Notice: Undefined index: understanding-swift in /var/sites/hackingwithswift.com/site/src/Controller/NewBooksReadController.php on line 89
Why does Swift use underscores with loops? - a free tutorial

NEW: Subscribe to Hacking with Swift+ and accelerate your learning! >>

Why does Swift use underscores with loops?

Paul Hudson    @twostraws   

If you want to loop over items in an array, you might write code such as this:

let names = ["Sterling", "Cyril", "Lana", "Ray", "Pam"]

for name in names {
    print("\(name) is a secret agent")
}

Every time the loop goes around, Swift will take one item from the names array, put it into the name constant, then execute the body of our loop – that’s the print() method.

However, some times you don’t actually need the value that is currently being read, which is where the underscore comes in: Swift will recognize you don’t actually need the variable, and won’t make the temporary constant for you.

Of course, Swift can really see that anyway – it can see whether or not you’re using name inside the loop, so it can do the same job without the underscore. However, using an underscore does something very similar for our brain: we can look at the code and immediately see the loop variable isn’t being used, no matter how many lines of code are inside the loop body.

So, if you don’t use a loop variable inside the body, Swift will warn you to rewrite it like this:

let names = ["Sterling", "Cyril", "Lana", "Ray", "Pam"]

for _ in names {
    print("[CENSORED] is a secret agent!")
}
Hacking with Swift is sponsored by Instabug

SPONSORED Are you tired of wasting time debugging your Swift app? Instabug’s SDK is here to help you minimize debugging time by providing you with complete device details, network logs, and reproduction steps with every bug report. All data is attached automatically, and it only takes a line of code to setup. Start your free trial now and get 3 months off exclusively for the Hacking with Swift Community.

Start your free trial!

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

BUY OUR BOOKS
Buy Pro Swift 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 (Vapor Edition) 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 Server-Side Swift (Kitura Edition) Buy Beyond Code

Was this page useful? Let us know!

Link copied to your pasteboard.