what is the simplest way to parse data(C++) from my text file and store it to variables?
I have a parse_data.txt with this text:
{"song":"Holiday","artist":"Green Day","Album":"American Idiot","Service":"Spotify"}
I need store Holiday to song variable, Green Day to artist variable...
so QString song = Holiday...
Can someone show me some examples?
Can we assume that you are using QT? If so, there is an response here that can show you an example on how to use here
Here is how you can apply to your code
QString jsonString = [your json string represenation];
QJsonDocument jsonResponse = QJsonDocument::fromJson(jsonString.toUtf8());
QJsonObject jsonObject = jsonResponse.object();
QJsonValue jsonValue = jsonObject["song"];
QString strValue = jsonValue.toString();
Related
I am trying to log data from 3 sensors to a json file. All I want to be able to accomplish is to write the Speed, Latitude and Longitude to a Json file, with an object containing each of the above. That is a json file that contains one route object, n sub objects each of which contain speed, latitude, longitude.
These 3 values I get from 3 global QList lists. Below is the json file which is stored locally. (The double values are not actual values, just for testing purposes)
{
"Sensordata": [
{
"Speed": 1,
"GPSLat":-12.5687,
"GPSLong":26.125546
},
{
"Speed": 1,
"GPSLat":-12.5687,
"GPSLong":26.125546
}
]
}
This is what the json must look like and when I add it must be formatted in the same way
void MainWindow::save_to_json() {
QFile file_obj(".../SensorData.json");
if(!file_obj.open(QIODevice::ReadOnly)){
qDebug()<<"Failed to open "<<"SensorData.json";
exit(1);
}
QTextStream file_text(&file_obj);
QString json_string;
json_string = file_text.readAll();
file_obj.close();
QByteArray data_json = json_string.toLocal8Bit();
QJsonDocument doc = QJsonDocument::fromJson(data_json);
QJsonObject rootObj = doc.object();
QJsonValue SensorData = rootObj.value("SensorData");
if(!SensorData.isArray())
{
// array expected - handle error
}
QJsonArray SensorDataArray = SensorData.toArray();
QJsonObject newObject;
newObject["Speed"] = speed_array.takeFirst();
newObject["GPSLat"] = gps_lat.takeFirst();
newObject["GPSLong"] = gps_long.takeFirst();
SensorDataArray.push_back(newObject);
}
ASSERT: "!isEmpty()" in file /home/username/Qt/5.12.1/gcc_64/include /QtCore/qlist.h, line 347
11:32:55: The program has unexpectedly finished.
11:32:55: The process was ended forcefully.
This is the error the above code creates.
To modify the data, given your example, you need to check if the contained data in the QJsonDocument is an array or a simple object. In your case, I suppose you want to append data to an array. Try something like this:
// Read the data
const QString filename = "example.json";
QJsonDocument doc = read(filename);
// Check that it's an array and append new data
QJsonValue sensorData = doc.value("SensorData");
if (!sensorData.isArray()) {
// if the doc is empty you may want to create it
}
// Get the array and insert the data
auto array = sensorData.array();
array.append(QJsonObject{
{"Speed", speed_array.takeFirst()},
{"GPSLat", gps_lat.takeFirst()},
{"GPSLong",gps_long.takeFirst(),
});
// Restore your sensor data
doc.setObject(QJsonObject{{"SensorData", array}});
// Write the new data
write(filename, doc);
A helper functions to read/write JSON documents may avoid the mistake of open/closing a file:
QJsonDocument read(const QString& filename) {
QFile file(filename);
file.open(QIODevice::ReadOnly | QIODevice::Text);
const QString val = file.readAll();
file.close();
return QJsonDocument::fromJson(val.toUtf8());
}
void write(const QString& filename, const QJsonDocument& document) {
QFile file(filename);
file.open(QFile::WriteOnly | QFile::Text | QFile::Truncate);
file.write(document.toJson());
file.close();
}
Updates
To not overwrite the original doc, you must update the field of the root object or use QJsonValueRef.
// Get a reference to your array
QJsonObject root = doc.object();
QJsonValueRef ref = root.find("SensorData").value();
// get the array and insert the data
QJsonArray array = ref.toArray();
array.append(QJsonObject{
{"Speed", speed_array.takeFirst()},
{"GPSLat", gps_lat.takeFirst()},
{"GPSLong",gps_long.takeFirst(),
});
// Update the ref with the new data
ref = array
// update the modified data in the json document
doc.setObject(root);
I'm trying to write a desktop client with Qt for an online dictionary site. I got stuck on something about JSON.
http://ac.tureng.co/?c=?&t=expensive
?(["expensive","expensive habits","expensive medical equipment","expensive question","expensive watch","expensive-looking","expensively","expensiveness"]);
I think the data which come from the above address is JSON array. json.org has the following description:
An array is an ordered collection of values. An array begins with [
(left bracket) and ends with ] (right bracket). Values are separated
by , (comma).*
Also a valid JSON according to the JSON Formatter if a string is written instead of the first question mark:
a([
"expensive",
"expensive habits",
"expensive medical equipment",
"expensive question",
"expensive watch",
"expensive-looking",
"expensively",
"expensiveness"
]);
But this change is not enough for the JSONLint site:
["expensive", "expensive habits", "expensive medical equipment", "expensive question", "expensive watch", "expensive-looking", "expensively", "expensiveness"]
I want to show suggestions to the user at the user input using the data from here. For now I can't extract JSON, so I've reached the desired behavior by acting it as plain text. Is there a way to do this properly by parsing JSON?
The codes I've written so far are:
QString turengOneriMetin = QString("http://ac.tureng.co/?c=?&t=%1").arg(arg1);
QUrl turengOneri(turengOneriMetin);
QNetworkAccessManager manager;
QNetworkReply *response = manager.get(QNetworkRequest(turengOneri));
QEventLoop event;
connect(response, SIGNAL(finished()), &event, SLOT(quit()));
event.exec();
QString content = response->readAll();
content.replace(0,1,"a");
content = content.replace("a([", "").replace("]);", "").replace("\"","");
QStringList wordList;
wordList << content.split(",");
ui->label->setText(content);
// https://stackoverflow.com/questions/24248606/how-to-accomplish-drop-down-word-suggestions-in-qt
QCompleter *completer = new QCompleter(wordList, this);
completer->setCaseSensitivity(Qt::CaseInsensitive);
ui->lineEdit->setCompleter(completer);
Update: After receiving the JSON correctly with help of answers and other resources I've done what I wanted with the following piece of code:
QJsonDocument document = QJsonDocument::fromJson(content.toUtf8());
QJsonArray documentArray = document.array();
QStringList wordList;
for (const QJsonValue &i : documentArray)
{
//qInfo() << i.toString() << endl;
wordList << i.toString();
}
I think that URL should be this - http://ac.tureng.co/?t=expensive
In this case you'll get valid JSON.
I need to serialize and parse multiple objects from my project, in order to save/load them when needed.
My objects will have exactly the same components : a QString name, an integer id, a QString description, and two integer x, y.
I'll need something like this :
{"name":"toto", "id":"42", "description":"tata", "x":"20", "y":"50"}
So I'll build my QJsonObject like this :
QJsonObject json;
json["id"] = object_to_serialize.get_id();
json["name"] = object_to_serialize.get_name();
json["description"] = object_to_serialize.get_description();
json["x"] = object_to_serialize.get_x();
json["y"] = object_to_serialize.get_y();
QJsonDocument filedoc(json);
file.write(filedoc.toJson);`
And in the file it will appear like this :
{"name":"toto", "id":"42", "description":"tata", "x":"20", "y":"50"}
{"name":"toto2", "id":"44", "description":"tata2", "x":"25", "y":"547"}
{"name":"toto3", "id":"46", "description":"tata3", "x":"21", "y":"580"}
My serialiser will take in parameter the object, the savefile name, and transform the object into a QJsonObject. It will need then to read the file to check if an object with the same id is here. If it is here, it will need to replace it, and if it is not, it will append it.
I'm a little lost between my serialization options and how to read it ;
Should I make a QJsonArray with multiple QJsonObject inside or QJsonObject with QJsonArrays ?
When I read it, I will need to check for the id ; but will a
foreach(object.value["id"] == 42)
//create the QJsonObject from the one with 42 and change it with the new data
will do to parse the object and not all of them ? Is there a better way ?
Thank you in advance for your answers.
You can have an array of json object, each of them having an ID so you can parse the relevant ones.
Although you could also parse all of them and add them in a map, as long as you don't have very heavy files it should be fine.
void parseJson(const QString &data)
{
QJsonDocument doc = QJsonDocument::fromJson(data.toUtf8());
if (doc.isNull())
{
war("invalid json document");
return;
}
QJsonArray jsonArray = doc.array();
foreach (const QJsonValue & value, jsonArray) {
QJsonObject obj = value.toObject();
if (obj.contains("id"))
{
if (obj["id"].toInt() == yourId) parseObject(obj);
}
}
}
void parseObject(const QJsonObject &obj)
{
if (obj.contains("valueA")) valueA = obj["valueA"].toDouble();
if (obj.contains("valueB")) valueB = obj["valueB"].toDouble();
}
This will work just fine if your file is not too big
Bigger Files
Now if you have very large file, it might be an issue to load it all in memory and parse it.
Since your structure is always the same and quite simple, JSON might not be the best choice, one more efficient method would be to do your own parser (or use probably some existing ones) that could read the file and process it as a stream.
Another method, would be to have one JSON entry per line preceded by an ID with a fixed number of digit. Load this in a QHash lookup and then only read id of interest from the file and only parse a small section.
// This code is not tested and is just to show the principle.
#define IDSIZE 5
QHash<int64, int64> m_lookup; // has to be global var
// For very large file, this might take some time and can be done on a separate thread.
// it needs to be done only once at startup (given the file is not modified externally)
void createLookup(const QString &fileName)
{
QFile inputFile(fileName);
if (inputFile.open(QIODevice::ReadOnly))
{
QTextStream in(&inputFile);
while (!in.atEnd())
{
int position = in.pos(); // store the position in the file
QString line = in.readLine();
int id = line.mid(0,IDSIZE).toInt(); // 5 digit id (like 00001, 00002, etc...
m_lookup[id] = position + IDSIZE;
}
inputFile.close();
}
}
QString getEntry(const QString &fileName, int64 id)
{
if (m_lookup.contains(id))
{
QFile inputFile(fileName);
if (inputFile.open(QIODevice::ReadOnly))
{
inputFile.seek(m_lookup[id]);
QString data = inputFile.readLine();
inputFile.close();
return data;
} else {
return QString(); // or handle error
}
} else {
return QString(); // or handle error
}
}
// use example
QString data = getEntry(id);
if (data.length() > 0)
{
QJsonDocument doc = QJsonDocument::fromJson(data.toUtf8());
if (!doc.isNull())
{
// assign your variables
}
}
and your data file looking like this:
00042{"name":"toto", "id":"42", "description":"tata", "x":"20", "y":"50"}
00044{"name":"toto2", "id":"44", "description":"tata2", "x":"25", "y":"547"}
00046{"name":"toto3", "id":"46", "description":"tata3", "x":"21", "y":"580"}
The advantage of this method, it will only read the entry of interest, and avoid having to load MB or GB of data in memory just to get a specific entry.
This could further be improved with a lookup table stored at the beginning of the file.
Alright, I found something I just don't understand. I am making a request to a web service using QtNetworkManager. For some reason I can't seem to go from the network response to a jsondoc directly, I have to cast it into a string and then BACK into uft8?
void WebAPIengine::handleNetworkData(QNetworkReply *networkReply)
{
//No network error
if (!networkReply->error()){
//Cast to string
QString strReply = (QString)networkReply->readAll();
//This works, jsonDoc will have the json response from webpage
QJsonDocument jsonDoc = QJsonDocument::fromJson(strReply.toUtf8());
//This doesn't work, networkReply->readAll() is said to return a QByteArray.
QJsonDocument jsonDoc2 = QJsonDocument::fromBinaryData(networkReply->readAll());
QJsonObject jsonObj = jsonDoc.object();
data = jsonObj;
}
//Network error
else{
data["Error"] = "WebAPIengine::handleNetworkData()";
}
Now I can not understand why jsonDoc is working and jsonDoc2 is not. Can someone explain?
Once you do a QNetworkReply->readAll(), the QNetworkReply object will be empty. So if you call the QNetworkReply->readAll() method again, you will not get anything.
Moreover I don't understand why you are converting the QByteArray returned by QNetworkReply->readAll() into a QString and then converting it back to QByteArray(by calling QString::toUtf8()) to give it to the QJsonDocument::fromJson function.
You can try doing this:
QByteArray temp = newReply->readAll();
QJsonDocument jsonDoc = QJsonDocument::fromJson(temp); // This should work
Also make sure to know what the content of the JSon document is, i.e. if it is a map (QJsonObject), array(QJSonArray), array of maps or map with an array as value.
I'm trying to parse this JSON Web-API using Qt5 and C++ using QJsonDocument and QJsonObject as seen here. But I fail to access the JSON value of the QJsonObject.
This is what I've tried so far:
// Contains the whole API as QString...
QString data = QString(reply->readAll());
// Reads the JSON as QJsonDocument...
QJsonDocument jsonResponse = QJsonDocument::fromJson(data.toUtf8());
// Reads the JSON as QJsonObject...
QJsonObject jsonObject = jsonResponse.object();
Now I have my object well prepared, but trying to access the values of the JSON somehow fails:
// This returns an empty string ""!?!
qDebug() << jsonObject.value("success").toString();
Well, maybe I got the keys wrong:
// Let's check the keys...
QStringList stringList = jsonObject.keys();
for (QStringList::Iterator it = stringList.begin(); it != stringList.end(); ++it)
{
// This returns "success" and "return" - huh!?!
qDebug() << *it;
}
OK, the keys are veryfied, why is it not working?
// Let's check the values by using the keys directly...
for (QStringList::Iterator it = stringList.begin(); it != stringList.end(); ++it)
{
// This returns empty strings "" and "" - now what?!?
qDebug() << jsonObject.value(*it).toString();
}
This again, makes no sense at all. I can't see why I can not access the value of the JSON object by the keys. Any idea?
I tried exactly the same code on other JSON APIs (for example this one) without any issues. I am totally stuck here.
Here's my solution for Qt5 Json parsing the Cryptsy API.
QEventLoop loopEvent;
QNetworkAccessManager namMNGR;
QObject::connect(&namMNGR, SIGNAL(finished(QNetworkReply*)), &loopEvent, SLOT(quit()));
QNetworkRequest req(QUrl(QString("http://pubapi.cryptsy.com/api.php?method=singlemarketdata&marketid=%1").arg(marketID)));
QNetworkReply *reply = namMNGR.get(req);
loopEvent.exec();
//Json API parsing begins.
QString jsonSTR = reply->readAll();
if (!(reply->error() == QNetworkReply::NoError)) {
delete reply; //API Connection Problem.
}
QJsonDocument jsonDocument = QJsonDocument::fromJson(jsonSTR.toUtf8());
QJsonObject obj1 = jsonDocument.object();
QJsonValue val1 = obj1.value(obj1.keys().first());
QJsonObject obj2 = val1.toObject();
QJsonValue val2 = obj2.value(obj2.keys().first());
QJsonObject obj3 = val2.toObject();
QJsonValue marketDataValue = obj3.value(obj3.keys().first());
QJsonObject marketDataObject = marketDataValue.toObject();
QJsonArray sellordersArray = marketDataObject["sellorders"].toArray();
Have you managed to get Authenticated POST API data from Qt5? I'm trying to figure out how to do it.