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MathGroup Archive 2012

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Amortization Schedule--Global Symbol Value Error

  • To: mathgroup at smc.vnet.net
  • Subject: [mg126581] Amortization Schedule--Global Symbol Value Error
  • From: waterhand <sandwaterhand at gmail.com>
  • Date: Sun, 20 May 2012 02:36:13 -0400 (EDT)
  • Delivered-to: l-mathgroup@mail-archive0.wolfram.com

Let me begin by saying that I am very new to Mathematica, so any help is appreciated.

I am trying to make an amortization schedule in a table format.  The code I have is below:


***

(* psv = Present Value, r = interest rate as a decimal, t = time in years, k = number of payments made *)
(* mortgage function calculates the monthly mortgage amount *)
mortgage[psv_, r_, t_] := N[psv*(r/12)/(1 - (1 + r/12)^(-12*t)), 2]

(* amountLeftOwed function calculates the amount left on the loan including interest *)
amountLeftOwed[psv_, r_, t_, k_] :=  N[psv*(1 + r/12)^k - mortgage[psv, r, t]*((1 + r/12)^k - 1)/(r/12), 2]

(* paidInInterest function calculates the amount of the mortgage payment that goes towards the interest *)
paidInInterest[psv_, r_, t_, k_] :=  N[amountLeftOwed[psv, r, t, k]*r/12, 2]

(* paidInPrincipal function calculates the amount of the mortgage payment that goes towards the principal *)
paidInPrincipal[psv_, r_, t_, k_] :=  N[mortgage[psv, r, t] - paidInInterest[psv, r, t, k], 2]

(* remainingBalance function gives the remaining balance after payment has been made *)
remainingBalance[psv_, r_, t_, k_] := N[amountLeftOwed[psv, r, t, k] - mortgage[psv, r, t], 2]

(* j is the number of months.  this converts months to years *)
t = j/12;

Manipulate[
 TableForm[
  Prepend[
   Table[{i, paidInInterest[psv, r, t, i], paidInPrincipal[psv, r, t, i], remainingBalance[psv, r, t, i]}, {i, 1, j}],
     {"Month",     "Interest", "Principal", "Balance"}]], 
  {{psv, 100000, "Loan Amount"}, 100, 200000, 100}, {{r, 0.05, "Interest Rate"}, 0.01, .1, 0.0005},
  {{j, 120, "Number of Months"}, {120, 240, 360}}
]

***

Here's the issues I am having:

1)  The variable j is coming up with the "Global symbols that have no assigned value" error despite it being assigned 120, 240, and 360 in the Manipulate[] function.  When I add a line like "j = 120", the code "works".  The reason for the quotations is explained in number 3 below.
2)  Despite the N[*, 2] function, the numbers are coming out as scientific notation.  Even with the line j = 120, the rounding is not to two places.
3)  The table should end with the ending balance being zero.  When I force the value of j (again, j = 120, 240, or 360), I am getting a negative value for the last two months.  Is there a rounding issue related to #2?

Again, very new to Mathematica.  I appreciate any help.

Thanks for reading,

Mike



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