What am I doing wrong with this mortgage formula? - c++

Question

#include <iostream>
#include <cmath>
using namespace std;
/* FINDS AND INITIALIZES TERM */
void findTerm(int t) {
int term = t * 12;
}
/* FINDS AND INITIALIZES RATE */
void findRate(double r) {
double rate = r / 1200.0;
}
/* INITALIZES AMOUNT OF LOAN*/
void findAmount(int amount) {
int num1 = 0.0;
}
void findPayment(int amount, double rate, int term) {
int monthlyPayment = amount * rate / ( 1.0 -pow(rate + 1, -term));
cout<<"Your monthly payment is $"<<monthlyPayment<<". ";
}
This is the main function.
int main() {
int t, a, payment;
double r;
cout<<"Enter the amount of your mortage loan: \n ";
cin>>a;
cout<<"Enter the interest rate: \n";
cin>>r;
cout<<"Enter the term of your loan: \n";
cin>>t;
findPayment(a, r, t); // calls findPayment to calculate monthly payment.
return 0;
}
I ran it over and over again, but it still gives me the incorrect amount.
My professor gave us an example that goes like this:
Loan=$200,000
Rate=4.5%
Term: 30 years
And the findFormula() function is supposed to produce $1013.67 for the mortgage payment. My professor gave us that code as well (monthlyPayment = amount * rate / ( 1.0 – pow(rate + 1, -term));). I'm not sure what's wrong with my code.

Answer 1

The formula may be fine, but you are not returning, nor using, any value from your conversion functions, so its inputs are wrong.
Consider this refactoring of your program:
#include <iostream>
#include <iomanip> // for std::setprecision and std::fixed
#include <cmath>
namespace mortgage {
int months_from_years(int years) {
return years * 12;
}
double monthly_rate_from(double yearly_rate) {
return yearly_rate / 1200.0;
}
double monthly_payment(int amount, double yearly_rate, int years)
{
double rate = monthly_rate_from(yearly_rate);
int term = months_from_years(years);
return amount * rate / ( 1.0 - std::pow(rate + 1.0, -term));
}
} // end of namespace 'mortgage'
int main()
{
using std::cout;
using std::cin;
int amount;
cout << "Enter the amount of your mortage loan (dollars):\n";
cin >> amount;
double rate;
cout << "Enter the interest rate (percentage):\n";
cin >> rate;
int term_in_years;
cout << "Enter the term of your loan (years):\n";
cin >> term_in_years;
cout << "\nYour monthly payment is: $ " << std::setprecision(2) << std::fixed
<< mortgage::monthly_payment(amount, rate, term_in_years) << '\n';
}
It still lacks any checking of the user inputs, but given the values of your example, it outputs:
Enter the amount of your mortage loan (dollars):
200000
Enter the interest rate (percentage):
4.5
Enter the term of your loan (years):
30
Your monthly payment is: $ 1013.37
The slightly difference from your expected output (1013,67) could be due to any sort of rounding error, even a different overload of std::pow choosen by the compiler (since C++11, the integral parameters are promoted to double).