Libraries: JSON Serialization and Deserialization (native MQL) - page 14

[seleand]

The library is very useful. But I encountered that when serialising values of double type there is a strong loss of precision.

Example:

double minutes_open[] = {0.95347, 0.95317, 0.95324}

for (int i=0; i<ArraySize(minutes_open); ++i) jv["minutes_open"].Add(minutes_open[i]);

After serialisation I get:

"minutes_open":[9.53e-01,9.53e-01,9.53e-01]

I.e. the last 2 significant characters are simply lost and all values in the array become simply equal to each other. As a result, all the calculations based on this data go in vain....

In the end I got round it by converting array values into string.

[Artem Maltsev]

Hi there!

Documentation of JAson library is not full at this site. I wrote documentation with more details and added unit tests: https://github.com/vivazzi/JAson. It may be useful for someone.

Add i created issue that has description of bug of using inner Json (CJAVal object): https://github.com/vivazzi/JAson/issues/1.

CJAVal object is not correctly assigned to another CJAVal object as object:

CJAVal data;

// simple structure
data["a"] = 12;

// nested structure
CJAVal nested_data;
nested_data["k1"] = 7;
nested_data["k2"] = "baz";

data["b"] = nested_data;

Print(data["b"]["k1"].ToInt());  // 0 instead of 7
Print(data["b"]["k2"].ToStr());  // "" instead of "baz"
Print(data.Serialize());  // {"a":12,"":{"k1":7,"k2":"baz"},"b":{}} instead of {"a":12,"b":{"k1":7,"k2":"baz"}}

As workaround, you need use Set() method:

data["b"].Set(nested_data);

Print(data["b"]["k1"].ToInt());  // 7
Print(data["b"]["k2"].ToStr());  // "baz"
Print(data.Serialize());  // {"a":12,"b":{"k1":7,"k2":"baz"}}

If anyone can help, then many MQL-developers will be grateful as this bug has been around for many years.

[Artem Maltsev]

Hi everyone!!!

The JAson documentation is not complete on this site, so I wrote a more detailed documentation https://github.com/vivazzi/JAson/ and added unit tests: maybe someone will find it useful.

And I described the problem of using a nested Json (CJAVal object): https://github.com/vivazzi/JAson/issues/1.

CJAVal object is not correctly saved if you use "=" sign (empty key is saved):

CJAVal data;

// simple structure
data["a"] = 12;

// nested structure
CJAVal nested_data;
nested_data["k1"] = 7;
nested_data["k2"] = "baz";

data["b"] = nested_data;

Print(data["b"]["k1"].ToInt());  // 0 instead of 7
Print(data["b"]["k2"].ToStr());  // "" instead of "baz"
Print(data.Serialize());  // {"a":12,"":{"k1":7, "k2": "baz"}, "b":{}} instead of {"a":12, "b":{"k1":7, "k2": "baz"}}

Yes, in the tests attached on the library page on this site, you can find that you need to use the Set() method:

data["b"].Set(nested_data);

Print(data["b"]["k1"].ToInt());  // 7
Print(data["b"]["k2"].ToStr());  // "baz"
Print(data.Serialize());  // {"a":12,"b":{"k1":7,"k2":"baz"}}

But if someone can fix the library so that it is possible to save nested Json structures via the "=" sign, it would help a lot of people. Since I've noticed that some people think the library has errors when they try to assign a nested CJAVal in the usual way via "=".

I hope this is even possible within the MQL language. I've tried to correct it myself, but I don't have a deep knowledge of the MQL language, and so far it hasn't worked to do assignment via "=".

[Laszlo Tormasi]

Артём Мальцев:

Hi there!

Documentation of JAson library is not full at this site. I wrote documentation with more details and added unit tests: https://github.com/vivazzi/JAson. It may be useful for someone.

Add i created issue that has description of bug of using inner Json (CJAVal object): https://github.com/vivazzi/JAson/issues/1.

CJAVal object is not correctly assigned to another CJAVal object as object:

As workaround, you need use Set() method:

If anyone can help, then many MQL-developers will be grateful as this bug has been around for many years.

The problem is with the Copy method. 

        virtual bool Copy(const CJAVal &a) {
            m_key=a.m_key;
            CopyData(a);
            return true;
    }

The key gets overwritten with the subobject's key in your example the "b" gets overwritten with the "nested_data"'s key which is empty. 

I used this simple solution, not fully tested: 

virtual bool Copy(const CJAVal &a) { if(m_key=="") m_key=a.m_key; CopyData(a); return true; }

[Artem Maltsev]

Thank you, Laszlo Tormasi - your fix works and all tests have passed!

I updated JAson with your fixes: https://github.com/vivazzi/JAson

[Artem Maltsev]

The problem of assigning nested Json structures via the "=" sign was solved by Laszlo Tormasi in the comment https://www.mql5.com/en/forum/65320/page3#comment_21453129.

JAson version with editsat https://github.com/vivazzi/JAson.

Thanks to everyone who tried to understand the problem and help!

[wei li]

How to manipulate generic serialisation and deserialisation?

[Mihail Matkovskij]

Fixed an error ("array out of range") that occurred with negative array indices. Now the [] operator returns NULL.

Processing looks like this.

void OnStart()
{
  CJAVal json, * jsC;

  // adding 
  json[0] = 0;
  json[1] = 1;
  json[2] = 2;
  
  // check
  if ((jsC = json[-1]) == NULL)
    Print("Error. jsC is null!");
}

Also, I added the ToIntI method for convenience. Since ToInt returns long.

int ToIntI() const { return (int)m_iv; }

[leonerd]

I don't understand how to read the nested json.

Here is a string like this:

string str=

{
  "key1": [
    {
      "key2": [
        {
          "key3": "val)"
        },
        {},
        {}
      ]
    }
  ]
}

How do I read it and access the elements?

CJAVal data;
data.Deserialize(str);
Print(data["key1"][0][0]["key3"].ToStr());

No matter how I try, it doesn't work.

[Mihail Matkovskij]

leonerd #:

I don't understand how to read the nested json.

Here is a string like this:

How do I read it and access the elements?

No matter how many times I try, I can't get it.

string str;
CJAVal data;

str=
"{"
"  \"key1\": ["
"    {"
"      \"key2\": ["
"        {"
"          \"key3\": \"val)\""
"        },"
"        {},"
"        {}"
"      ]"
"    }"
"  ]"
"}";

data.Deserialize(str);
Print(data["key1"][0]["key2"][0]["key3"].ToStr());

If you don't understand the structure of your json string or something goes wrong, do the following:

string str;
CJAVal data; 
CJAVal *value;

str=
"{"
"  \"key1\": ["
"    {"
"      \"key2\": ["
"        {"
"          \"key3\": \"val)\""
"        },"
"        {},"
"        {}"
"      ]"
"    }"
"  ]"
"}";

data.Deserialize(str);

if ((value = data["key1"]) == NULL) {
  Print("Value is NULL! Line: ", __LINE__);
  return;
}

if ((value = value[0]) == NULL) {
  Print("Value is NULL! Line: ", __LINE__);
  return;
}

if ((value = value["key2"]) == NULL) {
  Print("Value is NULL! Line: ", __LINE__);
  return;
}

if ((value = value[0]) == NULL) {
  Print("value is NULL! Line: ", __LINE__);
  return;
}

if ((value = value["key3"]) == NULL) {
  Print("Value is NULL! Line: ", __LINE__);
  return;
}

Print(value.ToStr());

To easily understand the structure of a json string use this site: https: //jsoneditoronline.org/.