Updating Strings for Swift 3

I wrote a Swift String Cheat Sheet last year to help me remember how to use one of the more complex API’s in the Swift standard library. It has undergone some significant changes with Swift 3 making for some painful code migration. This is partly down to the new API naming guidelines but also because of a new model for the way collections, indices and ranges work.

These are my notes on what I needed to change to update the Swift playground for Swift 3.

The Great Renaming

Applying the new API guidelines to the standard library has changed many of the String properties and methods. I will not mention every name change here as most are easily fixed by Xcode. Here are a couple of typical changes to give you the idea:

Initializing a String

The standard library replaces the String initializer init(count:repeatedValue) with init(repeating:count). The repeatedValue is also now a String instead of a character which makes it more flexible:

// Swift 2
let h = String(count:3, repeatedValue:"0") // "000"

// Swift 3
let h = String(repeating:"01", count:3)    // 010101

Converting to upper/lower case

The properties uppercaseString and lowercaseString are now functions and renamed to uppercased() and lowercased():

let mixedCase = "AbcDef"

// Swift 2
// let upper = mixedCase.uppercaseString // "ABCDEF"
// let lower = mixedCase.lowercaseString // "abcdef"

// Swift 3
let upper = mixedCase.uppercased()       // "ABCDEF"
let lower = mixedCase.lowercased()       // "abcdef"

I’ll cover some of the other name changes as we go.

Using Index to Traverse a Collection

One of the biggest changes impacting String in Swift 3 is the new model for collections and indices. To recap you cannot directly access an element in a String instead you need to use an index on one of the collection views:

let country = "EspaƱa"
country.characters       // characters
country.unicodeScalars   // Unicode scalar 21-bit codes
country.utf16            // UTF-16 encoding
country.utf8             // UTF-8 encoding

Unchanged with Swift 3 are the startIndex and endIndex properties on each collection view:

let hello = "hello"
let helloStartIndex = hello.characters.startIndex // 0

If you want the character view you can also omit the characters property:

let startIndex = hello.startIndex // 0
let endIndex = hello.endIndex     // 5
hello[startIndex]                 // "h"

What has changed is the way you advance an index to traverse over a string view. The successor(), predecessor() and advancedBy(n) functions are all gone.

// Swift 2
hello[hello.startIndex]                // "h"
hello[hello.startIndex.successor()]    // "e"
hello[hello.endIndex.predecessor()]    // "o"
hello[hello.startIndex.advancedBy(2)]  // "l"

In Swift 3 you now use index(after:) and index(before:) and index(_:offsetBy:) to achieve the same results:

// Swift 3
hello[hello.startIndex]                // "h"
hello[hello.index(after: startIndex)]  // "e"
hello[hello.index(before: endIndex)]   // "o"

hello[hello.index(startIndex, offsetBy: 1)]  // "e"
hello[hello.index(endIndex, offsetBy: -4)]   // "e"

You can also limit the offset to avoid an error when you run off the end of the index. The function index(_:offsetBy:limitedBy:) returns an optional which will be nil if you go too far:

if let someIndex = hello.index(startIndex,
                   offsetBy: 4, limitedBy: endIndex) {
  hello[someIndex] // "o"

To find the index of the first matching element (in this case a character):

let matchedIndex = hello.characters.index(of: "l") // 2
let nomatchIndex = hello.characters.index(of: "A") // nil

Finally the method to get the distance between two indices has been renamed:

// Swift 2
let distance = word1.startIndex.distanceTo(indexC)

// Swift 3
let distance = word1.distance(from: word1.startIndex, to: indexC)

Using Ranges

Ranges have changed in Swift 3. Suppose I have a start index (lowerBound) and end index (upperBound) on a characters view:

let fqdn = "useyourloaf.com"
let tldEndIndex = fqdn.endIndex
let tldStartIndex = fqdn.index(tldEndIndex, offsetBy: -3)

The full initializer to create a range from the lower and upper bounds:

let range = Range(uncheckedBounds: (lower: tldStartIndex, upper: tldEndIndex))
fqdn[range]  // "com"

The easier way to create a range is with the ..< and ... operators:

let endOfDomain = fqdn.index(fqdn.endIndex, offsetBy: -4)
let rangeOfDomain = fqdn.startIndex ..< endOfDomain
fqdn[rangeOfDomain] // useyourloaf

Checking for and returning the range of a matching substring:

if let rangeOfTLD = fqdn.range(of: "com") {
  let tld = fqdn[rangeOfTLD]                 // "com"


You can find the fully updated playground in my Code Examples repository. I have also updated the original post.

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