ios - Swift string formula into a real calculation

Question

I have several formulas stored in a Plist such as A * B. I'm trying to figure out how I could take this formula currently stored as a string in a Plist and use it as an actual calculation formula. I tried going the route of making the formula to (A) * (B) and then setting A and B before trying to use the formula but it did not work. Any suggestions?

example

let A = 5
let B = 2

println (formula)

actually printed out "(A) * (B)"

Answer 1

Xcode 8.3.1 ? Swift 3.1

extension String {
    var expression: NSExpression {
        return NSExpression(format: self)
    }
}

let a = 5
let b = 2
let intDictionary = ["a": a,"b": b]

var formula = "a * b"
if let timesResult = formula.expression.expressionValue(with: intDictionary, context: nil) as? Int {  
    print(timesResult) // 10
}
formula = "(a + b) / 2"
if let intAvgResult = formula.expression.expressionValue(with: intDictionary, context: nil) as? Int {
    print(intAvgResult)    // 3
}



let x = 5.0
let y = 2.0
let z = 3.0

let doubleDictionary = ["x": x, "y": y, "z": z]


formula = "(x + y + z) / 3"
if let doubleAvgResult = formula.expression.expressionValue(with: doubleDictionary, context: nil) as? Double {
    print(doubleAvgResult)
}