A lightweight collection of key-value pairs.
SDK
- Xcode 10.2+
Framework
- Swift Standard Library
Declaration
@frozen struct KeyValuePairs<Key, Value>
Overview
Use a Key instance when you need an ordered collection of key-value pairs and don’t require the fast key lookup that the Dictionary type provides. Unlike key-value pairs in a true dictionary, neither the key nor the value of a Key instance must conform to the Hashable protocol.
You initialize a Key instance using a Swift dictionary literal. Besides maintaining the order of the original dictionary literal, Key also allows duplicates keys. For example:
let recordTimes: KeyValuePairs = ["Florence Griffith-Joyner": 10.49,
"Evelyn Ashford": 10.76,
"Evelyn Ashford": 10.79,
"Marlies Gohr": 10.81]
print(recordTimes.first!)
// Prints "("Florence Griffith-Joyner", 10.49)"
Some operations that are efficient on a dictionary are slower when using Key. In particular, to find the value matching a key, you must search through every element of the collection. The call to first in the following example must traverse the whole collection to find the element that matches the predicate:
let runner = "Marlies Gohr"
if let index = recordTimes.firstIndex(where: { $0.0 == runner }) {
let time = recordTimes[index].1
print("\(runner) set a 100m record of \(time) seconds.")
} else {
print("\(runner) couldn't be found in the records.")
}
// Prints "Marlies Gohr set a 100m record of 10.81 seconds."
Key-Value Pairs as a Function Parameter
When calling a function with a Key parameter, you can pass a Swift dictionary literal without causing a Dictionary to be created. This capability can be especially important when the order of elements in the literal is significant.
For example, you could create an Int structure that holds a list of two-integer tuples and use an initializer that accepts a Key instance.
struct IntPairs {
var elements: [(Int, Int)]
init(_ elements: KeyValuePairs<Int, Int>) {
self.elements = Array(elements)
}
}
When you’re ready to create a new Int instance, use a dictionary literal as the parameter to the Int initializer. The Key instance preserves the order of the elements as passed.
let pairs = IntPairs([1: 2, 1: 1, 3: 4, 2: 1])
print(pairs.elements)
// Prints "[(1, 2), (1, 1), (3, 4), (2, 1)]"