Kotlin Tail Recursion

In a traditional recursion call, we perform our recursive call first, and then we take the return value of the recursive call and calculate the result. But in tail recursion, we perform the calculation first, and then we execute the recursive call, passing the results of the current step to the next recursive call. In the end, both recursion and tail recursion give the same output. The must-follow rule for tail recursion is that the recursive call should be the last call of the method.

Benefits of using tail recursion

1. In tail recursion, the function call is the last thing executed by the function, and nothing left in the current function to execute. So, there is no need to save the current function call in the stack memory, and the compiler can reuse that stack space for the next recursive call.
2. In tail recursion, we do not get the StackOverflowError during the execution of the program.

Example 1: Find the factorial of a number using tail recursion

// Kotlin program of factorial using tail-recursion
fun Fact(num: Int, x:Int):Long{

    return if(num==1)   // terminate condition
        x.toLong()
    else
        Fact(num-1,x*num)   //tail recursion
}

fun main() {
    var n = 1
    var result = Fact(5,n)
    println("Factorial of 5 is: $result")
}

Output:

Factorial of 5 is: 120

Working of the above programExample 2: Find the sum of elements of an array using tail-recursion 

// two parameters passed an array and size of array
fun sum(args: Array<Int> , index:Int, s : Int = 0 ):Int{
    return if(index<=0) s
    else sum(args ,index-1, s + args[index-1])     // tail-recursion
}

fun main() {
    // array initialization
    val array = arrayOf(1,2,3,4,5,6,7,8,9,10)
    // size of array
    val n = array.size
    val result = sum(array,n)             // normal function call
    println("The sum of array elements is: $result")
}

Output:

The sum of array elements is: 55

Explanation: Here, we have passed the array as an argument along with two other parameters to the sum() function. The default value of the (s) parameter is equal to zero. We are calculating the sum of elements from the last index of the array, with each recursive call. With the last recursive call, we will have the sum of all elements in s and return it when the condition is satisfied.