Ranges in Swift explained with code examples

Learn about the 4 different range operators available

Thanks to Lukas for the great illustration

Ranges in Swift allow us to select parts of Strings, collections, and other types. They’re the Swift variant of NSRange which we know from Objective-C although they’re not exactly the same in usage, as I’ll explain in this blog post.

Ranges allow us to write elegant Swift code by making use of the range operator. Your first time working with them might be because you needed to select a range of characters from a String but there’s a lot more you can do with it!

Types of ranges

There are multiple types of ranges in Swift you can use. The easiest way of working with them is by making use of the range operator. Let’s go over the different types available in Swift.

Closed range operator going from a…b

let range: ClosedRange = 0...10
print(range.first!) // 0
print(range.last!) // 10

A closed range operator going from a...b defines a range that includes both a and b in which a must not be greater than b.

The closed operator is useful if you’d like to use all the values. For example, if you’d like to iterate over all elements of a collection:

let names = ["Antoine", "Maaike", "Jaap"]
for index in 0...2 {
    print("Name \(index) is \(names[index])")
}
// Name 0 is Antoine
// Name 1 is Maaike
// Name 2 is Jaap

The different types of operators can also be used to select elements from a collection. For this, however, we need to make use of the CountableClosedRange type:

let names = ["Antoine", "Maaike", "Jaap"]
let range: CountableClosedRange = 0...2
print(names[range]) // ["Antoine", "Maaike", "Jaap"]

Obviously, Swift is smart enough to detect the countable variant by itself. Therefore, you could write the above code as follows:

let names = ["Antoine", "Maaike", "Jaap"]
print(names[0...2]) // ["Antoine", "Maaike", "Jaap"]

Half-open range operator going from a..<b

let range: Range = 0..<10
print(range.first!) // 0
print(range.last!) // 9

A half-open range defines a range going from a to b but does not include b. It's named half-open as it's containing its first value but not its final value. Just like with the closed range, the value of a must not be greater than b.

The half-open operator can be used to iterate over zero-based lists such as arrays and collections in Swift in which you want to iterate up to but not including the length of the list. It’s basically the same as the earlier code example but now we can make use of the count property:

let names = ["Antoine", "Maaike", "Jaap"]
print(names[0..<names.count]) // ["Antoine", "Maaike", "Jaap"]

If we would’ve done the same with a closed operator we would run into the following error:

Fatal error: Array index is out of range

One-sided operator going from a…

A one-sided range operator only defines one side of the bounds, for example, a... or ...b. A one-sided range goes as far as possible in one direction—for example, taking all the elements of an array from the start of the array to index 2:

let names = ["Antoine", "Maaike", "Jaap"]
print(names[...2]) // ["Antoine", "Maaike", "Jaap"]

Or taking all the elements starting from index 1 till the end of the array:

let names = ["Antoine", "Maaike", "Jaap"]
print(names[1...]) // ["Maaike", "Jaap"]

A one-sided range can be used for iteration but only if used with a starting value a.... Otherwise, it's unclear where the iteration should start. Iterating over a one-sided range requires you to manually check where the loop should end as it would otherwise continue indefinitely.

let neededNames = 2
var collectedNames: [String] = []
for index in 0... {
    guard collectedNames.count != neededNames else { break }
    collectedNames.append(names[index])
}
print(collectedNames) // ["Antoine", "Maaike"]

Converting a Range to an NSRange in Swift

Sooner or later you might run into an issue when you want to convert a Range into a NSRange type. For example, if you're working with an NSAttributedString in which you like to apply attributes to a specific range. In the following example, we'd like to apply an orange color to "Swift" in the title:

let title = "A Swift Blog"
let range = title.range(of: "Swift")
let attributedString = NSMutableAttributedString(string: title)
attributedString.setAttributes([NSAttributedString.Key.foregroundColor: UIColor.orange], range: range) // Cannot convert value of type 'Range<String.Index>?' to expected argument type 'NSRange' (aka '_NSRange')

As Range can't be converted to NSRange we're running into the following error:

Cannot convert value of type ‘Range?’ to expected argument type ‘NSRange’ (aka ‘_NSRange’)

We can fix this by making use of the available convenience initializer of an NSRange that takes a Swift Range:

let convertedRange = NSRange(range, in: title)

The final code will look as follows:

let title = "A Swift Blog"
let range = title.range(of: "Swift")!
let convertedRange = NSRange(range, in: title)
let attributedString = NSMutableAttributedString(string: title)
attributedString.setAttributes([NSAttributedString.Key.foregroundColor: UIColor.orange], range: convertedRange)
print(attributedString)
// A {
// }Swift{
//     NSColor = "UIExtendedSRGBColorSpace 1 0.5 0 1";
// } Blog{
// }

Ranges and Strings

Strings and ranges are a bit more special. As you might know, a String is actually a collection of characters. However, not every character is of the same size. We can demonstrate this by working with a NSRange and a NSString that contains an emoji:

let emojiText: NSString = "🚀launcher"
print(emojiText.substring(with: NSRange(location: 0, length: 2)))
// Expected: 🚀l
// Actually returns: 🚀

As you can see, the rocket emoji is more than 1 character long. Therefore, our substring is not returning the expected outcome.

Working with String indexes

The solution to this problem is to make use of Range<String.Index> instead of Range<Int>. The String.Index takes into account the actual size of a character. We can only make use of a half-open Range as that's required by the String subscript.

let emojiText = "🚀launcher"
let endIndex = emojiText.index(emojiText.startIndex, offsetBy: 2)
let range: Range<String.Index> = emojiText.startIndex..<endIndex
print(emojiText[range]) // 🚀l

Conclusion

That was it! Hopefully, you’ve learned a bit more about the possibilities that Swift gives us when working with ranges and collections. We used closed, half-open, and one-sided operators that all have their pros and cons.

If you like to improve your Swift knowledge, even more, check out the Swift category page. Feel free to contact me or tweet to me on Twitter if you have any additional tips or feedback.

Thanks!

Originally published at SwiftLee.

More posts and updates: @twannl