How to count an example in swift written to a variable?

Question

Swift

Variable of type String
var a = "1+2"
​​How to calculate

Answer 1

var numericExpression = "4 + 3 - 2"
let expression = NSExpression(format: numericExpression)
var result = expression.expressionValueWithObject(nil, context: nil) as NSNumber

Answer 2

This is an Assignment, not a Question. Try to implement it yourself (in the playground - as easy as shelling pears). If you run into misunderstandings about the programming part - come and ask.