• What is decimal 
      Decimal is a structure representing a base-10 number
  • What is the differences between decimal and float/double 
      Float and double are NOT accurate, especially when representing price.
  They are structure representing a base-2 number.
  When deal with currency, better use decimal than float/double
  • How to use decimal
    • Create a Decimal
    • Apply +/-/*/\ to decimals
    • Use numberFormmater to change decimal to string
  • How to get monthly price from a SKProduct subscription with period of one year 
      //Get price of product, which is the value in the currency of the related store of your iTunes Store id
  let price: Decimal = product.price
  let monthlyPrice = price / 12
  //Transform it into string using the local of product
  let numberFormmater = NumberFormatter()
  numberFormmater.local = product.local
  numberFormmater.numberStyle = .currency
  let result = numberFormmater.stringFromNumber(monthlyPrice as NSNumber)
  • How to enumrate dictionary, and what is the element 
      dic.map({(key, value) -> Any
    #do with key and value
    })
  for (key, value) in dic
  Every element is a tuple with key and associate value
  • What are the common ways to get sorted array
    • The element conform to Comparable protocol 
      arr.sorted()
arr.sorted(by: </>)
    • The element does not conform to Comparable protocol 
      arr.sorted(by: { v1, v2 -> Bool
  #compare two element and return true if v1 should be in front of v2
  })