Absolute Value (abs) of NSDecimalNumber without loss of precision

Question

I need to get the absolute value of an NSDecimalNumber without loss of precision. I can\'t seem to find a way to do this without converting to decimal or float (with a loss

Answer 1

You can use the built-in functions of the NSDecimalNumber class to compare the number to zero, then multiply by -1 as appropriate. For example:

- (NSDecimalNumber *)abs:(NSDecimalNumber *)num {
    if ([num compare:[NSDecimalNumber zero]] == NSOrderedAscending) {
        // Number is negative. Multiply by -1
        NSDecimalNumber * negativeOne = [NSDecimalNumber decimalNumberWithMantissa:1
                                                                          exponent:0
                                                                        isNegative:YES];
        return [num decimalNumberByMultiplyingBy:negativeOne];
    } else {
        return num;
    }
}

Since this method works solely with NSDecimalNumbers, there's no loss of precision.

Answer 2

There is a minor error in the answer: negativeOne should be a pointer. Here is a revised answer:

- (NSDecimalNumber *)abs:(NSDecimalNumber *)num {
    if [myNumber compare:[NSDecimalNumber zero]] == NSOrderedAscending) {
        // Number is negative. Multiply by -1
        NSDecimalNumber *negativeOne = [NSDecimalNumber decimalNumberWithMantissa:1
                                                                    exponent:0
                                                                  isNegative:YES];
        return [num decimalNumberByMultiplyingBy:negativeOne];
    } else {
        return num;
    }
}

And here is a version suitable to be used as a category on NSDecimalNumber:

- (NSDecimalNumber *)abs {
    if ([self compare:[NSDecimalNumber zero]] == NSOrderedAscending) {
        // Number is negative. Multiply by -1
        NSDecimalNumber *negativeOne = [NSDecimalNumber decimalNumberWithMantissa:1
                                                                         exponent:0
                                                                       isNegative:YES];
        return [self decimalNumberByMultiplyingBy:negativeOne];
    } else {
        return self;
    }
}

Answer 3

If you do not want to multiply it, zero subtracting the value will do the trick.

- (NSDecimalNumber *)abs:(NSDecimalNumber *)num {
    if ([num compare:[NSDecimalNumber zero]] == NSOrderedAscending) {
        // negative value
        return [[NSDecimalNumber zero] decimalNumberBySubtracting:num];
    } else {
        return num;
    }
}

Answer 4

Swift 4.0 variant:

extension NSDecimalNumber {
    func absoluteValue() -> NSDecimalNumber {
        if self.compare(NSDecimalNumber.zero) == .orderedAscending {
            let negativeValue = NSDecimalNumber(mantissa: 1, exponent: 0, isNegative: true)
            return self.multiplying(by: negativeValue)
        } else {
            return self
        }
    }
}

usage

let sum = NSDecimalNumber(value: -123.33)

sum.absoluteValue()