2

Working on a budget app in swift.

Essentially, there is a label that displays keypad textInput to currencyStyle and displays this number ($5.00), then waits for user to choose Debit or Withdraw. If debit, number is appended to Table, but I'm lost when it comes to Withdraw(-).

I would like to

  1. Convert NSDecimalNumber which is formatted as currency to a negative for appending to a tableView: String ($5.00 to -$5.00).

  2. Convert NSDecimalNumber value to a negative and add to an NSNumber array that is storing numbers to be calculated for an available balance (5.50 to -5.50)

For both I have tried using NSNumberFormatter negativePrefix and a few other properties in Apple Documentation to no avail. Even tried creating a negative NSNumber value = -100.0, but app crashed during division.

Any help would be appreciated.

aksh1t
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  • Check out this question: http://stackoverflow.com/questions/5057939/turn-a-nsdecimalnumber-negative – aksh1t Apr 15 '15 at 08:16
  • Does decimalNumberWithMantissa work in Swift? Can't find it in Xcode or Apple Swift Documentation... –  Apr 15 '15 at 08:26
  • You will need to initialise the -1 `NSDecimalNumber` like this in Swift: `NSDecimalNumber(mantissa: 1, exponent: 0, isNegative: true)` – aksh1t Apr 15 '15 at 08:28

3 Answers3

3

To easily negate the NSDecimalNumber create an extension in Swift like this:

extension NSDecimalNumber {
    func negative() -> NSDecimalNumber {
        return self.decimalNumberByMultiplyingBy(NSDecimalNumber(mantissa: 1, exponent: 0, isNegative: true));
    }
}

You can then just negate your NSDecimalNumber like this:

yourNumber = yourNumber.negative()
aksh1t
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2

You can also subtract its own value from zero and return it as follow:

extension NSDecimalNumber {
    var negative: NSDecimalNumber { NSDecimalNumber.zero.subtracting(self) }
}

NSDecimalNumber(value: 15.6).negative  // -15.6
Leo Dabus
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2

You could override the - operator. For example:

prefix func -(number: NSDecimalNumber) -> NSDecimalNumber {
    return number.decimalNumberByMultiplyingBy(NSDecimalNumber(mantissa: 1, exponent: 0, isNegative: true))
}

You can then use this like so:

let number = NSDecimalNumber(double: 5.00)
let negativeNumber = -number
ABakerSmith
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