define a function called what that gives the nth fibonacci number in swift

Here's the solution using recursion:

main.swift
func fibonacci(_ nth: Int) -> Int {
    if nth <= 1 {
        return nth
    } else {
        return fibonacci(nth - 1) + fibonacci(nth - 2)
    }
}

// example usage
fibonacci(6) // returns 8
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This function takes an integer nth as input and returns the corresponding Fibonacci number. It uses recursion to calculate the nth Fibonacci number by summing the previous two (n-1 and n-2) Fibonacci numbers. The base case is when nth <= 1, which returns nth itself since the Fibonacci sequence starts with 0 and 1.