/*
** amortize.c    Demonstration C program to calculate loan payments
**               on standard fixed rate loans
**
** 6-16-07, http://willus.com
**
*/
#include <stdio.h>
#include <stdlib.h>
#include <math.h>

int main(void)

    {
    char buf[64];
    double annual_rate_percent;
    double years;
    double months;
    double loan_amount;
    double principal;
    double monthly_rate_percent;
    double monthly_rate;
    double monthly_increase;
    double monthly_payment;
    int i,imod;

    printf("Fixed mortgage payment calculator from http://willus.com.\n\n");
    printf("Enter annual rate (%%, def=7.00):  ");
    fgets(buf,30,stdin);
    if (buf[0]=='\n')
        annual_rate_percent = 7.00;
    else
        annual_rate_percent = atof(buf);
    printf("Enter loan amount ($, def=150000):  ");
    fgets(buf,30,stdin);
    if (buf[0]=='\n')
        loan_amount = 150000.;
    else
        loan_amount = atof(buf);
    printf("Enter number of years (def=30):  ");
    fgets(buf,30,stdin);
    if (buf[0]=='\n')
        years = 30;
    else
        years = atof(buf);
    months = years * 12.;
    /*
    ** Note that the monthly rate is not, as you might expect,
    **
    ** 100 * ( ( ( 1 + annual_rate_percent / 100. ) ^ ( 1 / 12. ) ) - 1. )
    **
    ** ... which would, when compounded monthly, result in exactly the
    ** annual rate after one year.  Instead, the convention is simply
    ** to use the annual rate divided by 12, which is pretty close to
    ** the same value.  E.g. for an annual rate of 12.00 %, the monthly
    ** rate calculated by the formula above would be about 0.949 %,
    ** whereas the simpler "divide by 12" formula yields exactly 1.000 %.
    */
    monthly_rate_percent = annual_rate_percent / 12.;
    monthly_rate = monthly_rate_percent / 100.;
    monthly_increase = 1.0 + monthly_rate;

    /* The magic monthly payment formula */
    monthly_payment = loan_amount * monthly_rate * pow(monthly_increase,months)
                       / (pow(monthly_increase,months) - 1.);

    printf("\nAnnual Rate = %.3f %%\n",annual_rate_percent);
    printf("Loan Amount = $%g\n",loan_amount);
    printf("Number of Years = %g\n",years);
    printf("Monthly Payment = $%.2f\n",monthly_payment);
    printf("Payment #      Interest     Remaining Principal\n"
           "-----------------------------------------------\n");
    imod = (int)(months/10.+.1); // How often to print the values

    /* Loop to calculate remaining principal over time */
    principal=loan_amount;
    for (i=1;i<=months;i++)
        {
        double interest;

        interest = principal * monthly_rate;
        principal = principal + interest - monthly_payment;
        if (i==1 || i%imod==0)
            printf(" %4d         %9.2f         %9.2f\n",i,interest,principal);
        }
    printf("\nPress <Enter> to exit.\n");
    fgets(buf,30,stdin);
    return(0);
    }