Axiomatic Semantics - How to calculate a weakest precondition of a program

Question

Assuming the post-condition, how can I compute the weakest pre-condition of a program containing two statements?

For example :

 a=x;
 y = 0 
 {x = y + a}

Another example:

  y = x;
  y = x + x + y 
  {y = 3x ^ z> 0}

I tried to solve them but both questions resulted in pre-conditions or post-condition that are identical to the statement and I don't know if this is valid.

for example, the precondition of the last statement is "y=x" , thus it is the post condition of the preceding statement which is " y=x" as well

Answer 1

It is also possible to use DeserializeObject method which accepts a type. That way you don't need to write any type-specific parser, you just have to find out a type and provide it to that method. Something like this:

public Payload GetPayload(string type, string message)
    {
        switch (type)
        {
            case "fuel":
                return JsonConvert.DeserializeObject(message, typeof(Fuel));
            case "engine":
                return JsonConvert.DeserializeObject(message, typeof(Engine));
            default:
                return null;
        }
    }

You can then decorate your entity classes with JsonProperties attributes (mapping basically) as indicated here (plus use that coverter):

[JsonConverter(typeof(JsonPathConverter))]
class Fuel : Payload
{
   [JsonProperty("data.fueltype")]
   public string FuelType{ get; set; }

   [JsonProperty("data.mileage")]
   public float Mileage { get; set; }
}



string message = "{\r\n\t\"type\": \"fuel\",\r\n\t\"data\":\r\n\t\t{\r\n\t\t\t\"fueltype\": \"petrol\",\r\n\t\t\t\"mileage\": 23.76,\r\n\t\t\t\"tankcapacity\": 37\r\n\t\t}\r\n}";
var type = GetMessageType(); // get type from message i.e. with JObject
var payload = GetPayload(type, message);