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## Function to Calculate Monthly Mortgage Payments and Amortization Tables ##
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# Author: Thomas Girke
# Last update: Feb 27, 2007
# Utility: Calculates monthly and annual loan or mortgage payments, generates amortization tables and plots the results
# How to run the script:
# source("http://faculty.ucr.edu/~tgirke/Documents/R_BioCond/My_R_Scripts/mortgage.R")
# Definitions:
# P = principal, the initial amount of the loan
# I = annual interest rate
# L = length of the loan in years, or at least the length over which the loan is amortized.
# J = monthly interest in decimal form = I / (12 x 100)
# M = monthly payment; formula: M = P * ( J / (1 - (1 + J) ^ -N))
# N = number of months over which loan is amortized = L x 12
# see also: http://www.jeacle.ie/mortgage/instructions.html
mortgage <- function(P=500000, I=6, L=30, amort=T, plotData=T) {
J <- I/(12 * 100)
N <- 12 * L
M <- P*J/(1-(1+J)^(-N))
monthPay <<- M
cat("\nThe payments for this loan are:\n
Monthly payment: $", M, " (stored in monthPay)\n
Total cost: $", M*N, "\n\n", sep="")
# Calculate Amortization for each Month
if(amort==T) {
Pt <- P # current principal or amount of the loan
currP <- NULL
while(Pt>=0) {
H <- Pt * J # this is the current monthly interest
C <- M - H # this is your monthly payment minus your monthly interest, so it is the amount of principal you pay for that month
Q <- Pt - C # this is the new balance of your principal of your loan
Pt <- Q # sets P equal to Q and goes back to step 1. The loop continues until the value Q (and hence P) goes to zero
currP <- c(currP, Pt)
}
monthP <- c(P, currP[1:(length(currP)-1)])-currP
aDFmonth <<- data.frame(
Amortization=c(P, currP[1:(length(currP)-1)]),
Monthly_Payment=monthP+c((monthPay-monthP)[1:(length(monthP)-1)],0),
Monthly_Principal=monthP,
Monthly_Interest=c((monthPay-monthP)[1:(length(monthP)-1)],0),
Year=sort(rep(1:ceiling(N/12), 12))[1:length(monthP)]
)
aDFyear <- data.frame(
Amortization=tapply(aDFmonth$Amortization, aDFmonth$Year, max),
Annual_Payment=tapply(aDFmonth$Monthly_Payment, aDFmonth$Year, sum),
Annual_Principal=tapply(aDFmonth$Monthly_Principal, aDFmonth$Year, sum),
Annual_Interest=tapply(aDFmonth$Monthly_Interest, aDFmonth$Year, sum),
Year=as.vector(na.omit(unique(aDFmonth$Year)))
)
aDFyear <<- aDFyear
cat("The amortization data for each of the", N, "months are stored in \"aDFmonth\".\n\n")
cat("The amortization data for each of the", L, "years are stored in \"aDFyear\".\n\n")
}
if(plotData==T) {
barplot(t(aDFyear[,c(3,4)]),
col=c("blue", "red"),
main="Annual Interest and Principal Payments",
sub="The data for this plot is stored in aDFyear.",
xlab="Years", ylab="$ Amount",
legend.text=c("Principal", "Interest"),
ylim=c(0, max(aDFyear$Annual_Payment)*1.3))
}
}
cat("The monthly mortgage payments and amortization rates can be calculted with the mortgage() function like this: \n
mortgage(P=500000, I=6, L=30, amort=T, plotData=T)
P = principal (loan amount)
I = annual interest rate
L = length of the loan in years \n")