/*
ACTION 6-2 (ACT06_02.CPP)
OUTPUT:  A PROGRAM MENU THAT PROMPTS THE USER TO SELECT A 
	MONTHLY PAYMENT, TOTAL INTEREST, OR TOTAL LOAN AMOUNT
	CALCULATION OPTION.
	INVALID ENTRY MESSAGES AS REQUIRED.
	THE MONTHLY LOAN PAYMENT, TOTAL LOAN INTEREST,
	OR TOTAL LOAN AMOUNT, DEPENDING ON THE PROGRAM OPTION THAT THE
	USER SELECTS.
	INVALID ENTRY MESSAGES AS REQUIRED.

INPUT:	A USER RESPONSE TO THE MENU (P, I, T, OR Q).
	IF P IS SELECTED: USER ENTERS THE LOAN AMOUNT, INTEREST RATE, AND TERM.
	IF I IS SELECTED: USER ENTERS THE LOAN AMOUNT, INTEREST RATE, AND TERM.
	IF R IS SELECTED: USER ENTERS THE LOAN AMOUNT, INTEREST RATE, AND TERM.
	IF Q IS SELECTED: TERMINATE PROGRAM.

PROCESSING:  CALCULATE THE SELECTED OPTION AS FOLLOWS: 
	CASE V:	PAYMENT = PRINCIPLE * RATE/(1 - (1+RATE) -TERM)
	CASE I:	INTEREST = TERM * PAYMENT - PRINCIPLE
	CASE T:	TOTAL = PRINCIPLE + INTEREST
	CASE Q: QUIT THE PROGRAM.

	WHERE: 	PRINCIPLE IS THE AMOUNT OF THE LOAN.
	RATE IS A MONTHLY INTEREST RATE IN DECIMAL FORM.
	TERM IS THE NUMBER OF MONTHS OF THE LOAN.

*/

//PREPROCESSOR DIRECTIVES
#include <iostream.h>   //FOR cin AND cout
#include <math.h>   	 //FOR pow()

//MAIN FUNCTION
int main() 
{
 //DEFINE AND INITIALIZE VARIABLES
char choice = 'Q'; 				//USER MENU ENTRY
double payment = 0.0; 		//MONTHLY PAYMENT
double interest = 0.0;   	//TOTAL INTEREST FOR LIFE OF LOAN
double total = 0.0;     	//TOTAL LOAN AMOUNT = PRINCIPLE + INTEREST
double principle = 0.0; 	//LOAN AMOUNT
double rate = 0.0;       	//INTEREST RATE
int term = 0;        			//TERM OF LOAN IN MONTHS

//SET OUTPUT FORMAT
cout.setf(ios::fixed);
cout.setf(ios::showpoint);
cout.precision(2);

//DISPLAY PROGRAM DESCRIPTION MESSAGE
cout << "This program will calculate a monthly loan\n"
		 << "payment, total loan interest, or total loan amount." << endl;

//displayMenu() FUNCTION
cout  << "\n\n\t\t\tEnter P to get monthly payment" 
	 << "\n\t\t\tEnter I to get total loan interest"
	 << "\n\t\t\tEnter T to get total loan amount"
	 << "\n\t\t\tEnter Q to quit" << endl;

cout << "\n\n\tPlease enter your choice:  ";
  
//READ USER CHOICE
cin >> choice;

//loanCalculator() FUNCTION
switch (choice)
{
	case 'p':  //calculatePayment() FUNCTION
	case 'P' : cout << "\nEnter the amount of the loan: $";
		cin >> principle;
		cout << "\nEnter the duration of the loan in months: ";
		cin >> term;
		cout << "\nEnter the annual interest rate in percent: ";
		cin >> rate;

		//CHECK FOR INVALID ENTRY
		if ((rate <= 0) || (rate > 100))
		   cout << "\n\nThis is an invalid entry. Please"
						<< " run the program again." << endl;
		else
		{
		   rate = rate/12/100;
		   payment = principle * rate/(1 - pow((1+rate), -term));
		   cout << "\n\nThe monthly payment is $" << payment << endl;
		}//END ELSE
		break;

	case 'i': //calculateInterest() FUNCTION
	case 'I' :cout << "\nEnter the amount of the loan: $";
		cin >> principle;
		cout << "\nEnter the duration of the loan in months: ";
		cin >> term;
		cout << "\nEnter the annual interest rate in percent: ";
		cin >> rate;
		//CHECK FOR INVALID ENTRY
		if ((rate <= 0) || (rate > 100))
		    cout << "\n\nThis is an invalid entry. Please"
						 << " run the program again." << endl;
		else
		{
		    rate = rate/12/100;
		    payment = principle * rate/(1-pow((1+rate), -term));
		    interest = term * payment - principle;
		    cout << "\n\nThe total interest is $" << interest << endl;
		}//END ELSE
		break;
                
	case 't':	//calculateTotalLoanAmount() FUNCTION
	case 'T':cout << "\nEnter the amount of the loan: $";
		cin >> principle;
		cout << "\nEnter the duration of the loan in months: ";
		cin >> term;
		cout << "\nEnter the annual interest rate in percent: " ;
		cin >> rate;

		//CHECK FOR INVALID ENTRY
		if ((rate <= 0) || (rate > 100))
		    cout << "\n\nThis is an invalid entry. Please"
						 << " run the program again." << endl;
		else
		{
		    rate = rate/12/100;
		    payment = principle * rate/(1-pow((1+rate), -term));
		    interest = term * payment - principle;
		    total = principle + interest;
		    cout << "\n\nThe total loan amount is $" << total << endl;
		}//END ELSE
		break;

	case 'q': //QUIT THE PROGRAM
	case 'Q': cout << "Program terminated";
						break;
                  
	//DISPLAY INVALID ENTRY MESSAGE
	default : cout << "\n\nThis is an invalid entry. Please"
								 << " run the program again." << endl;
  } //END SWITCH

	//RETURN
	return 0;
} //END main()