Question

I have been tasked with creating a program that will generate an amortization schedule. I have only done a bit of research so far, but I need to calculate out payments, interest per payment and principal per payment. Can any one point me in the right direction to figure this out? While I will be writing this in RPG, I am sure others could make use of this algorithm in the future.

(Update) Okay, so how do I calculate based on a 365 day year?

Answer 1

Here is what I ended up creating. I have posted the whole test program. It is written in RPG, but should be easily be figured out for any other languages.

 H ActGrp(*caller) BndDir('MODULES') DftActGrp(*no)    //*********************************************************************    // Program . . . . . AMORT    //*********************************************************************    // Printer/Display Files  FAMORTDF   CF   E             WORKSTN sfile(SFL01:rrn01)    //*********************************************************************    // Named Constants   /copy modules/qcopysrc,statuscopy     // Named Indicators  D indicatorPtr    S               *   Inz(%Addr(*IN))  D                 DS                  Based(IndicatorPtr)   /copy modules/qcopysrc,scrncopy     // Subfile  Fields  D rrn01           S              4P 0 inz(0)     //*********************************************************************    // Misc Fields  D* Monthly Payment  D m               S             12P 2          D* Principal  D p               S             12P 2                                        D* Interest  D i               S              5P 3                                        D* Length (in Years)  D l               S              3P 0                                        D* Monthly Interest  D j               S             10P10                                        D* # of Months  D n               S              5P 0                                        D* Current Monthly Int.  D h               S             12P 2                                        D* Current Principal  D c               S             12P 2                                        D* New Balance  D q               S             12P 2                                           //*********************************************************************    // External Program Procedures     // Internal Subprocedures  D Init            PR  D Main            PR  D SubfileFilled   PR              N  D ClearScreen     PR  D IsValidData     PR              N  D LoanPayment     PR            12P 2  D  principal                    12P 2  D  interest                      5P 3  D  loanPeriod                    3P 0  D  paymentsYear                  3P 0     // External Subprocedures    ///copy modules/qsrvsrc,p.string     //*********************************************************************    // Entry Parms  D AMORT           PR                  extpgm('AMORT')  D AMORT           PI    //*********************************************************************   /free    Init();    Main();     *inlr = *on;   /end-free   P*--------------------------------------------------  P* Procedure name: Init  P* Purpose:  P* Returns:  P*--------------------------------------------------  P Init            B  D Init            PI    /free     pgm = 'AMORT';    sflDsp = *off;     return;    /end-free  P Init            E    P*--------------------------------------------------  P* Procedure name: Main  P* Purpose:  P* Returns:  P*--------------------------------------------------  P Main            B  D Main            PI    /free     dow (not F3) and (not F12);       write OVR01;      exfmt CTL01;      ClearScreen();       if (IsValidData()) and (not F3) and (not F12);         // Fill the header information        dPayment = LoanPayment(dLoanAmt:dIntRate:dLoanPrd:dPayYear);        dNumPaymnt = dLoanPrd * dPayYear;        m = dPayment + dExtraPay;        p = dLoanAmt;        q = p;         // Fill the table        if (SubfileFilled());          sflDsp = *on;        endif;      endif;     enddo;     return;    /end-free  P Main            E    P*--------------------------------------------------  P* Procedure name: SubfileFilled  P* Purpose: Fill the subfile  P* Returns:  P*--------------------------------------------------  P SubfileFilled   B  D SubfileFilled   PI              N   D isFilled        S               N  D x               S              4P 0  D intCume         S             12P 2  D extraPayCume    S             12P 2  D payDate         S               D  D payment         S             12P 2  D extraPayment    S             12P 2    /free     isFilled = *on;     sflClear = *on;    write CTL01;    sflClear = *off;    rrn01 = 0;    x = 0;     // Setup the work fields    payment = dPayment;    extraPayment = dExtraPay;    payDate = dStartDate;     // Create records until there is a zero balance    dow (q > 0);       x += 1;      eval(h) h = p * j; // Monthly Interest       // Adjust for final payment      if (p < m);        m = p + h;        payment = p;        extraPayment = h;      endif;       // Calulate Principal      c = m - h;      // Calulate the new balance      q = p - c;       // Accumulate the interest and extra payments      intCume += h;      extraPayCume += extraPayment;       // Determine the next pay date      select;        when dTerms = '1'; //Yearly          payDate += %years(1);        when dTerms = '2'; //Semi-Annual          payDate += %months(6);        when dTerms = '3'; //Quarterly          payDate += %months(3);        when dTerms = '4'; //Monthly          payDate += %months(1);        when dTerms = '5'; //Bi-Weekly          payDate += %days(14);      endsl;       // Fill the subfile      sPayNum = x;      sPayDate = payDate;      sBegBal = p;      sSchedPay = payment;      sExtraPay = extraPayment;      sTotPay = m;      sInterest = h;      sPrincipal = c;      sEndBal = q;      sCumeInt = intCume;       // Move the End balance to the beginning balance      p = q;       rrn01 += 1;      write SFL01;     enddo;     // Return the calculated information to the header    dActPaymnt = x;    dTotInt = intCume;    dTotEPay = extraPayCume;     if (rrn01 < 1);      isFilled = *off;    endif;     return isFilled;    /end-free  P SubfileFilled   E    P*--------------------------------------------------  P* Procedure name: ClearScreen  P* Purpose:  P* Returns:  P*--------------------------------------------------  P ClearScreen     B  D ClearScreen     PI    /free     c = 0;    h = 0;    i = 0;    j = 0;    l = 0;    m = 0;    n = 0;    p = 0;    q = 0;    dPayment = 0;    dNumPaymnt = 0;    dActPaymnt = 0;    dTotEPay = 0;    dTotInt = 0;     return;    /end-free  P ClearScreen     E    P*--------------------------------------------------  P* Procedure name: IsValidData  P* Purpose: Validate the data on the screen  P* Returns: True or False  P*--------------------------------------------------  P IsValidData     B  D IsValidData     PI              N   D isValid         S               N    /free     if (dLoanAmt <> 0) and (dIntRate <> 0) and (dLoanPrd <> 0) and       (dPayYear <> 0) and (dStartDate <> %date('0001-01-01'));      isValid = *on;    else;      isValid = *off;    endif;     return isValid;    /end-free  P IsValidData     E    P*--------------------------------------------------  P* Procedure name: LoanPayment  P* Purpose: Calculates the payment  P* Returns:  P*--------------------------------------------------  P LoanPayment     B  D LoanPayment     PI            12P 2  D  principal                    12P 2  D  interest                      5P 3  D  loanPeriod                    3P 0  D  paymentsYear                  3P 0   D retMonthlyPayment...  D                 S             12P 2    /free     eval(h) n = loanPeriod * paymentsYear;    eval(h) j = interest / (paymentsYear * 100);     eval(h) m = principal * (j / (1 - (1 + j) ** -n));     return m;    /end-free  P LoanPayment     E 

This code has been tested against our loan calculator. If test vs. Excel, it was off about 32 cents on a $200,000 loan. I am sure that is do to rounding issues.