Calculate "net present value" for economics assignment

calculateNetPresentValue2.swift

//: Playground - noun: a place where people can play

//:累乗をするための関数を定義する
func jyou(n : Double, x:Int) -> Double {
    var ans = 1.0
    for _ in 1...x{
        ans *= n
    }
    return ans
}

//:割引現在価値を求める関数を以下に定義する(コンソル債の時はendYear = nilとする)。StartとEndの間は利子のみが払われている
func calc(startYear : Int , endYear : Int?, price : Double, interest : Double, intRate : Double) -> Double{
    if endYear == nil{
        return(interest/intRate)
    }
    else {
        var X = 0
        var answer = 0.00
        for x in 1..<(endYear!-startYear+2){
            answer += interest / jyou(intRate+1, x: x)
            X = x
        }
        answer += price / jyou(intRate+1, x: X+1) //:返済される年
        return(answer)
    }
}
//:金利と額面を定義しておく
let interestPrice = 400.00
let backPrice = 10400.00

//:以下が解答
//:市場金利2%の場合
print(calc(2017, endYear: 2020, price: backPrice, interest: interestPrice, intRate: 0.02))
print(calc(2017, endYear: 2025, price: backPrice, interest: interestPrice, intRate: 0.02))
print(calc(2017, endYear: nil, price: backPrice, interest: interestPrice, intRate: 0.02))
//:市場金利４%の場合
print(calc(2017, endYear: 2020, price: backPrice, interest: interestPrice, intRate: 0.04))
print(calc(2017, endYear: 2025, price: backPrice, interest: interestPrice, intRate: 0.04))
print(calc(2017, endYear: nil, price: backPrice, interest: interestPrice, intRate: 0.04))
//:市場金利6%の場合
print(calc(2017, endYear: 2020, price: backPrice, interest: interestPrice, intRate: 0.06))
print(calc(2017, endYear: 2025, price: backPrice, interest: interestPrice, intRate: 0.06))
print(calc(2017, endYear: nil, price: backPrice, interest: interestPrice, intRate: 0.06))