[swift-users] Recommended way to collect elements into collection-valued Dictionary?

[Ben Cohen]

Hi – yes, there are a couple of things you can do:

You don’t have to rely on ExpressibleByArrayLiteral. RangeReplaceableCollection guarantees an empty init, so instead of [], you can write Value().

And the new 4.0 subscript that takes a default value allows you do avoid the if d[k] == nil dance for the initial value.

So you can write this as:

extension Dictionary where Value: RangeReplaceableCollection {
    mutating func append(value: Value.Element, for key: Key) {
        self[key, default: Value()].append(value)
    }
}




> On Jun 15, 2017, at 3:12 PM, Jens Persson via swift-users <swift-users at swift.org> wrote:
> 
> let question = """
>     Please see the extension to Dictionary below.
>     Is there a simpler or better way to accomplish the same thing?
>     """
> extension Dictionary where
>     Value: RangeReplaceableCollection,
>     Value: ExpressibleByArrayLiteral,
>     Value.Element == Value.ArrayLiteralElement
> {
>     mutating func append(value: Value.Element, for key: Key) {
>         if self[key]?.append(value) == nil {
>             self[key] = [value]
>         }
>     }
> }
> func usageExample() {
>     var characterIndexMap = [Character : [Int]]()
>     for (ci, c) in question.characters.enumerated() {
>         characterIndexMap.append(value: ci, for: c)
>     }
>     for (character, indices) in characterIndexMap
>         .sorted(by: { $0.1.count > $1.1.count })
>     {
>         print("\(character.debugDescription) occurs at indices:", indices)
>     }
> }
> usageExample()
> 
> 
> /* Will print:
> " " occurs at indices: [6, 10, 14, 24, 27, 38, 48, 54, 56, 64, 67, 74, 78, 81, 92, 96, 101]
> "e" occurs at indices: [2, 5, 8, 9, 13, 15, 18, 40, 51, 53, 62, 69, 72, 95, 100]
> "t" occurs at indices: [11, 17, 25, 31, 49, 70, 71, 79, 93, 102]
> "s" occurs at indices: [4, 7, 20, 47, 57, 90, 97]
> "o" occurs at indices: [22, 26, 33, 42, 65, 80, 85]
> "i" occurs at indices: [21, 29, 32, 58, 89, 104]
> "a" occurs at indices: [3, 35, 55, 76, 82, 98]
> "h" occurs at indices: [12, 50, 91, 94, 103]
> "r" occurs at indices: [36, 52, 63, 66, 73]
> "l" occurs at indices: [1, 41, 61, 88]
> "n" occurs at indices: [19, 23, 34, 105]
> "m" occurs at indices: [59, 86, 99]
> "c" occurs at indices: [30, 83, 84]
> "p" occurs at indices: [60, 87]
> "w" occurs at indices: [43, 75]
> "y" occurs at indices: [37, 77]
> "b" occurs at indices: [39, 68]
> "." occurs at indices: [44]
> "\n" occurs at indices: [45]
> "I" occurs at indices: [46]
> "x" occurs at indices: [16]
> "?" occurs at indices: [107]
> "g" occurs at indices: [106]
> "D" occurs at indices: [28]
> "P" occurs at indices: [0]
> */
> 
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