Extensions with the Swift standard library

Let's say that, in our application, we needed to calculate the factorial of some integers. A factorial is written as 5!. To calculate a factorial, we take the product of all the positive integers that are less than or equal to the number. The following example shows how we would calculate the factorial of five:

5! = 5*4*3*2*1 
5! = 120 

We could very easily create a global function to calculate the factorial, and we would do that in most languages, however, in Swift, extensions give us a better way to do this. The Integer type in Swift is implemented as a structure which we can extend to add this functionality directly to the type itself. The following example shows how we can do this:

extension Int { 
  func factorial() -> Int {  
    var answer = 1 
    for x in (1...self).reversed() {  
      answer *= x 
    } 
    return...