|HELP| : How to extract Data from Json file ?

Elmehdi

I have a Json file wich has the format starting with a "[" , and has objects inside it, each object has its own attributes. My purpose is to retrieve these data of each object and insert it in a database, that's another talk but for the moment I want just to print them. I was struggling with this for about 4 days without finding what I need.
here is an example of my file :

[
 {
    "updated":1592183850402,
    "country":"Morocco",
    "countryInfo":
    {"_id":504,
        "iso2":"MA",
        "iso3":"MAR",
        "lat":32,
        "continent":"Africa",
        "activePerOneMillion":22.12
    }
},
{
    "updated":159388477402,
    "country":"France",
    "countryInfo":
    {"_id":504,
        "iso2":"FR",
        "iso3":"FRA",
        "lat":332,
        "continent":"Europe",
        "activePerOneMillion":66.12
    }
}
]

And I really need help, since I'm new to both Qt and Json I can't go so far by myself. if any one can help me with a piece of code.
this is what I wrote, but whenever I reache the foreach loop I can't figure out my next move. I have tried different ways inside the loop but none of it worked.

    QTextStream file_text(&my_json);
    QString json_string;
    json_string = file_text.readAll();
    my_json.close();
    QByteArray json_bytes = json_string.toLocal8Bit();
    auto json_doc = QJsonDocument::fromJson(json_bytes);
    QJsonObject json_obj = json_doc.object();
    QJsonArray jsonArray = json_obj["activePerOneMillion"].toArray();

    foreach (const QJsonValue & value, jsonArray) { ... }

Elmehdi

Thank you all. And a special thanks to @LeLev who helped me in private. apparently the base-ranged for loop helped me since it reconizes the type of objects that are stored in the QjsonArray so it access them all. This was what worked for me:

 QFile countries_file("countries.json");
    QFile morocco_file("morocco.json");
    QString json_string;

    if(countries_file.open(QIODevice::ReadOnly | QIODevice::Text)){
        json_string = countries_file.readAll();
        countries_file.close();
    }
    else
        qDebug()<< "file not found";
    auto json_doc = QJsonDocument::fromJson(json_string.toUtf8());

    QJsonArray jArr = json_doc.array();
QJsonValue val;
    for(auto jsonObj : jArr)
    {
        val = jsonObj.toObject().value("country");
        _country = val.toString();
        qDebug() << "Country: "<< _country;

        val = jsonObj.toObject().value("cases");
        _cases = val.toInt();
        qDebug() << "Cases: "<< _cases;

        val = jsonObj.toObject().value("deaths");
        _deaths = val.toInt();
        qDebug() << "Deaths: "<< _deaths;
   }


        qDebug() << Qt::endl;
        qDebug() << Qt::endl;

mrjj

Hi and welcome to the forums.
You seem off to a good start.
What you have is a json array with objects in it.

So in you for loop you will get QJsonObject and not a
QJsonValue if i read it correctly :=)
Nope. @JonB is right :)

Pablo J. Rogina

@Elmehdi said in |HELP| : How to extract Data from Json file ?:

I'm new to both Qt and Json

Just in case, have you read the documentation?

There's even a good example...

JonB

@Elmehdi
With all die respect to my learned friend @mrjj, you do get QJsonValues as you iterate through QJsonArray jsonArray.

The point is, a QJsonValue can hold many data types (see https://doc.qt.io/qt-5/qjsonvalue.html#details). In your case those will be QJsonObjects. The first thing you should write in your code is:

foreach (const QJsonValue & value, jsonArray)
{
    qDebug() << value.type();
}

and look at the values per https://doc.qt.io/qt-5/qjsonvalue.html#Type-enum.

From there, you will probably also want to look at QJsonObject QJsonValue::toObject() and/or QVariant QJsonValue::toVariant().

If you know about recursion, that will be pretty handy when parsing JSON data.

mrjj

Hi
I think what confused me is this
QJsonArray jsonArray = json_obj["cases"].toArray();
i dont see "cases" in the data sample.

JonB

@mrjj
I did indeed intend to mention to @Elmehdi : if that is the complete JSON file, nothing there matches with the json_obj["cases"] in your code, I don't know where your idea of the "cases" comes from.

ODБOï

it is not valide json format, you have to delete the semicolon at the end of last lines "activePerOneMillion":66.12,

mrjj

@LeLev
Thats good spotting!

Elmehdi

Thank you guys, I didn't expect the fast answers, that was great. @mrjj sorry there is no field with that name I corrected it.
@JonB I will check that immediately thanks.

Elmehdi

@LeLev you are right I just made a mistake while copying since it was not the whole file it was just a part of it, I fixed it now.

Elmehdi

Thank you all. And a special thanks to @LeLev who helped me in private. apparently the base-ranged for loop helped me since it reconizes the type of objects that are stored in the QjsonArray so it access them all. This was what worked for me:

 QFile countries_file("countries.json");
    QFile morocco_file("morocco.json");
    QString json_string;

    if(countries_file.open(QIODevice::ReadOnly | QIODevice::Text)){
        json_string = countries_file.readAll();
        countries_file.close();
    }
    else
        qDebug()<< "file not found";
    auto json_doc = QJsonDocument::fromJson(json_string.toUtf8());

    QJsonArray jArr = json_doc.array();
QJsonValue val;
    for(auto jsonObj : jArr)
    {
        val = jsonObj.toObject().value("country");
        _country = val.toString();
        qDebug() << "Country: "<< _country;

        val = jsonObj.toObject().value("cases");
        _cases = val.toInt();
        qDebug() << "Cases: "<< _cases;

        val = jsonObj.toObject().value("deaths");
        _deaths = val.toInt();
        qDebug() << "Deaths: "<< _deaths;
   }


        qDebug() << Qt::endl;
        qDebug() << Qt::endl;