How to create a struct for this json data in swift4?

Question

I create a very simple json data for practice, but it always decode error when JSONDecoder().decode. I try some way to modify my struct, but all gets same error(prints "error0"). The code is at below.

struct ss : Codable {
    var a : String
    var b : String
}


let js = "[{\"a\":\"1\",\"b\":\"2\"},{\"c\":\"3\",\"d\":\"4\"}]"
let data = js.data(using: .utf8)

let a = [ss].self
do {
    if let s = try? JSONDecoder().decode(a, from : data!) {
        print(s[0].a)
    }else{
        print("error0")
    }
}catch{
    print("error1")
}

Catch the error. There is a try that you don't listen to. And print the caught error. — Larme
– Larme, Commented Jan 3, 2019 at 9:33
It should say that there is no value found for key "a". That's because on the second dictionary, the keys are "c" and "d", not "a" and "b". — Larme
– Larme, Commented Jan 3, 2019 at 9:34
Your should fix the JSON. the second item in the JSON array does not have the a and b properties. Or you can make the properties optional and will be nil. — Milander
– Milander, Commented Jan 3, 2019 at 9:42
print("error1"): No, do print("error1: \(error)"). There is an error object that can have information, usefull information and can differ. So read it, no "error1" only. — Larme
– Larme, Commented Jan 3, 2019 at 9:49

Dharmesh Kheni · Accepted Answer · 2019-01-03 09:49:35Z

3

There is a problem with your JSON replace

let js = "[{\"a\":\"1\",\"b\":\"2\"},{\"c\":\"3\",\"d\":\"4\"}]"

with

let js = "[{\"a\":\"1\",\"b\":\"2\"},{\"a\":\"3\",\"b\":\"4\"}]"

The other dictionary don't have keys a and b and that's why JSONDecoder is not able to decode Now your update code will be:

struct ss : Codable {
    var a : String
    var b : String
}


let js = "[{\"a\":\"1\",\"b\":\"2\"},{\"a\":\"3\",\"b\":\"4\"}]"
let data = js.data(using: .utf8)

let a = [ss].self

do {
    let jsonDecoder = JSONDecoder()
    let s = try jsonDecoder.decode(a, from: data!)
    print(s[0].a) //"1\n"
} catch {
    print(error)
}

PS: As @Milander suggested If you don't want to fix JSON you can make optional properties in your Struct like

struct ss : Codable {
    let a, b, c, d: String?
}

edited Jan 3, 2019 at 9:49

answered Jan 3, 2019 at 9:40

Dharmesh Kheni

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1 Comment

Dharmesh Kheni Over a year ago

Happy to help you!!

SPatel · Accepted Answer · 2019-01-03 10:06:25Z

You can define additional keys like below:

No optional No replacement

struct ss : Codable {
    var a : String
    var b : String

    init(from decoder: Decoder) throws {
        if let con = try? decoder.container(keyedBy: CodingKeys.self), let a = try? con.decode(String.self, forKey: .a), let b = try? con.decode(String.self, forKey: .b) {
            self.a = a
            self.b = b
        } else if let con = try? decoder.container(keyedBy: AdditionalInfoKeys.self), let c = try? con.decode(String.self, forKey: .c), let d = try? con.decode(String.self, forKey: .d) {
            a = c
            b = d
        } else {
            throw NSError(domain: "Decoding error", code: 123, userInfo: nil)
        }
    }

    enum AdditionalInfoKeys: String, CodingKey {
        case c, d
    }
}

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