gistfile1.txt

import UIKit

var output = [String: Any]()
for index in 0..<5 {
 
    let mail = String(format: "%d@mail.com", index)
    let name = String(format: "name_%d", index)
    let id = NSNumber.init(value: index)
    let item: [String: Any] = ["name": name, "id": id]
    
    output[mail] = item
}

let jsonData = try? JSONSerialization.data(withJSONObject: output, options: .prettyPrinted)
let jsonString = String(data: jsonData!, encoding: String.Encoding.utf8)
print(jsonString!)