Create Loop for Amortization Schedule in Swift - ios

Question

I'm looking to figure out a simple loop in order to calculate an amortization schedule in Swift.
So far, here is my setup on Playground:
let loanAmount: Double = 250000.00
let intRate: Double = 4.0
let years: Double = 30.0
var r: Double = intRate / 1200
var n: Double = years * 12
var rPower: Double = pow(1 + r, n)
var monthlyPayment: Double = loanAmount * r * rPower / (rPower - 1)
var annualPayment: Double = monthlyPayment * 12
For the actual loop, I'm unsure how to fix the code below.
for i in 0...360 {
var interestPayment: Double = loanAmount * r
var principalPayment: Double = monthlyPayment - interestPayment
var balance: Double; -= principalPayment
}
Looking to generate a monthly schedule. Thanks in advance for any tip.

Answer 1

I'm guessing you mean to declare the balance variable outside the loop, and to decrement it inside the loop:
// stylistically, in Swift it's usual to leave
// off the types like Double unless you have a
// reason to be explicit
let loanAmount = 250_000.00
let intRate = 4.0
let years = 30.0
// since these are one-off calculations, you
// should use let for them, too. let doesn't
// just have to be for constant numbers, it just
// means the number can't change once calculated.
let r = intRate / 1200
let n = years * 12
let rPower = pow(1 + r, n)
// like above, these aren't changing. always prefer let
// over var unless you really need to vary the value
let monthlyPayment = loanAmount * r * rPower / (rPower - 1)
let annualPayment = monthlyPayment * 12
// this is the only variable you intend to "vary"
// so does need to be a var
var balance = loanAmount
// start counting from 1 not 0 if you want to use an open
// (i.e. including 360) range, or you'll perform 361 calculations:
for i in 1...360 {
// you probably want to calculate interest
// from balance rather than initial principal
let interestPayment = balance * r
let principalPayment = monthlyPayment - interestPayment
balance -= principalPayment
println(balance)
}
This should print out the correct balances going down to zero for the final balance (well actually 9.73727765085641e-09 – but that's a whole other question).
If you wanted to create a monthly balance, say in an array, you could add an additional array variable to store that in:
var balance = loanAmount
//array of monthly balances, with the initial loan amount to start with:
var monthlyBalances = [balance]
for i in 1...360 {
let interestPayment = balance * r
let principalPayment = monthlyPayment - interestPayment
balance -= principalPayment
monthlyBalances.append(balance)
}
Advanced version for anyone who's interested
You might wonder if there's a way to declare monthlyBalances with let rather than var. And there is! You could use reduce:
let monthlyBalances = reduce(1...360, [loanAmount]) {
payments, _ in
let balance = payments.last!
let interestPayment = balance * r
let principalPayment = monthlyPayment - interestPayment
return payments + [balance - principalPayment]
}
However this is a bit nasty for a couple of reasons. It would much much nicer if the Swift standard library had a slightly different version of reduce called accumulate that generated an array out of a running total, like this:
let monthlyBalances = accumulate(1...360, loanAmount) {
balance, _ in
let interestPayment = balance * r
let principalPayment = monthlyPayment - interestPayment
return balance - principalPayment
}
And here's a definition of accumulate:
func accumulate<S: SequenceType, U>
(source: S, var initial: U, combine: (U, S.Generator.Element) -> U)
-> [U] {
var result: [U] = []
result.append(initial)
for x in source {
initial = combine(initial, x)
result.append(initial)
}
return result
}