Swift, reducing numeric error using Decimal type

user_12906445 0
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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

swift
1 Answers

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.

user_973370
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