How to decode JSON nested dictionary array with extension:decodable?

Question

I’ve got JSON like this:

{
  "states" : [
    "C",
    "A",
    "B",
    "Sink",
  ],
  "symbols" : [
    "c",
    "a",
    "b"
  ],
  "transitions" : [
    {
      "with" : "c",
      "to" : "B",
      "from" : "C"
    },
    {
      "with" : "c",
      "to" : "Sink",
      "from" : "C"
    },
    {
      "with" : "b",
      "to" : "B",
      "from" : "B"
    },
    {
      "with" : "b",
      "to" : "Sink",
      "from" : "B"
    },
    {
      "with" : "c",
      "to" : "C",
      "from" : "A"
    },
    {
      "with" : "c",
      "to" : "Sink",
      "from" : "A"
    },
    {
      "with" : "a",
      "to" : "A",
      "from" : "A"
    },
    {
      "with" : "a",
      "to" : "Sink",
      "from" : "A"
    }
  ],
  "initialState" : "A",
  "finalStates" : [
    "B"
  ]
}

I’m unable to decode transitions part the JSON(I need to decode it way that apple does in here). What I’ve got so far is this(commented part is leading error typeMismatch(Swift.Dictionary<Swift.String, Any>, Swift.DecodingError.Context(codingPath: [], debugDescription: "Expected to decode Dictionary<String, Any> but found an array instead.", underlyingError: nil)) )

public struct FiniteAutomata {
    let states:[String]
    let symbols:[String]
    let initialState:String
    let finalStates:[String]
    /*
    let with:[String]
    let from:[String]
    let to:[String]
     */

    enum CodingKeys: String, CodingKey {
        case states
        case symbols
        case initialState
        case finalStates
        case transitions
    }
    /*
    enum transitionInfoKeys: String, CodingKey{
        case with
        case to
        case from
    }*/
}

extension FiniteAutomata: Decodable {
    public init(from decoder: Decoder)throws{
        let decoderContainer = try decoder.container(keyedBy: CodingKeys.self)
        states = try decoderContainer.decode([String].self, forKey: .states)
        symbols = try decoderContainer.decode([String].self, forKey: .symbols)
        initialState = try decoderContainer.decode(String.self, forKey: .initialState)
        finalStates = try decoderContainer.decode([String].self, forKey: .finalStates)
        /*
        let nestedContainer = try decoderContainer.nestedContainer(keyedBy: transitionInfoKeys.self, forKey: .transitions)
        with = try nestedContainer.decode([String].self, forKey: .with)
        to = try nestedContainer.decode([String].self, forKey: .to)
        from = try nestedContainer.decode([String].self, forKey: .from)
         */
    }
}

It's unclear what you want in the end. Why do you want to flatten transitions? Why note have struct Transition: Codable { let with: String; let to: String; let from: String } and in FiniteAutomata: let transitions: [Transition] ? let with:[String] is not a good idea, because you'll loose the synchronization with from and to. — Larme
– Larme, Commented Mar 22, 2021 at 11:36

Joakim Danielson · Accepted Answer · 2021-03-22 11:47:44Z

1

You need

struct Transition: Decodable {
    let with,to,from:String
}

Then

let transitions : [Transition]

Also Remove

public init(from decoder: Decoder)throws{
    let decoderContainer = try decoder.container(keyedBy: CodingKeys.self)
    states = try decoderContainer.decode([String].self, forKey: .states)
    symbols = try decoderContainer.decode([String].self, forKey: .symbols)
    initialState = try decoderContainer.decode(String.self, forKey: .initialState)
    finalStates = try decoderContainer.decode([String].self, forKey: .finalStates)
    /*
    let nestedContainer = try decoderContainer.nestedContainer(keyedBy: transitionInfoKeys.self, forKey: .transitions)
    with = try nestedContainer.decode([String].self, forKey: .with)
    to = try nestedContainer.decode([String].self, forKey: .to)
    from = try nestedContainer.decode([String].self, forKey: .from)
     */
}

as this will occur automatically no need to write it manually

edited Mar 22, 2021 at 11:47

Joakim Danielson

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answered Mar 22, 2021 at 11:37

Shehata Gamal

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Comments

vadian · Accepted Answer · 2021-03-22 11:36:56Z

1

The value for key transition is an array of dictionaries which becomes an array of a struct

struct Transition : Decodable {
   let with, to, from : String
}

Then decode

let transitions : [Transition]

All CodingKeys and the custom init method is not needed

answered Mar 22, 2021 at 11:36

vadian

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1 Comment

Makaveli_O Over a year ago

Thanks I had no idea that init is being executed automatically

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