dictEncode.swift

let dict: [Float : String] = [
    18.0: "ff0000",
    20.0: "00ff00",
    21.0: "0000ff"
]

let encoder = JSONEncoder()
encoder.outputFormatting = [.prettyPrinted, .sortedKeys]
let encoded = try! encoder.encode(dict)
let jsonText = String(decoding: encoded, as: UTF8.self)
print(jsonText)

//[
//  21,
//  "0000ff",
//  18,
//  "ff0000",
//  20,
//  "00ff00"
//]