main.cpp

#include <iostream>
#include <iomanip>
#include <string>
#include <cmath>
using namespace std;

// declaring the functions
double findInt( double intRate, int yearlyPayments );
double loanAmountFunc( double i, double y, double periodicPayment );
double monthlyPaymentFunc( double i, double y, double periodicPayment );


int main()
{
  double salary;
	double intRate;
	int loanLength = -30;
	int yearlyPayments = 12;
	double periodicPayment = 0;

	// input the salary and interest rate
	cout << "Please enter your yearly gross salary: ";
	cin >> salary;
	cout << "Please enter an interest rate: ";
	cin >> intRate;

	// turn interest rate into a decimal
	intRate = intRate * 0.01;

	// find the PeriodicPayment
	periodicPayment = ( ( salary / 12 ) * .3 );
	
	// Calculate "y" 
	double y = -( yearlyPayments * loanLength );

	// Call function to calculate "i" 
	double i = findInt( intRate, yearlyPayments );

	// Call function for monthly payment
	double monthlypayment = monthlyPaymentFunc( i, y, periodicPayment );

	// Call function for loan amount
	double loanAmount = loanAmountFunc( i, y, periodicPayment );

	// two decimal precision
	cout.precision(2);
	cout << fixed << showpoint;

	// Output to Terminal
	cout << "The target (30% of monthly salary) monthly payment range is: $" << ( periodicPayment - 10 )
		<< " - \n$" << periodicPayment << endl;
	cout << "Your max target house price is: $" << loanAmount << endl;
	cout << "with a monthly payment of: $" << monthlypayment << endl;

	system( "pause" );
}

// i Function
double findInt( double intRate, int yearlyPayments )
{
	double i = 0;
	i = intRate / yearlyPayments;
	return i;
}

// Loan Amount Function
double loanAmountFunc( double i, double y, double periodicPayment )
{
	double loanAmount = 0;
	double monthlyPayment = 0;
	while ( periodicPayment > monthlyPayment && monthlyPayment < ( periodicPayment - 10 ) )
	{
		loanAmount += 2000;
		monthlyPayment = ( loanAmount * i / ( 1 - pow ( ( 1 + i ),-( y ) ) ) );
	}
	return loanAmount;
}

// Monthly Payment Function
double monthlyPaymentFunc( double i, double y, double periodicPayment )
{
	double loanAmount = 0;
	double monthlyPayment = 0;
	while ( periodicPayment > monthlyPayment && monthlyPayment < ( periodicPayment - 10 ) )
	{
		loanAmount += 2000;
		monthlyPayment = ( loanAmount * i / ( 1 - pow ( ( 1 + i ),-( y ) ) ) );
	}
	return monthlyPayment;
}