CODEWITHSUNDEEP
c++
Kevin Schultz
Kevin Schultz

Reputation: 884

Compute car payment amount not calculating properly

I am creating a small program for a homework assignment. The program runs correctly except the calculation is not correct.

The formula I am using to calculate the payment amount is:

Payment = (intRate * (1+intRate)^N / ((1+intRate)^N-1)) * L

Where "N" is the number of payments and "L" is the principal. The code I wrote to do this is:

monthlyPayment = (intRate * pow ((1 + intRate), numberPayments) / (intRate * pow ((1 +     intRate), numberPayments)-1))*principal;

The complete code is below.

#include <iostream>
#include <string>
#include <cmath>
#include <iomanip>

using namespace std;

int main()
{
    double principal, intRate, paybackAmount, interestPaid,  monthlyPayment;
    int numberPayments;

    // Change the panel color.
    system ("color F0");

    cout << "\n";
    cout << "This application will calculate your loan amounts." << endl;
    cout << "Please enter the data below." << endl;
    cout << "\n";

    cout << "Loan Amount: ";
    cin >> principal;
    cout << "Monthly Interest Rate: ";
    cin >> intRate;
    cout << "Number of Payments: ";
    cin >> numberPayments;


    cout << "\n\n\n";

    monthlyPayment = (intRate * pow ((1 + intRate), numberPayments) / (intRate *     pow ((1 + intRate), numberPayments)-1))*principal;
    paybackAmount = monthlyPayment * numberPayments;

    cout << fixed << setprecision(2) << showpoint << left << setw(24) << "Loan     Amount:" << "$" << setw(11) << right << principal << endl;
    cout << fixed << setprecision(1) << showpoint<< left << setw(24) << "Monthly     Interest Rate:" << setw(11) << right << intRate << "%" << endl;
    cout << fixed << setprecision(0) << left << setw(24) << "Number of Payments:"     << setw(12) << right << numberPayments << endl;
    cout << fixed << setprecision(2) << showpoint<< left << setw(24) << "Monthly     Payment:" << "$" << setw(11) << right << monthlyPayment << endl;
    cout << fixed << setprecision(2) << showpoint<< left << setw(24) << "Amount     Paid Back:" << "$" << setw(11) << right << paybackAmount << endl;
    cout << fixed << setprecision(2) << showpoint<< left << setw(24) << "Interest     Paid:" << "$" << right << setw(11) << paybackAmount - principal << "\n\n" << endl;


    system("pause");
}

Thanks in advance for the help!

Upvotes: 0

Views: 756

Answers (3)

Joseph Mansfield
Joseph Mansfield

Reputation: 110698

You're multiplying both the numerator and denominator by intRate, when you should only be multiplying the numerator, according to the your equation.

You're also subtracting 1 from the result of the second pow rather than from numberPayments.

(intRate * pow ((1 + intRate), numberPayments)-1)
//  ^ Why is this here?            Wrong place ^

What you actually want is:

monthlyPayment = (intRate * pow(1+intRate, numberPayments) /
                            pow(1+intRate, numberPayments-1)) * principal;

Upvotes: 1

Loki Astari
Loki Astari

Reputation: 264571

Seems like an extra argument has been slipped in:

((1+intRate)^N-1)) * L => (intRate * pow ((1 + intRate), numberPayments)-1))*principal
//                         ^^^^^^^^^  Look Odd.

Additionally:

(1+intRate)^N-1

You represent this as:

pow ((1 + intRate), numberPayments)-1

I would have done that differently as:

pow ((1 + intRate), (numberPayments-1))

Upvotes: 0

Arpit
Arpit

Reputation: 12797

i think you have extra intRate

  `(intRate * pow ((1 +     intRate), numberPayments)-1))*principal;`
   ^^^^^^^^

Upvotes: 0

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