function(myVector)
{    
  max <- maxNumber(myVector) # calling the maxNumber() function 
  min <- minNumber(myVector) # calling the minNumber() function 
    
  print(paste("The maximum value in the vector is: ", max), quote = FALSE)
  print(paste("The minimum value in the vector is: ", min), quote = FALSE)
}
# Driver Code
input <- c(2, 5, 4, 10)
maxMinNumber(input) # calling the maxMinNumber() function

In this code, we have split the task of finding the maximum number in maxNumber() function and the task of finding the minimum number in minNumber() function.

Later, we call both these functions in another function called minMaxNumber().

Defining a Function in Another Function

We can also create one function in another function and use it in the same function. How cool is that!

Syntax for defining one function in another

functionName1 <- function(argument1, argument2, ..., argumentN) 
{
  # Creating a Function Inside a Function
  functionName2 <- function(argument1, argument2, ..., argumentN) 
  {
    # Statements
  }

  # Using the created function
  functionName2(argument1, argument2, ..., argumentN)
}

Let’s explore how we can implement this in R:

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driverCode <- function()
{
  input <- c(2, 5, 4, 10)
  
  # Creating the maxNumber function inside the driverCode function
  maxNumber <- function(myVector) 
  {
    max <- -1 
    for(v in myVector)
    {
      if(max < v) 
      {
        max = v
      }
    }
    max 
  }
  
  # Creating the minNumber function inside the driverCode function
  minNumber <- function(myVector) 
  {
    min <- Inf
    for(v in myVector)
    {
      if(min > v) 
      {
        min = v
      }
    }
    min
  }
  
  # Creating the maxMinNumber function inside the driverCode function
  maxMinNumber <- function(myVector)
  {    
    max <- maxNumber(myVector) # calling the maxNumber() function 
    min <- minNumber(myVector) # calling the minNumber() function 
    
    print(paste("The maximum value in the vector is: ", max), quote = FALSE)
    print(paste("The minimum value in the vector is: ", min), quote = FALSE)
  }
  
  # calling the maxMinNumber function from inside the driverCode function
  maxMinNumber(input) # calling the maxMinNumber() function
}
driverCode() # Calling the driverCode function. 
     # This is where actual execution of code starts

As you can see from the code above, the functions maxNumber(), minNumber() and maxMinNumber() are all created in the function driverCode().

We also call the function maxMinNumber() from the driverCode() function which later goes on to call maxNumber() function and minNumber() function.

Let’s have a look at an illustration:

Nested functions
Nested functions

In the above illustration, the dotted line shows the function, whereas the solid lines indicate function calls. For example, maxNumber() and minNumber() function are called from inside maxMinNumber() function.

Also, the drivercode() function is called from outside all these functions. In the driverCode() function, all the other functions are called and created.