BLACK FRIDAY SALE: Save big on all my Swift books and bundles! >>

Why does Swift have default values for dictionaries?

Paul Hudson    @twostraws   

Updated for Xcode 14.1

Whenever you read a value from a dictionary, you might get a value back or you might get back nil – there might be no value for that key. Having no value can cause problems in your code, not least because you need to add extra functionality to handle missing values safely, and that’s where dictionary default values come in: they let you provide a backup value to use for when the key you ask for doesn’t exist.

For example, here’s a dictionary that stores the exam results for a student:

let results = [
    "english": 100,
    "french": 85,
    "geography": 75
]

As you can see, they sat three exams and scored 100%, 85%, and 75% for English, French, and Geography. If we wanted to read their history score, how we do it depends on what we want:

  1. If a missing value means the student failed to take the test, then we could use a default value of 0 so that we always get an integer back.
  2. If a missing value means the student has yet to take the exam, then we should skip the default value and instead look for a nil value.

So, it’s not like you always need a default value when working with dictionaries, but when you do it’s easy:

let historyResult = results["history", default: 0]
Hacking with Swift is sponsored by RevenueCat

SPONSORED In-app subscriptions are a pain to implement, hard to test, and full of edge cases. RevenueCat makes it straightforward and reliable so you can get back to building your app. Oh, and it's free if your app makes less than $10k/mo.

Learn more

Sponsor Hacking with Swift and reach the world's largest Swift community!

BUY OUR BOOKS
Buy Pro Swift Buy Pro SwiftUI Buy Swift Design Patterns Buy Testing Swift Buy Hacking with iOS Buy Swift Coding Challenges Buy Swift on Sundays Volume One Buy Server-Side Swift Buy Advanced iOS Volume One Buy Advanced iOS Volume Two Buy Advanced iOS Volume Three Buy Hacking with watchOS Buy Hacking with tvOS Buy Hacking with macOS Buy Dive Into SpriteKit Buy Swift in Sixty Seconds Buy Objective-C for Swift Developers Buy Beyond Code

Was this page useful? Let us know!

Average rating: 4.6/5

 
Unknown user

You are not logged in

Log in or create account
 

Link copied to your pasteboard.