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

Why does Swift use underscores with loops?

Paul Hudson    @twostraws   

Updated for Xcode 14.0 beta 1

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, sometimes 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 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: 4.5/5

 
Unknown user

You are not logged in

Log in or create account
 

Link copied to your pasteboard.