Strava API returns null as first value of powerstream - Swift

Question

When trying to use the Strava API with wattages per second I keep running into the problem of the first value always being null instead of a number.

struct streamsElement: Decodable {
    struct Stream: Decodable {
      let data: [Double]?
      let series_type: String
      let original_size: Int
      let resolution: String
      
    }

//decoding
if let wattsData = streams.watts?.data {
    for watts in wattsData {
        wattsArr.append("watts: \(watts)" )
    }
}

"watts":{"data":[
null,
144,
98,
95, etc.

This works fine for all the other streams.

Error

Error decoding JSON data: valueNotFound(Swift.Double, Swift.DecodingError.Context(codingPath: [CodingKeys(stringValue: "watts", intValue: nil), CodingKeys(stringValue: "data", intValue: nil), _JSONKey(stringValue: "Index 0", intValue: 0)], debugDescription: "Expected Double but found null value instead.", underlyingError: nil))

How can i turn this Null value into a zero so I don't get a JSON decoding error?

I tried to skip the first value in the decoding but this did not work.

Please add what you have tried so far. Also an example of the problematic JSON would be helpful. — burnsi, Jan 19 '23 at 20:54
Use a custom `init(from decoder: Decoder) throws`, and check if the value is there, and if not, give it 0 value. — Larme, Jan 19 '23 at 23:13
Show a minimal reproducible code that produces your error, see: https://stackoverflow.com/help/minimal-reproducible-example Also show the complete error message and on what line in your code. Show also some representative json data and how you are trying to decode it. — workingdog support Ukraine, Jan 20 '23 at 06:56

score 0 · Accepted Answer · answered Jan 20 '23 at 10:10

You don´t have to turn the null JSON value into a 0 just to prevent the crash. Change the the type of the array to an optional and you are good to go. E.g.:

struct Stream: Decodable {
    let data: [Double?] // add a ? if the array itself is optional too
    let series_type: String
    let original_size: Int
    let resolution: String
}

I don´t know why and where you do what you described as decoding. But if you want your data Double´s to be nonoptional you could do something like this in a custom decoder:

struct streamsElement: Decodable {
    struct Stream: Decodable {
        let data: [Double]
        
        enum CodingKeys: CodingKey{
            case data
        }
        
        init(from decoder: Decoder) throws {
            let container = try decoder.container(keyedBy: CodingKeys.self)
            // decode the data property to the array of optionals
            let data = try container.decode([Double?].self, forKey: .data)
            // replace nil values with 0
            self.data = data.map{$0 ?? 0}
        }
    }
    
    var watts: Stream
}

I´ve implemented only the problematic property. Of course you would need to expand this to the others.

Thanks a lot, the custom decoder works and I am now able to form the array with watts. — Lucas, Jan 20 '23 at 10:32

Strava API returns null as first value of powerstream - Swift

1 Answers1