Swif, reducing numeric error using Decimal type-CodePudding

I found that making operand in Decimal can reduce numeric error in Swift

0.1   0.2 = 0.30000000000000004
Decimal(0.1)   Decimal(0.2) = 0.3

So I tried to make a function that calculate calculation String in Decimal like this:

   func calculate(expression: String) -> Decimal {
    let expression = NSExpression(format: expression)
    let value = expression.expressionValue(with: nil, context: nil) as? Decimal
    return value ?? 0.0
}

But value property keep getting nil value and function always returning 0.0. Can I get some any help on this?

Thanks

CodePudding user response：

The Decimal type does not reduce numeric error. It just computes values in decimal. That can increase or decrease error, depending on the calculation. 1/10 happens to be bad in binary for the same reason that 1/3 is bad in decimal. Your code doesn't actually compute anything in Decimal. It's just trying to convert the final Double value to Decimal at the end, which introduces binary-to-decimal rounding (making it less accurate).

That said, expressionValue(with:context:) returns an NSNumber. You can't convert that to Decimal with as?. You need to use .decimalValue:

let number = expression.expressionValue(with: nil, context: nil) as? NSNumber
let value = number?.decimalValue

This will compute the value in Double and then round it to a Decimal.

But if you want to do calculations in Decimal, I don't believe that NSExpression can do that.