Am I using the power function correctly?

[ArgentGrace]

Thanks!

[Naughty Albatross]

payment = (((amount_of_loan)*(rate_of_interest))*(pow((1+(rate_of_interest)),(number_of_years)))/((pow((1+(rate_of_interest)),(number_of_years)))-1));

This formula should be :
payment = pow((amount_of_loan)*(interest_rate) * (1+interest_rate), (number_of_years)) / pow((1+interest_rate), (number_of_years)) - 1;

[Naughty Albatross]

Does that help? :)

[ArgentGrace]

Unfortunately it did not :-(.

I copied and paste your formula code over mine, and there is still an error.

The below is what it says:

`payment' undeclared (first use this function)

In function `float calculatePayment(double&, double&, double&, double&, double, int&)':

[Naughty Albatross]

Then here is this one :
int final_payment = pow((amount_of_loan)*(rate_of_interest) * (1+rate_of_interest), (number_of_years)) / (pow((1+rate_of_interest), (number_of_years)) - 1);

Anyway, can you let us know about your assignment sample output?

[E0p9LyTq]

Change the prototype:
void calculatePayment(double&, double&, double&, double&, double, int&, double&);

Change the definition of the function:
void calculatePayment(double& selling_price, double& rate_of_interest, double& down_payment, double& amount_of_loan, double downpayment_percentage, int& number_of_years, double& payment)

Change your function call in main:
calculatePayment(selling_price, rate_of_interest, down_payment, amount_of_loan, downpayment_percentage, number_of_years, payment);

Add an output line to show your monthly payment:

   cout << "Amount of Loan: " << amount_of_loan << endl;
   cout << "Down Payment: " << down_payment << endl;
   cout << "Monthly Payment: " << payment << "\n";

What is the selling price? :
10000
What is the rate of interest?:
15
What is the number of years in loan?:
20

Output Test:
Selling Price: 10000
Rate of Interest: 15
Number of Years: 20
Amount of Loan: 8000
Down Payment: 2000
Monthly Payment: 120000

Purpose: Monthly House Cost Calculation

Yeah, I'd say your monthly payment algorithm is seriously whacked! ;)

[E0p9LyTq]

And closed account's algorithm is even worse, with the corrected code:

What is the selling price? :
10000
What is the rate of interest?:
15
What is the number of years in loan?:
20

Output Test:
Selling Price: 10000
Rate of Interest: 15
Number of Years: 20
Amount of Loan: 8000
Down Payment: 2000
Monthly Payment: 3.83376e+101

Purpose: Monthly House Cost Calculation

[E0p9LyTq]

Using the way to calculate loan payments here, http://www.wikihow.com/Calculate-Loan-Payments I did the loan calculation in intermediate steps instead of one huge indecipherable statement:

void calculatePayment(double& selling_price, double& rate_of_interest, double& down_payment, double& amount_of_loan, const double& downpayment_percentage, int& number_of_years, double& payment)
{
   down_payment = (selling_price * downpayment_percentage);
   amount_of_loan = (selling_price - down_payment);

   double J = (rate_of_interest / 100.0) / 12.0;

   int N = 12 * number_of_years;

   double intermediate_step = (1.0 / (std::pow((1.0 + J), static_cast<double>(N))));

   double second_intermediate = J / (1.0 - intermediate_step);

   payment = second_intermediate * amount_of_loan;
}

What is the selling price? :
10000
What is the rate of interest?:
15
What is the number of years in loan?:
20

Output Test:
Selling Price: 10000
Rate of Interest: 15
Number of Years: 20
Amount of Loan: 8000
Down Payment: 2000
Monthly Payment: 105.343

Purpose: Monthly House Cost Calculation

[ArgentGrace]

Thank you all,

I took FurryGuy's recommendation to do intermediate steps and it worked perfectly.

Thanks!

[E0p9LyTq]

Glad I could help. :)

As you can see, intermediate steps makes it less error prone. They make understanding what the code does MUCH easier as well! :D

Easier to read means easier to maintain.

Using intermediate steps in a complex calculation makes it easier to track down errors. Printing out the results of each step would help pinpoint where the error occurs.