How to create a negative zero

gistfile1.m

// Call this method with [NSDecimalNumber zero] to get negative zero, or NaN.
- (NSDecimalNumber *)negativeAmount:(NSDecimalNumber *)anAmount
{
    if (anAmount == nil) {
        return [NSDecimalNumber zero];
    }
    
    NSDecimal decimalAmount = [anAmount decimalValue];
    decimalAmount._isNegative = 1;
	
    return [NSDecimalNumber decimalNumberWithDecimal:decimalAmount];
}

// Here's a version that handles that situation properly:
- (NSDecimalNumber *)negativeAmount:(NSDecimalNumber *)anAmount
{
    if (anAmount == nil || [anAmount isZero]) {
        return [NSDecimalNumber zero];
    }
    
    NSDecimal decimalAmount = [anAmount decimalValue];
    decimalAmount._isNegative = 1;
	
    return [NSDecimalNumber decimalNumberWithDecimal:decimalAmount];
}

The isZero method is from a category we have on NSDecimalNumber.