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What are KeyValuePairs?

Swift version: 5.2

Paul Hudson    @twostraws   

KeyValuePairs, somewhat confusingly known as DictionaryLiteral before Swift 5.0, is a useful data type that provides dictionary-like functionality with a handful of benefits:

  • Your keys don’t need to conform to Hashable.
  • You can add items with duplicate keys.
  • The order in which you add items is preserved.

However, because KeyValuePairs doesn’t require its keys to be hashable, you don’t get the fast key look up of a regular dictionary – it’s O(n) rather than O(1) if you like Big O notation. Instead, you access them like an array, using numerical indexes.

As an example, here’s a KeyValuePairs instance holding the name of a singer:

let singer: KeyValuePairs = ["firstName": "Taylor", "lastName": "Swift"]

If we wanted to access the key name and value for the first item, we’d write something like this:

print("\(singer[0].key) is set to \(singer[0].value)")

Even better, because KeyValuePairs inherits from the Sequence protocol we can loop over all keys and values, like this:

for value in singer {
    print("\(value.key) is set to \(value.value)")
}
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