StructureSwift

KeyValuePairs

A lightweight collection of key-value pairs.

@frozen struct KeyValuePairs<Key, Value>

Overview

Use a KeyValuePairs 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 KeyValuePairs instance must conform to the Hashable protocol.

You initialize a KeyValuePairs instance using a Swift dictionary literal. Besides maintaining the order of the original dictionary literal, KeyValuePairs 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 "(key: "Florence Griffith-Joyner", value: 10.49)"

Some operations that are efficient on a dictionary are slower when using KeyValuePairs. In particular, to find the value matching a key, you must search through every element of the collection. The call to firstIndex(where:) 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 KeyValuePairs 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 IntPairs structure that holds a list of two-integer tuples and use an initializer that accepts a KeyValuePairs instance.

struct IntPairs {
    var elements: [(Int, Int)]

    init(_ elements: KeyValuePairs<Int, Int>) {
        self.elements = Array(elements)
    }
}

When you’re ready to create a new IntPairs instance, use a dictionary literal as the parameter to the IntPairs initializer. The KeyValuePairs 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)]"

Members

Typealiases

  • typealias Element

    The element type of a KeyValuePairs: a tuple containing an individual key-value pair.

  • typealias Index

    A type that represents a position in the collection.

  • typealias Indices

    A type that represents the indices that are valid for subscripting the collection, in ascending order.

  • typealias SubSequence

    A collection representing a contiguous subrange of this collection’s elements. The subsequence shares indices with the original collection.

Initializers

Instance Subscripts

Instance Properties

Instance Methods

Conforms To

Removed Members