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

[Kenny Leung]

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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