carpayment.cpp

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// Tutorial 20: CarPayment.cpp
// Calculate different billing plans for a car loan.
#include <iostream> // required to perform C++ stream I/O
#include <iomanip> // required for parameterized stream manipulators
#include <cmath> // required to use the C++ math library functions
#include <string> // required to access string functions
#include <exception> // required to use exceptions

using namespace std; // for accessing C++ Standard Library members

// function prototypes
double calculateMonthlyPayment( double, int, int );
void processData( int, int, double );

// function main begins program execution
int main()
{
   // define variables
   int price; // price of the car
   int downPayment; // the down payment
   double interest; // the annual interest rate
   bool errorOccurred = false; // true if an input error occurred

   // temporarily store one line of invalid input
   string invalidInput;
   
   // do while cin has not successfully read the car price
   do
   {
      // try to retrieve the car price
      try 
      {
         // cause cin to throw exceptions when errors occur
         cin.exceptions( ios_base::failbit );

         // prompt user for and input the car price 
         cout << "\nEnter the car price: ";
         cin >> price; // get the car price

         errorOccurred = false; // value obtained successfully

      } // end try
      catch ( ios_base::failure &inputError )
      {
         cin.clear(); // clear fail flag

         // remove all characters from cin
         getline( cin, invalidInput );
            
         errorOccurred = true; // indicate that an error occurred
               
         cout << "\nError: Please enter an integer for the "
              << "car price." << endl;
      } // end catch
   
   }
   while ( errorOccurred == true );  // end do...while

   // prompt user for and input the down payment
   // and the annual interest rate (no exception handling)
   cout << "Enter the down payment: ";
   cin >> downPayment; // get the down payment

   cout << "Enter the annual interest rate: ";
   cin >> interest; // get the annual interest rate

   // create table of options over different periods of time
   processData( price, downPayment, interest); 

   return 0; // indicate that program ended successfully

} // end function main

// function calculates payments for each time interval
void processData( int price, int downPayment, double interest )
{
   // calculate loan amount and monthly interest
   int loanAmount = price - downPayment;
   double monthlyInterest = interest / 1200;
   
   // set precision for output values
   cout << fixed << setprecision( 2 );

   // add header to output
   cout << endl << left << setw( 10 ) << "Months" 
        << "Monthly Payments" << endl;

   // calculate and display payments
   for ( int years = 2; years <= 5; years++ )
   {
      // convert payment period to months
      int months = 12 * years; 
      
      // get monthlyPayment         
      double monthlyPayment = calculateMonthlyPayment( 
         monthlyInterest, months, loanAmount );

      // insert result into output stream
      cout << left << setw( 10 ) << months 
           << "$" << monthlyPayment << endl;

   } // end for

} // end function processData

// calculate monthlyPayment
double calculateMonthlyPayment( double monthlyInterest,
   int months, int loanAmount )
{
   double base = pow( 1 + monthlyInterest, months );

   return loanAmount * monthlyInterest / ( 1 - ( 1 / base ) );

} // end function calculateMonthlyPayment

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