fib.swift

func fib(idx:Int) {
    let result = (0...idx).reduce([Int]()) {(var t, e) in
        t.append(t.count < 2 ? e : t[t.count-2] + t[t.count-1])
        return t
    }.last
    print(result ?? "no value found")
}

_ = Process.arguments
    .map{Int($0)}.flatMap{$0}.filter{$0>=0}.map(fib)

/// usage:
/// $ swift fib.swift 1 2 3 4 5 6 7 8 9 10
/// 1
/// 1
/// 2
/// 3
/// 5
/// 8
/// 13
/// 21
/// 34
/// 55

finding fibonacci number by index