# The Big Nerd Ranch Guide to Structures and Classes in Swift

In this chapter from Cocoa Programming for OS X: The Big Nerd Ranch Guide, 5th Edition, you will build a simple 2D physics simulation. You will create your own structure and a few classes, and you will learn about the differences between them.
This chapter is from the book

### This chapter is from the book 

At this point you should be somewhat familiar with using Swift’s standard types: strings, arrays, enums, etc. It is time to move on to bigger and better things: defining your own types. In this chapter, you will build a simple 2D physics simulation. You will create your own structure and a few classes, and you will learn about the differences between them.

## Structures

In Cocoa, structures are typically used to represent groupings of data. For example, there is NSPoint, which represents a point in 2D space with an X and a Y value. As your first structure you will create a 2D vector structure.

Create a new playground. From Xcode’s File menu, select New...Playground. Name the playground Physics and save it with the rest of your projects.

Start by defining the Vector structure:

```import Cocoa

struct Vector {
var x: Double
var y: Double
}
```

Much like C structures, Swift structures are composite data types. They are composed of one or more fields, or properties, each of which has a specified type. A few lines down, create an instance of Vector and access its properties:

```let gravity = Vector(x: 0.0, y: -9.8)    // {x 0, y -9.800000000000001}
gravity.x                                   // 0
gravity.y                                   // -9.800000000000001
```

You just used Swift’s automatic initializer to create an instance of this structure. The automatic initializer has a parameter for each property in the structure. If you were to add a z field, this code would cause a compiler error because it lacks a z parameter. (Do not worry about the zeros; that is just typical floating point fun.)

You can provide your own initializers, but when you do, the automatic initializer is no longer provided. Go back to Vector and add an initializer that takes no parameters and initializes x and y to 0.

```struct Vector {
var x: Double
var y: Double

init() {
x = 0
y = 0
}
}
```

Initializers in Swift use the init keyword, followed by the parameter list, and then the body of the initializer. Within the body, the x and y properties are assigned directly.

An initializer must initialize all of the properties of its structure.

As we warned, defining this initializer has caused the automatic one to vanish, causing an error in the playground. You can easily define it manually, however:

```struct Vector {
var x: Double
var y: Double

init() {
x = 0
y = 0
}

init(x: Double, y: Double) {
self.x = x
self.y = y
}
}
```

A Swift programmer would say that this initializer takes two parameters, x and y, both of type Double.

What is self? It represents the instance of the type that is being initialized. Using self.propertyName is usually unnecessary (you did not use it in init()), but because the initializer’s parameter names match the names of the properties you must use self to tell the compiler that you mean the property and not the parameter.

Before continuing, let’s make an improvement. As the Vector structure stands, its two initializers have independent code paths. It would be better to have them use one code path by having the parameterless initializer call the initializer which takes both x and y.

```struct Vector {
var x: Double
var y: Double

init() {
x = 0
y = 0
self.init(x: 0, y: 0)
}

init(x: Double, y: Double) {
self.x = x
self.y = y
}
}
```

A single code path for initialization is not required for structures, but it is a good habit to get into as you will use it when working with classes.

### Instance methods

Methods allow you to add functionality to your data types. In Swift, you can add methods to structures as well as classes (and enums!). Instance methods operate within the context of a single instance of the type. Add an instance method for multiplying a vector by a scalar:

```struct Vector {
...

init(x: Double, y: Double) {
self.x = x
self.y = y
}

func vectorByAddingVector(vector: Vector) -> Vector {
return Vector(x: self.x + vector.x,
y: self.y + vector.y)
}
}
```

The func keyword in the context of a structure indicates that this is a method. It takes a single parameter of type Double and returns an instance of Vector.

Try this new method out:

```let gravity = Vector(x: 0.0, y: -9.8)       // {x 0, y -9.800000000000001}
gravity.x
gravity.y
let twoGs = gravity.vectorByAddingVector(gravity)  // {x 0, y -19.6}
```

What is the name of this method? In conversation you would call it vectorByAddingVector, but in this text we include parameters, like this: vectorByAddingVector(_:). By default, the first parameter of a method is not named – thus the underscore.

Why not name the first parameter? Because the convention – inherited from Objective-C and Cocoa – is that the base name of the method includes the name of the first parameter, in this case Vector. Suppose you added another parameter to that method. What would it look like?

```func vectorByAddingVector(vector: Vector, numberOfTimes: Int) -> Vector {
var result = self
for _ in 0..<numberOfTimes {
...
```

This method would be called vectorByAddingVector(_:numberOfTimes:). Note that there is a colon for each parameter.

This can lead to verbose method names, but the code actually becomes very readable. No guessing or relying on the IDE to tell you what the third parameter is!

By default, each parameter’s internal name is the same as its external name (except the first parameter, that is). In vectorByAddingVector(_:numberOfTimes:), the second parameter is named numberOfTimes. That is certainly very descriptive, but you might prefer to use a shorter name (like times) within the method. In that case you would explicitly set the internal parameter name like this:

```func vectorByAddingVector(vector: Vector, numberOfTimes times: Int) -> Vector {
var result = self
for _ in 0..<times {
...
```

The method’s signature has not changed. For those calling it, its name is still vectorByAddingVector(_:numberOfTimes:), but internally you have the satisfaction of using the name you want.

#### Using self in instance methods

As in initializers, self represents the instance that the method is being called on. As long as there is no conflict with named parameters or local variables, however, it is entirely optional, so we prefer to leave it off. Make this change to vectorByAddingVector(_:).

```struct Vector {
...

func vectorByAddingVector(vector: Vector) -> Vector {
return Vector(x: self.x + vector.x,
y: self.y + vector.y)
return Vector(x: x + vector.x,
y: y + vector.y)
}
}
```

### InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.

## Overview

Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

## Collection and Use of Information

To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

### Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

### Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.

### Surveys

Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

### Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.

If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

### Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

### Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

## Other Collection and Use of Information

### Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

### Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

### Do Not Track

This site currently does not respond to Do Not Track signals.

## Security

Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.

## Children

This site is not directed to children under the age of 13.

## Marketing

Pearson may send or direct marketing communications to users, provided that

• Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
• Such marketing is consistent with applicable law and Pearson's legal obligations.
• Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
• Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

## Correcting/Updating Personal Information

If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.

## Choice/Opt-out

Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

## Sale of Personal Information

Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

## Supplemental Privacy Statement for California Residents

California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

## Sharing and Disclosure

Pearson may disclose personal information, as follows:

• As required by law.
• With the consent of the individual (or their parent, if the individual is a minor)
• In response to a subpoena, court order or legal process, to the extent permitted or required by law
• To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
• In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
• To investigate or address actual or suspected fraud or other illegal activities
• To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
• To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
• To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.

This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.