On 18-02-2013, at 10:34, Prakasit Singkateera <asltjoey.rs...@gmail.com> wrote:
> Hi all, > > Firstly, it is not a homework. I am working for a hotel booking company in > Thailand but I don't want to explain a complex equation and concept here so > I keep it simple and closely related to what I am trying to solve.I apology > if my question is not clear enough. > > I am new to R and previously this problem can be solved easily in Excel > using the "Goal Seek" tool. An example related to my question is when we > use the PMT formula (in Excel) to find the loan payment amount for the > given values of parameters i.e. interest rate, total number of payments, > and principal amount of the loan. > > loan_payment_amt_of_each_period = > PMT(interest_rate,total_number_of_payments,principal_amt) > > The question is when you know exactly on a monthly basis that you can > afford only X amount of money to pay and you want to know how many months > you have to do the payment given your monthly affordable money, the fixed > interest rate, and the principal amount of loan. Using Goal Seek tool in > Excel, it is like a backward solving for X given Y by not having to > transform anything from the original equation. Simply put the > loan_payment_amt_of_each_period you want and let the software calculate the > total_number_of_payments for you. > You can find R versions of the Excel functions here: http://factbased.blogspot.nl/2013/02/some-of-excel-finance-functions-in-r.html The R code is here: http://pastebin.com/q7tyiEmM I do not know if these are a correct translation of what's in Excel or Calc. For you application to find the number of payments, you can use the R function uniroot for solving a single equation with one unknown. Small example # R version of Excel PMT function (as in LibreOffice Calc) pmt <- function(rate, nper, pv, fv=0, type=0) { rr <- 1/(1+rate)^nper res <- (-pv-fv*rr)*rate/(1-rr) return(res/(1+rate*type)) } # here x is the number of payments Rpmt <- function(x, xrate, xpmt, xpv) xpmt - pmt(xrate,x,xpv) irate <- .05 xpmt <- -10 xpv <- 100 # testing pmt(irate,10,xpv) pmt(irate,5,xpv) pmt(irate,20,xpv) Rpmt(xpv/xpmt/2,xrate=irate, xpmt=xpmt, xpv=xpv) Rpmt(xpv/xpmt*2,xrate=irate, xpmt=xpmt, xpv=xpv) # find number of payments z <- uniroot(Rpmt,lower=1,upper=100, xrate=irate, xpmt=xpmt, xpv=xpv) z #number of payments z$root # check Rpmt(z$root,xrate=irate, xpmt=xpmt, xpv=xpv) pmt(irate,z$root,xpv) Should you desire higher accuracy of the solution, use tol=1e-8 in the uniroot() call. Berend ______________________________________________ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
