I assume the math is right, but what are your types?

If they're floats or doubles, you've got literals in the code that that'll become ints, rather than floats. If they're not floats or doubles, they probably should be.

I don't remember the rules for automatic typecasting, so this might not be it, but you might try tacking a ".0" to the end of each number in the code (i.e. 1200 -> 1200.0).

Instead of setting balanceLeft to a long series of calculations, try putting each calculation into an individual variable and print them out separately so that you can verify that they'll be what you're expecting.

Regardless, this:

pow((1+ interestRate),(paymentNum - numOfPayments))

will always be over 1 as long as interestRate is over 0. And since that is greater than 1, this:

1 - (pow((1+ interestRate),(paymentNum - numOfPayments)))

will be subtracting from 1 a number greater than 1. This will give you a negative number. Since you then divide by interestRate and multiply by paymentAmt, you will get a negative result if both interestRate and paymentAmt are positive (or both negative).

Thanks for the tips, Yoyo and Quintin. The professor gave us another week on this one (I'm guessing a lot of people had trouble), so I'm still in the loop.

I'm going to post all my code here, with your tips. However, now when I try to compile, I get this "Id returned 1 exit status" error, and it won't fully compile. It started after I broke down the calculations further, so I don't know what the heck I'm doing wrong. Edit: What I get are a mass of linker errors that I can't decipher, and then at the end, I get a Build Error 1, and that's it.

Anyhow, here's my code, in its entirety (and again, much appreciated):

#include
#include
#include
#include

using namespace std;

int main(int argc, char *argv[])
{
/* This program calculates the balance on a loan after
a certain number of payments.
*/

// Declaration of variables
int paymentNum; // Payment number - INPUT
float paymentAmt; // Amount of payment - INPUT
float interestRate; // Interest rate - INPUT
int numOfPayments; // Number of payments - INPUT
float amtOfLoan; // Original amount of loan - INPUT
float balanceLeft; // Balance left over - CALC & OUTPUT
float powerCalc; // Calculation of "1 - pow(x,y)" section - CALC
float intRatePlusOne; // Calculation of 1 + interest rate - CALC
float paymentsRemaining; // Calculation of number of remaining payments - CALC

// Get inputs
cout << "Original Loan Amount: ";
cin >> amtOfLoan;
cout << "Monthly Payment: ";
cin >> paymentAmt;
cout << "Annual Interest Rate: ";
cin >> interestRate;
cout << "Term of Loan (Years): ";
cin >> numOfPayments;
cout << "Show Balance After Payment #: ";
cin >> paymentNum;

// Translate term of loan from years to months

numOfPayments *= 12;

// Translate Annual Interest Rate into monthly interest rate

interestRate = float(interestRate) / 1200.0;

// Calculate monthly interest rate + 1 raised to the power of (number of payments made - total number of payments)

intRatePlusOne = 1 + interestRate;

// Calculate the number of payments remaining

paymentsRemaining = paymentNum - numOfPayments;

// Calculate the 1 - pow(x,y) section of the equation
powerCalc = 1 - pow(intRatePlusOne, paymentsRemaining);

// Calculate remaining balance

balanceLeft = paymentAmt * (powerCalc/interestRate);
cout << "\nBalance After Payment #" << paymentNum << ": $" << fixed << setprecision(2) << balanceLeft << endl << endl;

system("PAUSE");
return EXIT_SUCCESS;
}

Are you typing that in as you do it? It might be more helpful to cut and paste here (that way you are not inadvertently adding/removing mistakes.)

also, shouldn't you subtract the total # of payments from the number of payments to be calculated?, otherwise the payments Remaining will be a negative number.

Linker errors? That's pretty interesting. It would mean that it went through the compilation stage just fine, but then when it tried to find the right libraries for the functions used in the program, it got confused.

But it's hard to diagnose a problem like this without more details - there's a myriad things that could cause linker errors...

The second batch of code (minus the break statement) compiles just fine for me, one thing that "system("PAUSE"):" call wont work cross platform (if you are specifically doing it for windows you are fine, otherwise use "cin.get();")

Also in your original code snippet you had a precidence issue:

balanceLeft = (paymentAmt * (1 - (pow((1+ interestRate),(paymentNum - numOfPayments))) / interestRate));

is not the same as:

// Calculate the 1 - pow(x,y) section of the equation
powerCalc = 1 - pow(intRatePlusOne, paymentsRemaining);

// Calculate remaining balance

balanceLeft = paymentAmt * (powerCalc/interestRate);

it is the same as

// Calculate the 1 - (pow(x,y)/interest rate) section of the equation
powerCalc = 1 - (pow(intRatePlusOne, paymentsRemaining)/interestRate);

// Calculate remaining balance

balanceLeft = paymentAmt * powerCalc;

just like in normal math * & / come before + & -, but after () & ^

Got it...once I made those switches to that last set of code, and ran it through a different compiler (was using Dev-C++, switched to TextPad and a Borland C++ compiler), all's well.

Nos, I was wondering if maybe my math was going strange on me the first go around...would explain all the problems I was having.

Minase, regarding the commenting and such, my teacher this semester isn't a stickler for it, but next semester, my professor will kick my ever loving butt around the school twice if I don't do exactly what I'm doing now (at least by reputation she will), so I figure it's better to just get it ingrained in my head now. She wrote the textbooks we're using, so if she says to comment liberally, I shall comment liberally with pleasure. (Helps me out, too, so I don't go back later and go "huh?")

Also, with your other suggestions, we haven't touched upon unsigned yet, but that's a cool one to know, so I'll file that away in my mind for a bit. No talk about Hungarian Notation in the text, so I haven't done that. I'll check with her on that, though.

Finally, I'm guessing I should've written intRatePlusOne = interestRate++...d'oh. I knew that...

Thanks, all, for your help. Much, MUCH appreciated.

I know about them, and I even somewhat remember when to use which one. I just hadn't thought about it at that point when I was first writing the program.

Personally, I'm not really keen on using them outside of loops, or other places where iteration is necessary, but that's me. On the other hand, I'll use += and its relatives until the cows come home, since it saves a lot of time and typing.