what the benefit of convenience initializer if do same job with initialize the class members with default values

Convenience initializers give you incredible control over how something is initialized. Check out my example... it uses cats! Everyone loves cats.

Example: A program that creates four cats, and outputs their color and age. Depending on what parameters are provided at the moment of creation, the correct convenience initializer is used.

• Otto - color: "white" // age: 10
• Muffins - color: NOT GIVEN // age: 5
• Bruno - color: "red" -// age: NOT GIVEN
• Pico - NOTHING GIVEN

class Cat {
     var color: String
     var age: Int

     //If both parameters color and age are given:
     init (color: String, age: Int) {
          self.color = color
          self.age = age
     }

    //If only age is given:
     convenience init (age: Int) {
          self.init(color: "black", age: age)
     }

     //if only color is given:
     convenience init (color: String) {
          self.init(color: color, age: 1)
     }

     //if nothing given
     convenience init () {
          self.init(color: "black", age: 1)
     }
}

//create Otto:
var Otto = Cat(color: "white", age: 10)
println("Otto is \(Otto.color) and \(Otto.age) year(s) old!")

//create Muffins:
var Muffins = Cat(age: 5)
println("Muffins is \(Muffins.color) and \(Muffins.age) year(s) old!")

//create Bruno:
var Bruno = Cat(color: "red")
println("Bruno is \(Bruno.color) and \(Bruno.age) year(s) old!")

//create Pico:
var Pico = Cat()
println("Pico is \(Pico.color) and \(Pico.age) year(s) old!")

CONSOLE OUTPUT:

Otto is white and 10 year(s) old!

Muffins is black and 5 year(s) old!

Bruno is red and 1 year(s) old!

Pico is black and 1 year(s) old!

This can be answered a bit in the course but i will try to give a relative easy answer:

If we have a class "car" that has three variables: color, wheels and doors:

class Car {
     var wheels: Int
     var doors: Int
     var color: String
}

If we would set these variables as default values then every car would look the exactly same. But if we can do a initializer then we can customize every car:

class Car {
     var wheels: Int
     var doors: Int
     var color: String

     init(wheels: Int, doors: Int, color: String) {
           self.wheels = wheels
           self.doors = doors
           self.color = color
     }
}

A convenience initializer could also be added to easy create a common car:

class Car {
     var wheels: Int
     var doors: Int
     var color: String

     init(wheels: Int, doors: Int, color: String) {
           self.wheels = wheels
           self.doors = doors
           self.color = color
     }

      convenience init (color: String) {
           self.wheels = 4
           self.doors = 2
           self.color = color
      }
}

This could also be done by default values to the three variables but if some of the values would need to be manipulated by a function or method Xcode would complain about trying to access a method that is not accessible so initialize the values in a initializer creates more possibilities to manipulate values

class Car {
     var wheels: Int
     var doors: Int
     var color: String

     init(wheels: Int, doors: Int, color: String) {
           self.wheels = wheels
           self.doors = doors
           self.color = color
     }

      convenience init (color: String) {
           self.wheels = 4
           self.doors = 2
           self.color = createColor(color)
      }

      func createColor(color: String) -> UIColor {
             return UIColor(red: 1, green: 1, blue: 1, alpha: 1)
      }
}