other/02_helper.R

In pgstevenson/borrowr: Home loan helper

# roxygen2::roxygenise()
# devtools::document()
# devtools::load_all()

# source: http://biostatmatt.com/archives/895

payment  <- function(loan, apr, months) {
  rate <- 1 + apr / 100 / 12
  loan * rate^months * (rate - 1) / (rate^months - 1)
}

amortize <- function(loan, payment, apr, months) {
  rate <- 1 + apr / 100 / 12
  month <- 0:months
  balance <- loan * rate^month - payment * (rate^month - 1) / (rate - 1)
  complete  <- match(TRUE, balance <= 0)
  balance <- ifelse(month < month[complete], balance, 0)
  principal <- loan - balance
  interest  <- payment * month - principal
  interest  <- ifelse(month < month[complete], interest,
                      interest[complete-1])
  amrt <- list(month = month[-1], balance=balance[-1],
               principal = principal[-1], interest = interest[-1],
               paid = principal[-1] + interest[-1], loan=loan,
               payment=payment, apr=apr)
  class(amrt) <- "amortization"
  return(amrt)

}

summary.amortization <- function(amrt) {
  cat("  loan amount: ", amrt$loan, "\n")
  cat("          APR: ", amrt$apr, "\n")
  cat("      payment: ", amrt$payment, "\n")
  cat("       months: ", match(TRUE, amrt$balance <= 0), "\n")
  cat("interest paid: ", max(amrt$interest), "\n")
  cat("   total paid: ", max(amrt$paid), "\n")
}

plot.amortization <- function(amrt) {
  year <- amrt$month / 12
  plot(year, amrt$balance, type="l", ylab="Dollars ($)",
       main=paste("Amortization ($", format(amrt$loan, big.mark=","),
                  ", ", amrt$apr, "%, $", format(amrt$payment, big.mark=",",
                                                 nsmall=2, digits=2), "/mo)", sep=""), ylim=c(0,max(amrt$paid)),
       xlab="Year")
  lines(year, amrt$interest, col="red", lty=2)
  lines(year, amrt$principal, col="green", lty=2)
  lines(year, amrt$paid, col="blue", lty=2)
  legend(x=1, y=max(amrt$paid),
         legend=c("balance", "interest", "principal", "paid"),
         col=c("black","red","green","blue"), lty=c(1,2,2,2))
}