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

[Loïc Lecrenier]

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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>>> 
>>> -- 
>>> 
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>> 
>> 
>> 
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