Using Swift decodable on nested arrays of strings

Question

I am attempting to decode an array of strings, where in the returned JSON is an array of strings but also contains nested arrays

Like:

{ "people": ["Alice", "Bob"], 
"departments": [["Accounts", "Sales"]]
}

My Swift code:

let decoder = JSONDecoder()
let model = try decoder.decode([String:[String]].self, from: dataResponse)
print(model as Any)

I want to be able to decode the departments, but each time I do it complains that:

Error typeMismatch(Swift.String, Swift.DecodingError.Context(codingPath: [_DictionaryCodingKey(stringValue: "departments", intValue: nil), _JSONKey(stringValue: "Index 0", intValue: 0)], debugDescription: "Expected to decode String but found an array instead.", underlyingError: nil))

I understand that this is because the decoder is expecting a string with an array of strings

I am wondering if I can also tell it to expect multiple, nested arrays of strings.

Pasting your JSON data into app.quicktype.io gives you a very reasonable start on how to model your Swift structs — Gereon
– Gereon, Commented Mar 3, 2019 at 17:30
I was not aware of this tool, but it looks great and much appreciated — zardon
– zardon, Commented Mar 3, 2019 at 17:52

Leo Dabus · Accepted Answer · 2019-03-03 17:20:11Z

3

You just need to create the appropriate structure and pass it to the decoder:

struct Root: Decodable {
    let people: [String]
    let departments: [[String]]
}

let decoder = JSONDecoder()
do {
    let model = try decoder.decode(Root.self, from: dataResponse)
     print(model.people)      // ["Alice", "Bob"]\n"
     print(model.departments) // [["Accounts", "Sales"]]\n"
} catch {
    print(error) 
}

answered Mar 3, 2019 at 17:20

Leo Dabus

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1 Comment

zardon Over a year ago

Ah I see. this has been bugging me for a while how decoder worked on multiple arrays - i can do it for single arrays, but it is appreciated and I certainly need to brush up on my skills

Paul B · Accepted Answer · 2021-10-27 19:38:06Z

1

If you don't want to create structs (need just a piece of data, for example) here is an approach to consider.

let jsonData =  """
{ "people": ["Alice", "Bob"],
"departments": [["Accounts", "Sales"]],
"stores": [["Atlanta", "Denver"]]
}
""".data(using: .utf8)

if let jsonObject = try? JSONSerialization.jsonObject(with: jsonData!, options: []) as? [String: Any] {
    if let people = jsonObject["people"] as? [String] {
        print(people)
    }
    if let departments = jsonObject["departments"] as? [[String]] {
        print(departments)
    }
}

edited Oct 27, 2021 at 19:38

answered Mar 3, 2019 at 18:21

Paul B

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2 Comments

Leo Dabus Over a year ago

let jsonData =  Data(""" { "people": ["Alice", "Bob"], "departments": [["Accounts", "Sales"]], "stores": [["Atlanta", "Denver"]] } """.utf8)

. Btw ptions: [] can be simply omitted and to unwrap jsonObject properly you need to add a parenthesis if let jsonObject = (try? JSONSerialization.jsonObject(with: jsonData)) as? [String: Any] {

Paul B Over a year ago

Good idea, @Leo. Brevity is good as log as clarity doesn't suffer.

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