Recursion in Kotlin

Kotlin supports recursive functions. Here’s a classic example.

import kotlin.math.pow

// This `fact` function calls itself until it reaches the
// base case of `fact(0)`.
fun fact(n: Int): Int {
    return if (n == 0) {
        1
    } else {
        n * fact(n - 1)
    }
}

fun main() {
    println(fact(7))

    // In Kotlin, we can define local functions which can also be recursive
    fun fib(n: Int): Int {
        return if (n < 2) {
            n
        } else {
            fib(n - 1) + fib(n - 2)
        }
    }

    println(fib(7))
}

This program demonstrates two recursive functions:

1. The fact function, which calculates the factorial of a number.
2. The fib function, which calculates the nth Fibonacci number.

Both functions use recursion, calling themselves with smaller inputs until they reach a base case.

In Kotlin, we can define local functions within other functions. This is demonstrated with the fib function inside main. Unlike in some other languages, we don’t need to declare the function before defining it - Kotlin allows us to use the function immediately after its definition.

When you run this program, it will output:

5040
13

The first number (5040) is the factorial of 7, and the second number (13) is the 7th Fibonacci number.

Recursion can be a powerful tool, but it’s important to use it judiciously. For large inputs, recursive solutions can lead to stack overflow errors. In such cases, iterative solutions or tail-recursive functions (which Kotlin can optimize) might be more appropriate.

Next, we’ll explore more Kotlin concepts and features.