Invert keys for values in Swift dictionary, collecting duplicate values - swift

Question

I have the following dictionary:
let dict = ["key1": "v1", "key2": "v1", "key3": "v2"]
I want to swap the values for keys so that the result to be:
result = ["v1": ["key1", "key2"], "v2": ["key3"]]
How can I do this without using for loops (i.e. in a functional way)?

Answer 1

In Swift 4, Dictionary has the init(_:uniquingKeysWith:) initializer that should serve well here.
let d = [1 : "one", 2 : "two", 3 : "three", 30: "three"]
let e = Dictionary(d.map({ ($1, [$0]) }), uniquingKeysWith: {
(old, new) in old + new
})
If you did not have duplicate values in the original dict that needed to be combined, this would be even simpler (using another new initializer):
let d = [1 : "one", 2 : "two", 3 : "three"]
let e = Dictionary(uniqueKeysWithValues: d.map({ ($1, $0) }))

Answer 2

You can use grouping initializer in Swift 4:
let dict = ["key1": "v1", "key2": "v1", "key3": "v2"]
let result = Dictionary(grouping: dict.keys.sorted(), by: { dict[$0]! })
Two notes:
You can remove .sorted() if the order of keys in resulting arrays is not important.
Force unwrap is safe in this case because we're getting an existing dictionary key as the $0 parameter

Answer 3

This is a special application of the commonly-needed ability to group key-value pairs by their keys.
public extension Dictionary {
/// Group key-value pairs by their keys.
///
/// - Parameter pairs: Either `Swift.KeyValuePairs<Key, Self.Value.Element>`
/// or a `Sequence` with the same element type as that.
/// - Returns: `[ KeyValuePairs.Key: [KeyValuePairs.Value] ]`
init<Value, KeyValuePairs: Sequence>(grouping pairs: KeyValuePairs)
where
KeyValuePairs.Element == (key: Key, value: Value),
Self.Value == [Value]
{
self =
Dictionary<Key, [KeyValuePairs.Element]>(grouping: pairs, by: \.key)
.mapValues { $0.map(\.value) }
}
/// Group key-value pairs by their keys.
///
/// - Parameter pairs: Like `Swift.KeyValuePairs<Key, Self.Value.Element>`,
/// but with unlabeled elements.
/// - Returns: `[ KeyValuePairs.Key: [KeyValuePairs.Value] ]`
init<Value, KeyValuePairs: Sequence>(grouping pairs: KeyValuePairs)
where
KeyValuePairs.Element == (Key, Value),
Self.Value == [Value]
{
self.init( grouping: pairs.map { (key: $0, value: $1) } )
}
}
With that, just flip each key-value pair and go to town.
Dictionary( grouping: dict.map { ($0.value, $0.key) } )