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iOS

Howie F
Howie F
1,710 Points

Better explanation/tutorial for initializers?

Can someone please point me (a beginner with basically no programming experience) to a better explanation/tutorial to help me understand initializers? I've listened to Amit's videos 2-3 times and still have no idea why they are needed, when different types of initializers should be used, etc. I think Amit is omitting some basic notions.

For example, he doesn't offer a precise definition of the "self" keyword. I'd like to see a precise definition along with its use in a statement as such:

self.sides = sides [1] [2] [3]

[1] = definition of "self" keyword and what its function is [2] = definition of "sides", and what it's referring to [3] = definition of "sides", and what it's referring to

Thank you!

3 Answers

Jhoan Arango
Jhoan Arango
14,575 Points

Hello Howie :

The following paragraph is from the e-book The Swift Programming Language and I recommend downloading it in iBooks.

Initialization is the process of preparing an instance of a class, structure, or enumeration for use. This process involves setting an initial value for each stored property on that instance and performing any other setup or initialization that is required before the new instance is ready for use

Apple Inc. “The Swift Programming Language (Swift 2 Prerelease).” iBooks. https://itun.es/us/k5SW7.l

What does that mean ? Let’s see it in code.

/* 
  We are going to create a class, and this class has 
  three store properties.
*/

class CarFactory {
        let wheels: Int
        let doors: Int
        let colors: String  
}

/*
  Currently this class has no initializers or initial values, therefore we can not
  create an instance of it. We have to give those store properties 
  some values, by means of an initilizer. 
*/

class CarFactory {
    let wheels: Int
    let doors: Int
    let colors: String

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

// There we have an initializer, but what does that do ? 

/*
 When creating an instance of the class CarFactory, we will
 give values to the store properties, therefore we will successfully 
 create an instance. 
*/

Notice how self is used to distinguish the name property from the name argument to the initializer. The arguments to the initializer are passed like a function call when you create an instance of the class.

Apple Inc. “The Swift Programming Language (Swift 2 Prerelease).” iBooks. https://itun.es/us/k5SW7.l

/* 
The word self is use here so that you can distinguish a property, 
from a named argument. I’m going to change the name of one of the parameters
in the initializer, so that you can see the difference.
*/

class CarFactory {
    let wheels: Int
    let doors: Int
    let colors: String

    init(wheels: Int, doors: Int, chooseColor: String) { // see the change?
        self.wheels = wheels  
        self.doors = doors
        colors = chooseColor  // the word self is not required here         
    }
}

/*
The word self is not required there since the name of the parameter
is different from the name of the property. So the system knows which 
is which.
*/

Now let’s create that instance

// Instance
let fordFiesta = CarFactory(wheels: 4, doors: 2, colors: "Black”)

// Now you can access the properties of this instance like this

fordFiesta.wheels // this will show 4
fordFiesta.doors // this will show 2

// we can also use the print method

println(fordFiesta.color) // this will print “Black"

Take it one step at a time, re-watch the videos as many times, read the book from apple, and trust me, little by little you will start making sense of everything, I hope this gives you a different view of initializers and you are able to continue your learning.

Good luck.

Howie F
Howie F
1,710 Points

Thank you very much Jhoan. This is very helpful!

I probably still have this wrong, so a few questions:

  1. Does the "wheels" in "self.wheels" then refer to the wheels in "let wheels: int", like: http://imgur.com/EwgggRN
  2. I don't understand what it means to initialize with "wheels", which is itself a variable (I think). I can get my head around initializing "wheels" with a value (e.g., 4), but what does it mean to initialize with "wheels"?
  3. If you initialized "color" with "chooseColor" as you did in the example, how would that change creating instances? Does that mean the instance would be: let fordFiesta = carFactory(wheels: 4, doors: 2, chooseColor: "Black")?
  4. What's the difference between "Initializer", "Initial Value", and "Default Value"?
  5. At the most basic level, what's happening during a initialization? Can you explain what the compiler needs so I can better understand why initialization needs to happen?

Sorry for all the questions, I do appreciate it!

Jhoan Arango
Jhoan Arango
14,575 Points

Hello, I will answer your questions once I'm in a laptop, I'm currently on my phone and is a pain to type on here.

But I'd like to know something before I know how to answer your questions. Have you gone through the whole swift course ? From the beginners part ?

I ask you this because I want to know how detailed I have to be with my answer.

Thanks

Howie F
Howie F
1,710 Points

Jhoan,

Thanks for responding. I'm going through the "iOS Development with Swift" track sequentially and am currently in the "Object Oriented Swift" section.

Howie

Jhoan Arango
Jhoan Arango
14,575 Points

Hello :

To answer your questions I will go one by one..

Answer to Question 1

Yes, that is correct.

class CarFactory {
    let wheels: Int <---- Refers to this
    let doors: Int
    let colors: String

    init(wheels: Int, doors: Int, colors: String) {
This --->   self.wheels = wheels  
                 self.doors = doors
                 self.colors = colors        
    }
}


class CarFactory {
    let wheels: Int
    let doors: Int
    let colors: String
                          // here
    init(wheels: <-- Refers to this Int, doors: Int, colors: String) {
        self.wheels = wheels   <--- This
        self.doors = doors
        self.colors = colors        
    }
}

Answer to Question 2

Initializing is GIVING a constant or a variable a value.

var someVariable: String    // has no initial value
var someVariable: String = "Value"   // This one has an initial value

/*
 Here we are declaring a new variable called “ someVariable” of 
 type String. BUT it does not have ANY value. We can initialize it by
 giving it a value by means of an initializer. 
*/

func AnyFunction(someVariable: String) {
    var someVariable: String <-- has no value //

    init(someVariable: String) {
        self.someVariable = someVariable
    }
}

let instance = AnyFunction(someVariable: "Value") <-- giving it a value //

instance.someVariable // This is how we access the value 

Answer to Question 3

Yes you got it. That is exactly how the instance would look like!

Answer to Question 4

1) Initializer is what is used to give values to store properties in a class.

2) Initial value is the first value you assign to a constant or variable

3)

class CarFactory {
    let wheels: Int = 4 // Default
    let doors: Int
    let color: String

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


let fordFiesta = CarFactory(doors: 2, color: "Black”) // “wheels” is not here

fordFiesta.wheels // but we can still access “wheels” here and it will return 4
fordFiesta.color
fordFiesta.doors

Notice how let wheels: Int = 4 has a value already ? Well since it already has a value it becomes a default value, and it no longer needs to be initialized by the initializer. But when you create an instance of CarFactory, you can then access the value of “wheels”.

Now if all of the store properties had “Default values” Then we would not need an initializer method.

class CarFactory {
    let wheels: Int = 4
    let doors: Int = 2
    let color: String = "Black"

  // No initializer method

}

let fordFiesta = CarFactory() // No need of initializer

// But we can still access those “default” values.

println(fordFiesta.wheels) // this will print 4
println(fordFiesta.color)  // this will print “Black”
println(fordFiesta.doors) // this will print 2

Answer to Question 5

This question is answered by all of the above answers.

I hope you understand now.

Good luck :)

Howie F
Howie F
1,710 Points

Jhoan,

This is much clearer to me now. Thanks for taking the time to explain this to me! Very helpful.