Steps

1. How to define an Array of type String

Let's start by defining an Array of type String to hold some donut items.

println("Step 1: How to define an Array of type String")
val arrayDonuts: Array[String] = Array("Vanilla Donut", "Strawberry Donut", "Plain Donut", "Glazed Donut")

2. How to define a tail recursive function

Next, let's define a tail recursive function named search() which will have the following input parameters:

• donutName parameter of type String for the donut item to search within the Array
• donuts parameter of type String Array for the donut items Array
• index parameter of type Int for the index within the Array on which to run the search

println("\nStep 2: How to define a tail recursive function")
@annotation.tailrec
def search(donutName: String, donuts: Array[String], index: Int): Option[Boolean] = {
  if(donuts.length == index) {
    None
  } else if(donuts(index) == donutName) {
    Some(true)
  } else {
    val nextIndex = index + 1
    search(donutName, donuts, nextIndex)
  }
}

NOTE:

• The body of the function could have inlined elegantly but we are showing each step verbosely so that it is more clear what the search function is doing.
• If you have never seen or written a tail recursive function, you can review each step of the function by debugging through. Follow the Debugging Tutorial if you need help with debugging in IntelliJ.
• The very first expression if(donuts.length == index) is the exit call for the search function because we have looped through all elements within the Array.
• The second expression else if(donuts(index) == donutName) will exit the search function as we have found the donut item in the Array.
• In the third expression shown below

val nextIndex = index + 1
search(donutName, donuts, nextIndex)

we increment the current index by 1 and call the search function itself (this is the recursive bit) but for the next item in the Array.

• The @annotation.tailrec instructs the compiler to add any optimisations with regards to stack frame management as this function is recursive.

3. How to call a tail recursive function

Calling a tail recursive function is not so different than calling any other function. You simply call the function by its name and pass it the corresponding parameters.

println("\nStep 3: How to call a tail recursive function")
val found = search("Glazed Donut", arrayDonuts, 0)
println(s"Find Glazed Donut = $found")

val notFound = search("Chocolate Donut", arrayDonuts, 0)
println(s"Find Chocolate Donut = $notFound")

You should see the following output when you run your Scala application in IntelliJ:

Step 3: How to call a tail recursive function
Find Glazed Donut = Some(true)
Find Chocolate Donut = None