Using NSDecimalNumber in for Loop?

Question

I have a loop i am trying to run with NSDecimalNumber values but the value returned is always the same. I understand NSDecimalNumber isn't mutable but i originally used double values and was getting the wrong result at the end which I assume is some floating point error/rounding error. Here is the code:

double balanceAmount = loanAmountValue;
        double rtemp = r / (n * 12);
        double intA = balanceAmount * rtemp;
        double principalA = payfinal - intA;
        double principal = balanceAmount - principalA;
        NSDecimalNumber *balDeciminal = (NSDecimalNumber *) [NSDecimalNumber numberWithDouble:balanceAmount];
        NSDecimalNumber *rTempDecimal = (NSDecimalNumber *) [NSDecimalNumber numberWithDouble:rtemp];
        NSDecimalNumber *payFinalDecimal = (NSDecimalNumber *) [NSDecimalNumber numberWithDouble:payfinal];
        NSDecimalNumber *principalDecimal = (NSDecimalNumber *) [NSDecimalNumber numberWithDouble:principalA];
        for (n = n * 12; n != 0; --n) {
            NSDecimalNumber *realBalanceDecimal = [balDeciminal decimalNumberBySubtracting:principalDecimal];

            NSDecimalNumber *interestDecimal = [balDeciminal decimalNumberByMultiplyingBy:rTempDecimal];
            NSDecimalNumber *principalDecimalAmount = [payFinalDecimal decimalNumberBySubtracting:interestDecimal];



        NSString *tempInterest = [NSString stringWithFormat:@"$%@", interestDecimal];
        [interestLabels addObject:tempInterest];
        NSString *tempPrincipal = [NSString stringWithFormat:@"$%@", principalDecimalAmount];
        [pricipalLabels addObject:tempPrincipal];
        NSString *tempBalance = [NSString stringWithFormat:@"$%@", balDeciminal];
        [balanceLabels addObject: tempBalance];
    }
    NSLog(@"%@", balanceLabels);
    NSLog(@"%@", pricipalLabels);
    NSLog(@"%@", interestLabels);

If NSDecimalNumber doesn't allow me to make these sort of calculations could someone suggest something else that will return a result that is accurate?

Thanks!

EDIT : Double Code

double r = interestAmountValue/200;
    //NSLog(@"%f", r);
    double n = yearAmountValue;
    double rPower = pow(1+r, 0.166666666);
    double tophalf = rPower - 1;
    double nPower = (-12 * n);
    double bothalf = pow(rPower, nPower);
    double bothalffinal = 1 - bothalf;
    double tempfinal = tophalf / bothalffinal;
    double payfinal = loanAmountValue * tempfinal;
    double totalPaymentd = payfinal * n * 12;

    double totalInterestd = totalPaymentd - loanAmountValue;

for (n = n * 12; n != 0; --n) {



        double realBalance = balanceAmount - principalA;
        double interest = balanceAmount * rtemp;
        NSLog(@"%f", interest);
        double principalAmount = payfinal - interest;





        balanceAmount -= principalA;
}

Answer 1

Hi the problem you were having is that you weren't checking was the loan payed off and you were entering negative numbers meaning the underlying problem is with the spreading of payments.

double loanAmountValue = 100000;
double balanceAmount = loanAmountValue;
double n = 30;
double interestAmountValue = 8;
double r = interestAmountValue/1200;
double rtemp = r/(n*12);
double intA = balanceAmount * rtemp;


//NSLog(@"%f", r);
double rPower = pow(1+r, 0.166666666);
double tophalf = rPower - 1;
double nPower = (-12 * n);
double bothalf = pow(rPower, nPower);
double bothalffinal = 1 - bothalf;
double tempfinal = tophalf / bothalffinal;
double payfinal = loanAmountValue * tempfinal;
double principalA = balanceAmount - intA;
double totalPaymentd = payfinal * n * 12;

double totalInterestd = totalPaymentd - loanAmountValue;

for (n = n * 12; n != 0; --n) {



    double realBalance = balanceAmount - principalA;
    double interest = balanceAmount * rtemp;
    NSLog(@"interest: %f", interest);
    NSLog(@"rtemp: %f",rtemp);
    double principalAmount = payfinal - interest;



    // Check for negative balance
    if (balanceAmount < principalAmount) {

        NSLog(@"Balance Amount: %f",balanceAmount);
        NSLog(@"Months Left: %f",n);

        break;
    }

    balanceAmount -= principalAmount;


    NSLog(@"balanceAmount %f",balanceAmount);
}

Using your code with the values of n = 30 years, loanAmountValue = 100000, and interestAmountValue = 8%. I had the loan payed off with 63 months left. I assume r = interestAmountValue/1200 instead of 200 to get percentage per month? Regarding use of NSDecimal number I don't know what impact floating point precision will have on your corrected mortgage calculator, but the problem you were having was not with using double data types it is with the spreading of payments.