Load JSON file

[neda]

Hi,
I have been trying to read a JSON file.
But this code does not work.

JSON file is the best choice for save points? (Reading speed is very important)
Please guide me.
Thanks

file:

{
     "point":{
           "x":1.12
           "y":1.19
         },
     "point":{
           "x":1.15
           "y":1.22
         },
....
....
}

QVector<QPointF> listPoints;    
    QJsonObject json=loadJson(fileName);
    QJsonValue value=json.value("point");
    QJsonArray array=value.toArray();
    foreach(const QJsonValue & val,array){
        double x=val.toObject().value("x").toDouble();
        double y=val.toObject().value("y").toDouble();
        listPoints.append(QPointF(x,y));        
    }

[mrjj]

Hi
if reading speed is of great concern, a binary file is faster.
Where does points come from. Can you change how its saved ?
That said, how many points re we talking about ?

[mranger90]

First, as @mrjj stated, if there are a lot of points, then you should consider other means besides json.
As for your current status, the json is ill-formed: you should have something like:

{
    "points" : [
        {
             "x" : 1.12,
             "y" : 1.19
         },
        {
             "x" : 1.15,
             "y" : 1.22
         },
        {
             "x" : 1.18,
             "y" : 1.25
         }
    ]
}

Code to parse it is something like:

    QFile inFile("/home/joem/tmp/mypoints.json");
    inFile.open(QIODevice::ReadOnly|QIODevice::Text);
    QByteArray data = inFile.readAll();
    inFile.close();

    QJsonParseError errorPtr;
    QJsonDocument doc = QJsonDocument::fromJson(data, &errorPtr);
    if (doc.isNull()) {
        qDebug() << "Parse failed";
    }
    QJsonObject rootObj = doc.object();
    QVector<QPointF> listPoints;
    QJsonArray ptsArray = rootObj.value("points").toArray();
    qDebug() << "There are " << ptsArray.size() << "sets of points in the json file";
    foreach(const QJsonValue & val, ptsArray){
        double x=val.toObject().value("x").toDouble();
        double y=val.toObject().value("y").toDouble();
        listPoints.append(QPointF(x,y));
    }

    qDebug() << "Finished parsing, heres the data";
    for(auto pt: listPoints) {
        qDebug() << pt.x() << "," << pt.y();
    }

[neda]

@mrjj said in Load JSON file:

Where does points come from.

Thanks for your reply.
I read data from "serialPort" and save as point in "QVector<QPointF>".
"QVector<QPointF>" is the best choice for hold points? (I have a lot of points (more than 1,000,000 points) and a lot of processes on points)
I use these points for draw real time chart (QML).

Can you change how its saved ?

What do you suggest?

[mrjj]

Hi

Like this ( test code)
I would test the speed of both since @mranger90 was so kind to provide the json sample.

    auto const s = 10000000L;
    auto const path = QStringLiteral("/path/to/file");
    
        QVector<int> x;
        x.resize(s);
// save
        QFile f(path);
        if (!f.open(QIODevice::WriteOnly))
            return ;
        QDataStream ds(&f);
        ds.setVersion(QDataStream::Qt_4_5);
        ds << x;

 // load 
        QFile f(path);
        if (!f.open(QIODevice::ReadOnly))
            return ;
        QDataStream ds(&f);
        ds.setVersion(QDataStream::Qt_4_5);
        ds >> x;

        qDebug() << x.size();

Anyway, QVector is fine. especially if it can be used directly by the plotting part
so you don't need to copy data around.

[neda]

@mrjj
Thank you so much for your help I really appreciate it.