[swift-evolution] map-like operation that returns a dictionary
Kenny Leung
kenny_leung at pobox.com
Wed Jan 13 10:26:43 CST 2016
Hmm…
Now that I look seriously at it, I have no idea how the first function works! Back to the books…
-Kenny
> On Jan 12, 2016, at 10:11 AM, Thorsten Seitz <tseitz42 at icloud.com> wrote:
>
> 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
>>>>>>
>>>>>>
>>>>>>
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>>>>>
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