Decoding and converting string to double in codable

Question

How can I decode the following JSON to a Codable object in Swift?

{"USD":"12.555", "EUR":"11.555"}

Here's the struct i'm using:

struct Prices: Codable {
    var USD: Double
    var EUR: Double
    
    private enum CodingKeys: String, CodingKey {
        case USD = "USD"
        case EUR = "EUR"
    }
    
    init() {
        self.USD = 0.0
        self.EUR = 0.0
    }
    
    init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)
        self.USD = try container.decode(Double.self, forKey: .USD)
        self.EUR = try container.decode(Double.self, forKey: .EUR)
    }
}

The error I'm getting is

Error typeMismatch(Swift.Double, Swift.DecodingError.Context(codingPath: [CodingKeys(stringValue: "USD", intValue: nil)], debugDescription: "Expected to decode Double but found a string/data instead.", underlyingError: nil))

score 2 · Accepted Answer · answered Jul 11 '20 at 11:22

I think your struct is incorrect and will be hard to maintain if you want to download more currency rates so I suggest a different approach. First create a struct for holding a currency rate

struct CurrencyRate {
    let currency: String
    let rate: Decimal?
}

Then decode the json as a dictionary and use map to convert it into an array of CurrencyRate

var rates = [CurrencyRate]()
do {
    let result = try JSONDecoder().decode([String: String].self, from: json)
    rates = result.map { CurrencyRate(currency: $0.key, rate: Decimal(string: $0.value))}

} catch {
    print(error)
}

Two notes about CurrencyRate

You have two currencies in a rate so normally you also have another property named baseCurrency or otherCurrency or something similar but if that other currency always is the same you can omit it.
Depending on your use case it might also be a good idea to make a new type for currency properties, Currency

score 1 · Answer 2 · answered Jul 11 '20 at 10:11

The values you're getting are Strings, so your struct should simply be

struct Prices: Codable {
    let USD: String
    let EUR: String
}

Depending you how you inted to use those price values, converting them to double may be inadvisable, because floating-point math is weird and Decimals may be better suited.

Frankenstein · Answer 3 · 2020-07-11T10:18:28.417

You have to decode the values as String and then cast it to Double. So, either Cast String to Double and provide a default value, like this:

init(from decoder: Decoder) throws {
    let container = try decoder.container(keyedBy: CodingKeys.self)
    self.USD = Double(try container.decode(String.self, forKey: .USD)) ?? 0
    self.EUR = Double(try container.decode(String.self, forKey: .EUR)) ?? 0
}

Or make the properties optional and remove the default values:

struct Prices: Codable {
    var USD, EUR: Double?

init(from decoder: Decoder) throws {
    let container = try decoder.container(keyedBy: CodingKeys.self)
    self.USD = Double(try container.decode(String.self, forKey: .USD))
    self.EUR = Double(try container.decode(String.self, forKey: .EUR))
}

Note: Also, you don't need CodingKeys as the property names are the same as keys in the data.

Alternatively, you can use the computed property method, like this:

struct Prices: Codable {
    var USDString: String
    var EURString: String

    private enum CodingKeys: String, CodingKey {
        case USDString = "USD"
        case EURString = "EUR"
    }

    var USD: Double? { Double(USDString) }
    var EUR: Double? { Double(EURString) }
}

Decoding and converting string to double in codable

3 Answers3