> I.e. I suggest to implement and mapKeys() also. It could be also useful in
> some situations.
`mapKeys` is much more dangerous, because you could end up mapping many values
into a single key. You kind of need to combine the values somehow. Perhaps:
extension Dictionary {
func mapValues<OutValue>(_ valueTransform: @noescape Value
throws -> OutValue) rethrows -> [Key: OutValue] { … }
func mapKeys<OutKey: Hashable>(_ keyTransform: @noescape Key
throws -> OutKey) rethrows -> [OutKey: [Value]] { … }
// Possibly flatMap variants, too?
}
extension Dictionary where Value: Sequence {
func reduceValues<OutValue>(_ initial: OutValue, combine:
@noescape (OutValue, Value.Iterator.Element) throws -> OutValue) rethrows ->
[Key: OutValue] {
return mapValues { $0.reduce(initial, combine: combine)
}
}
}
Which you would end up using like this:
let wordFrequencies: [String: Int] = …
let firstLetterFrequencies: [Character: Int] = wordFrequencies.mapKeys
{ $0.characters.first! }.reduceValues(0, combine: +)
--
Brent Royal-Gordon
Architechies
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