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You create PartialRangeFrom instances by using the postfix range operator (postfix ...).

let atLeastFive = 5...

You can use a partial range to quickly check if a value is contained in a particular range of values. For example:

atLeastFive.contains(4)
// false
atLeastFive.contains(5)
// true
atLeastFive.contains(6)
// true

You can use a partial range of a collection's indices to represent the range from the partial range's lower bound up to the end of the collection.

let numbers = [10, 20, 30, 40, 50, 60, 70]
print(numbers[3...])
// Prints "[40, 50, 60, 70]"

Using a Partial Range as a Sequence

When a partial range uses integers as its lower and upper bounds, or any other type that conforms to the Strideable protocol with an integer stride, you can use that range in a for-in loop or with any sequence method that doesn't require that the sequence is finite. The elements of a partial range are the consecutive values from its lower bound continuing upward indefinitely.

func isTheMagicNumber(_ x: Int) -> Bool {
    return x == 3
}
 
for x in 1... {
    if isTheMagicNumber(x) {
        print("\(x) is the magic number!")
        break
    } else {
        print("\(x) wasn't it...")
    }
}
// "1 wasn't it..."
// "2 wasn't it..."
// "3 is the magic number!"

Because a PartialRangeFrom sequence counts upward indefinitely, do not use one with methods that read the entire sequence before returning, such as map(_:), filter(_:), or suffix(_:). It is safe to use operations that put an upper limit on the number of elements they access, such as prefix(_:) or dropFirst(_:), and operations that you can guarantee will terminate, such as passing a closure you know will eventually return true to first(where:).

In the following example, the asciiTable sequence is made by zipping together the characters in the alphabet string with a partial range starting at 65, the ASCII value of the capital letter A. Iterating over two zipped sequences continues only as long as the shorter of the two sequences, so the iteration stops at the end of alphabet.

let alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
let asciiTable = zip(65..., alphabet)
for (code, letter) in asciiTable {
    print(code, letter)
}
// "65 A"
// "66 B"
// "67 C"
// ...
// "89 Y"
// "90 Z"

The behavior of incrementing indefinitely is determined by the type of Bound. For example, iterating over an instance of PartialRangeFrom<Int> traps when the sequence's next value would be above Int.max.

let numbers = [10, 20, 30, 40, 50, 60, 70]
let upToFour = ..<4
 
let r1 = upToFour.relative(to: numbers)
// r1 == 0..<4

let numbersSuffix = numbers[2...]
// numbersSuffix == [30, 40, 50, 60, 70]
 
let r2 = upToFour.relative(to: numbersSuffix)
// r2 == 2..<4

let numbersPrefix = numbers[upToFour]
// numbersPrefix == [10, 20, 30, 40]