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

[Jo Albright]

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
>>>> 
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>>>> https://lists.swift.org/mailman/listinfo/swift-evolution
>>>> 
>>>> 
>>>> 
>>>> -- 
>>>> 
>>>> Owen Zhao
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>>> 
>>> 
>>> 
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>> 
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