JKQtPlotter/examples/simpletest_datastore

Example (JKQTPlotter): Advanced Usage of JKQTPDatastore

This project (see ./examples/simpletest_datastore/) explains several advanced options of JKQTPDatastore, which is the class used to centrally store the data for (most) graphs on a JKQTPlotter widget.

The source code of the main application can be found in jkqtplotter_simpletest_datastore.cpp. This tutorial cites parts of this code to demonstrate different ways of working with data for the graphs.

In every code-segment below, we will use these two declarations from the code to access the internal datastore of the JKQTPlotter instance:

    // 1. create a plotter window and get a pointer to the internal datastore (for convenience)
    JKQTPlotter plot;
    JKQTPDatastore* datastore=plot.getDatastore();

Copy Data from a Vector into a column of the JKQTPDatastore

First we fill data into a QVector for a simple plot (a sine curve) and add a plot using this data:

    QVector<double> X, Y;
    const int Ndata=100;
    for (int i=0; i<Ndata; i++) {
        const double x=double(i)/double(Ndata)*8.0*M_PI;
        X<<x;
        Y<<sin(x);
    }

Now the contents of the vector is copied into the datastore using JKQTPDatastore::addCopiedColumn(). After this operation, the contents of the vectors X and Y can be altered, as the JKQTPDatastore inside the JKQTPlotter now hosts its own copy of the data.

    plot.addGraph(linegraph=new JKQTPXYLineGraph(&plot));
    linegraph->setXColumn(ds->addCopiedColumn(X, "x"));
    linegraph->setYColumn(ds->addCopiedColumn(Y, "y"));
    linegraph->setTitle(QObject::tr("sine graph"));

Note that you could also use a std::vector instead, as JKQTPDatastore::addCopiedColumn() is a template function that only requires the container to support C++ standard iterations (with begin() and end()), as well as the function size() to determine the number of items in the container. Also other datatypes than double are possible, as long as they can be converted to double (the function jkqtp_todouble() is used to perform the conversion, notably all integer and floating-point types, as well as bool are supported out of the box).

Of course if you have your data in a C-array, you can use the same syntax:

    #define NDATA 5
    double X[NDATA]= { 1, 2, 3, 4, 5 };
    double Y[NDATA]= { 1, 2, 3, 2, 1 };
    linegraph->setXColumn(datastore->addCopiedColumn(X, NDATA, "x"));
    linegraph->setYColumn(datastore->addCopiedColumn(Y, NDATA, "Y"));

Reference External Data in a column of the JKQTPDatastore

As an alternative to the method of copying data (see above), you could also just link the data. For this to work, the data has to reside in a C-array of type double, as this is the internal datatype of the JKQTPDatastore. You can simply replace the two lines with JKQTPDatastore::addCopiedColumn() in the example above by (we exploit the fact that QVector<double>::data() returns a pointer to the internal C-array of the vector:

    linegraph->setXColumn(datastore->addColumn(X.data(), X.size(), "x"));
    linegraph->setYColumn(datastore->addColumn(Y.data(), Y.size(), "Y"));

Of course if you have your data in a C-array, you can use the same syntax:

    #define NDATA 5
    double XCA[NDATA]= { 1, 2, 3, 4, 5 };
    double YCA[NDATA]= { 1, 0, 1, 0, 1 };
    linegraph->setXColumn(datastore->addColumn(XCA, NDATA, "x"));
    linegraph->setYColumn(datastore->addColumn(YCA, NDATA, "Y"));

This method is especially useful, when you have large datasets (e.g. images) that you don't want to copy around.

In addition to the variants of JKQTPDatastore::addColumn(), that do not transfer ownership of the data to the JKQTPDatastore, you can also use JKQTPDatastore::addInternalColumn(), which tells the JKQTPDatastore to use the external data array and also take over its owner-ship. This implies that the array is freed when the JKQTPDatastore is destroyed, by calling free() in the array. Therefor data for this method nees to be allocated using malloc() or calloc():

    #define NDATA 5
    double* XCA=(double*)malloc(NDATA, sizeof(double));
    ...
    linegraph->setXColumn(datastore->addInternalColumn(XCA, NDATA, "x"));