[swift-evolution] map-like operation that returns a dictionary

Thorsten Seitz tseitz42 at icloud.com
Tue Jan 12 12:11:55 CST 2016 

struct Person {
    var id: Int
    var name: String
}
let tom   = Person(id: 1, name: "Tom")
let dick  = Person(id: 2, name: "Dick")
let harry = Person(id: 3, name: "Harry")
let people = [tom, dick, harry]

extension SequenceType {
    
    func toDict<Key: Hashable>(@noescape withKey extractKey: Self.Generator.Element -> Key) 
	-> [Key:Self.Generator.Element]
    {
        return toDict { element in (extractKey(element), element) }
    }
    
    func toDict<Key: Hashable, Value>(@noescape mapping: Self.Generator.Element -> (Key, Value))
        -> [Key:Value]
    {
        var result: [Key:Value] = [:]
        for element in self {
            let (key, value) = mapping(element)
            result.updateValue(value, forKey: key)
        }
        return result
    }
}

let cache = people.toDict { $0.id }
cache

I named the method(s) „toDict“ instead of „map“ because map normally returns a collection which is either the same as the receiver or a simple one.
The second version is more general and allows to do things like

let dict = ["Tom", "Dick", "Harry"].enumerate().toDict { (index, value) in (index + 1, value) }


-Thorsten


> Am 11.01.2016 um 17:56 schrieb Kenny Leung via swift-evolution <swift-evolution at swift.org>:
> 
> I would like to keep this lightweight for the consumer, so you could write:
> 
> let people :[Person] = fetchPeople()
> let cache :[Int:Person] = people.map {return $0.id}
> 
> -Kenny
> 
> 
>> On Jan 10, 2016, at 2:24 PM, Jo Albright via swift-evolution <swift-evolution at swift.org> wrote:
>> 
>> Possible solution to do what you are wanting. 
>> 
>> extension Array {
>> 
>>    func map<T>(var d: [String:T] = [:], @noescape transform: (Int,Array.Generator.Element) throws -> [String:T]) rethrows -> [String:T] {
>> 
>>        for (i,item) in self.enumerate() {
>> 
>>            try d += transform(i,item)
>> 
>>        }
>> 
>>        return d
>> 
>>    }
>> 
>> }
>> 
>> func += <T>(inout lhs: [String:T], rhs: [String:T]) -> [String:T] {
>> 
>>    for (k,v) in rhs { lhs[k] = v }; return lhs
>> 
>> }
>> 
>> 
>> let names = ["Jo","Jenna","Jake","Julie"]
>> let ages = [32,20,46,39]
>> 
>> let namesInfo1 = names.map { ["\($0)": $1] }
>> namesInfo1 // ["2": "Jake", "1": "Jenna", "0": "Jo", "3": "Julie"]
>> 
>> let namesInfo2 = names.map(["6":"Jim"]) { ["\($0)": $1] }
>> namesInfo2 // ["2": "Jake", "1": "Jenna", "0": "Jo", "6": "Jim", "3": "Julie"]
>> 
>> let namesInfo3 = names.map { [$1: ages[$0]] }
>> namesInfo3 // ["Jenna": 20, "Jo": 32, "Jake": 46, "Julie": 39]
>> 
>> Designer . Developer .  Nerd 
>> Jo Albright
>> 
>> 
>>> On Jan 10, 2016, at 4:25 PM, Loïc Lecrenier via swift-evolution <swift-evolution at swift.org> wrote:
>>> 
>>> I think this is O(n^2) though, so it’s not really usable.
>>> (It is destroying and copying the dictionary each time)
>>> 
>>> There really isn’t anything like map for dictionaries. 
>>> I am not convinced we should add it to the standard library though.
>>> 
>>>> On Jan 10, 2016, at 10:19 PM, Ian Ynda-Hummel via swift-evolution <swift-evolution at swift.org> wrote:
>>>> 
>>>> I think this probably wants to be a `reduce`. Given the above example:
>>>> 
>>>>   ["John", "Mike", "Amy", "Kavin"].enumerate().reduce([Int: String]()) { (var dictionary, data) in
>>>>       dictionary[data.index] = data.element
>>>>       return dictionary
>>>>   }
>>>> 
>>>> Which means you create an immutable dictionary with `let`. This could probably look nicer, but I think it illustrates the idea, at least.
>>>> 
>>>> -Ian
>>>> 
>>>> On Sun, Jan 10, 2016 at 2:46 PM Craig Cruden via swift-evolution <swift-evolution at swift.org> wrote:
>>>> There is no equivalent to something like 
>>>> 
>>>> array.zipWithIndex.toMap (or in this case array.zipWithIndex.toDictionary // where zipWithIndex creates tuples with the index - a specialized case of zip for Arrays with indexes.
>>>> 
>>>> 
>>>> 
>>>>> On 2016-01-11, at 2:41:59, Ross O'Brien via swift-evolution <swift-evolution at swift.org> wrote:
>>>>> 
>>>>> Or an enumeration followed by a forEach.
>>>>> ["John", "Mike", "Amy", Kavin"].enumerate().forEach {
>>>>>   dic[$0] = $1
>>>>> }
>>>>> That said, it requires creating a dictionary var first, not a let. If there was an initialiser for Dictionary which took an Array or EnumerateSequence, that might be useful. I'm not sure how I'd attempt to write such an initialiser though.
>>>>> 
>>>>> 
>>>>> On Sun, Jan 10, 2016 at 7:08 PM, Donnacha Oisín Kidney <swift-evolution at swift.org> wrote:
>>>>> I think that use of map is generally discouraged. forEach would probably be more explicit, or a for-loop.
>>>>> 
>>>>>> On 10 Jan 2016, at 18:58, 肇鑫 via swift-evolution <swift-evolution at swift.org> wrote:
>>>>>> 
>>>>>> You can use dictionary in a map. You just ignore the return value of the map.
>>>>>> 
>>>>>> var dic = [Int:String]()
>>>>>> var index = 0
>>>>>> 
>>>>>> ["John", "Mike", "Amy", "Kavin"].map {
>>>>>>   dic.updateValue($0, forKey: index)
>>>>>>   index += 1
>>>>>> }
>>>>>> 
>>>>>> print(dic) // [2: "Amy", 0: "John", 1: "Mike", 3: "Kavin"]
>>>>>> 
>>>>>> zhaoxin
>>>>>> 
>>>>>> On Mon, Jan 11, 2016 at 1:50 AM, Kenny Leung via swift-evolution <swift-evolution at swift.org> wrote:
>>>>>> Hi All.
>>>>>> 
>>>>>> I find that instead of using map() on arrays, I more often use an operation that returns a dictionary from an array. A common case is fetching an array of data, then creating a local cache of it indexed by ID.
>>>>>> 
>>>>>> Is there a name for this operation? Is this something that others would like to see added to the standard library?
>>>>>> 
>>>>>> -Kenny
>>>>>> 
>>>>>> _______________________________________________
>>>>>> swift-evolution mailing list
>>>>>> swift-evolution at swift.org
>>>>>> https://lists.swift.org/mailman/listinfo/swift-evolution
>>>>>> 
>>>>>> 
>>>>>> 
>>>>>> -- 
>>>>>> 
>>>>>> Owen Zhao
>>>>>> _______________________________________________
>>>>>> swift-evolution mailing list
>>>>>> swift-evolution at swift.org
>>>>>> https://lists.swift.org/mailman/listinfo/swift-evolution
>>>>> 
>>>>> 
>>>>> 
>>>>> _______________________________________________
>>>>> swift-evolution mailing list
>>>>> swift-evolution at swift.org
>>>>> https://lists.swift.org/mailman/listinfo/swift-evolution
>>>>> 
>>>>> 
>>>>> _______________________________________________
>>>>> swift-evolution mailing list
>>>>> swift-evolution at swift.org
>>>>> https://lists.swift.org/mailman/listinfo/swift-evolution
>>>> 
>>>> 
>>>> _______________________________________________
>>>> swift-evolution mailing list
>>>> swift-evolution at swift.org
>>>> https://lists.swift.org/mailman/listinfo/swift-evolution
>>>> _______________________________________________
>>>> swift-evolution mailing list
>>>> swift-evolution at swift.org
>>>> https://lists.swift.org/mailman/listinfo/swift-evolution
>>> 
>>> _______________________________________________
>>> swift-evolution mailing list
>>> swift-evolution at swift.org
>>> https://lists.swift.org/mailman/listinfo/swift-evolution
>> 
>> 
>> _______________________________________________
>> swift-evolution mailing list
>> swift-evolution at swift.org
>> https://lists.swift.org/mailman/listinfo/swift-evolution
> 
> _______________________________________________
> swift-evolution mailing list
> swift-evolution at swift.org
> https://lists.swift.org/mailman/listinfo/swift-evolution

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.swift.org/pipermail/swift-evolution/attachments/20160112/66a907d3/attachment.html>

More information about the swift-evolution mailing list