R inside Qt: A simple RInside application

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The RInside package makes it pretty simple and straightforward to embed R, the wonderful statistical programming environment and language, inside of a C++ application. This uses both the robust embedding API provided by R itself, and the higher-level abstractions from our Rcpp package. A number of examples are shown on this blog both here and here; and the source package actually contains well over a dozen complete examples which cover anything from simple examples to parallel use via MPI for parallel computing.

Beginning users sometimes ask about how to use RInside inside larger projects. And as I had meant to experiment with embedding inside of the powerful Qt framework anyway, I started to dabble a little. A first result is now in the SVN sources of RInside.

My starting point was the classic tkdensity demo that comes with R itself. It is a good point of departure as Tcl/Tk makes it very portable—in fact it should run on every platform that runs R—and quite expressive. And having followed some of the GUI experiments around R over the years, I have also seen various re-implementations using different GUI frameworks. And so I am adding mine to this body of work:

Example of embedding R via RInside into a Qt C++ application: density estimation for a mixture

The problem I addressed first was actual buildability. For the RInside examples, Romain and I provide a Makefile that just works by making calls to R itself to learn about flags for R, Rcpp and RInside such that all required headers and libraries are found. That is actually relatively straightforward (and documented in our vignettes) but a little intimidating at first—which is why a ready-made Makefile is a good thing.

Qt of course uses qmake and the .pro files to encode / resolve dependencies. So task one was to map what our Makefile does into its variables. Turns out that wasn’t all that hard:

## -*- mode: Makefile; c-indent-level: 4; c-basic-offset: 4;  tab-width: 8; -*-
## Qt usage example for RInside, inspired by the standard 'density
## sliders' example for other GUI toolkits
## Copyright (C) 2011  Dirk Eddelbuettel and Romain Francois

TEMPLATE =              app
HEADERS =               qtdensity.h 
SOURCES =               qtdensity.cpp main.cpp

QT +=                   svg

## comment this out if you need a different version of R, 
## and set set R_HOME accordingly as an environment variable
R_HOME =                $$system(R RHOME)

## include headers and libraries for R 
RCPPFLAGS =             $$system($$R_HOME/bin/R CMD config --cppflags)
RLDFLAGS =              $$system($$R_HOME/bin/R CMD config --ldflags)
RBLAS =                 $$system($$R_HOME/bin/R CMD config BLAS_LIBS)
RLAPACK =               $$system($$R_HOME/bin/R CMD config LAPACK_LIBS)

## if you need to set an rpath to R itself, also uncomment
#RRPATH =               -Wl,-rpath,$$R_HOME/lib

## include headers and libraries for Rcpp interface classes
RCPPINCL =              $$system($$R_HOME/bin/Rscript -e \'Rcpp:::CxxFlags\(\)\')
RCPPLIBS =              $$system($$R_HOME/bin/Rscript -e \'Rcpp:::LdFlags\(\)\')

## for some reason when building with Qt we get this each time
## so we turn unused parameter warnings off
RCPPWARNING =           -Wno-unused-parameter 
## include headers and libraries for RInside embedding classes
RINSIDEINCL =           $$system($$R_HOME/bin/Rscript -e \'RInside:::CxxFlags\(\)\')
RINSIDELIBS =           $$system($$R_HOME/bin/Rscript -e \'RInside:::LdFlags\(\)\')

## compiler etc settings used in default make rules

## addition clean targets
QMAKE_CLEAN +=          qtdensity Makefile
The double dollar signs and escaping of parentheses are a little tedious, but hey it works and expands the compiler and linker flags such that everything .

The code itself is pretty straightforward too. We instantiate the RInside object as well as the main Qt application object. We then instantiate a new object of class QtDensity that will launch the main widget; it is given a reference to the RInside object.

// -*- mode: C++; c-indent-level: 4; c-basic-offset: 4;  tab-width: 8; -*-
// Qt usage example for RInside, inspired by the standard 'density
// sliders' example for other GUI toolkits
// Copyright (C) 2011  Dirk Eddelbuettel and Romain Francois

#include <QApplication>

#include "qtdensity.h"

int main(int argc, char *argv[])
    RInside R(argc, argv);  		// create an embedded R instance

    QApplication app(argc, argv);
    QtDensity qtdensity(R);
    return app.exec();

The definition of the main object is pretty simple: a few private variables, and a few functions to interact with the GUI and get values from the radio buttons, slider or input field—as well as functions to update the chart or re-draw the random variables.

// -*- mode: C++; c-indent-level: 4; c-basic-offset: 4;  tab-width: 8; -*-
// Qt usage example for RInside, inspired by the standard 'density
// sliders' example for other GUI toolkits
// Copyright (C) 2011  Dirk Eddelbuettel and Romain Francois


#include <RInside.h>

#include <QMainWindow>
#include <QHBoxLayout>
#include <QSlider>
#include <QSpinBox>
#include <QLabel>
#include <QTemporaryFile>
#include <QSvgWidget>

class QtDensity : public QMainWindow

    QtDensity(RInside & R);

private slots:
    void getBandwidth(int bw);
    void getKernel(int kernel);
    void getRandomDataCmd(QString txt);
    void runRandomDataCmd(void);

    void setupDisplay(void);    // standard GUI boilderplate of arranging things
    void plot(void);            // run a density plot in R and update the
    void filterFile(void);      // modify the richer SVG produced by R

    QSvgWidget *m_svg;          // the SVG device

    RInside & m_R;              // reference to the R instance passed to constructor
    QString m_tempfile;         // name of file used by R for plots
    QString m_svgfile;          // another temp file, this time from Qt
    int m_bw, m_kernel;         // parameters used to estimate the density
    QString m_cmd;              // random draw command string


Lastly, no big magic in the code either (apart from the standard magic provided by RInside). A bit of standard GUI layouting, and then some functions to pick values from the inputs as well as to compute / update the output. One issue is worth mentioning. The screenshot and code show the second version of this little application. I built a first one using a standard portable network graphics (png) file. That was fine, but not crisp as png is a pixel format so I went back and experimented with scalable vector graphics (svg) instead. One can create svg output with R in a number of ways, one of which is the cairoDevice package by Michael Lawrence (who also wrote RGtk2 and good chunks of Ggobi). Now, it turns out that Qt displays the so-called SVG tiny standard whereas R creates a fuller SVG format. Some discussion with Michael reveals that one can modify the svg file suitably (which is what the function filterFile below does) and it all works. Well: almost. There is a bug (and Michael thinks it is the SVG rendering) in which the density estimate does not get clipped to the plotting region.

// -*- mode: C++; c-indent-level: 4; c-basic-offset: 4;  tab-width: 8; -*-
// Qt usage example for RInside, inspired by the standard 'density
// sliders' example for other GUI toolkits -- this time with SVG
// Copyright (C) 2011  Dirk Eddelbuettel and Romain Francois

#include <QtGui>

#include "qtdensity.h"

QtDensity::QtDensity(RInside & R) : m_R(R)
    m_bw = 100;                 // initial bandwidth, will be scaled by 100 so 1.0
    m_kernel = 0;               // initial kernel: gaussian
    m_cmd = "c(rnorm(100,0,1), rnorm(50,5,1))"; // simple mixture
    m_R["bw"] = m_bw;           // pass bandwidth to R, and have R compute a temp.file name
    m_tempfile = QString::fromStdString(Rcpp::as<std::string>(m_R.parseEval("tfile <- tempfile()")));
    m_svgfile = QString::fromStdString(Rcpp::as<std::string>(m_R.parseEval("sfile <- tempfile()")));


void QtDensity::setupDisplay(void)  {
    QWidget *window = new QWidget;
    window->setWindowTitle("Qt and RInside demo: density estimation");

    QSpinBox *spinBox = new QSpinBox;
    QSlider *slider = new QSlider(Qt::Horizontal);
    spinBox->setRange(5, 200);
    slider->setRange(5, 200);
    QObject::connect(spinBox, SIGNAL(valueChanged(int)), slider, SLOT(setValue(int)));
    QObject::connect(slider, SIGNAL(valueChanged(int)), spinBox, SLOT(setValue(int)));
    QObject::connect(spinBox, SIGNAL(valueChanged(int)), this, SLOT(getBandwidth(int)));

    QLabel *cmdLabel = new QLabel("R command for random data creation");
    QLineEdit *cmdEntry = new QLineEdit(m_cmd);
    QObject::connect(cmdEntry,  SIGNAL(textEdited(QString)), this, SLOT(getRandomDataCmd(QString)));
    QObject::connect(cmdEntry,  SIGNAL(editingFinished()), this, SLOT(runRandomDataCmd()));

    QGroupBox *kernelRadioBox = new QGroupBox("Density Estimation kernel");
    QRadioButton *radio1 = new QRadioButton("&Gaussian");
    QRadioButton *radio2 = new QRadioButton("&Epanechnikov");
    QRadioButton *radio3 = new QRadioButton("&Rectangular");
    QRadioButton *radio4 = new QRadioButton("&Triangular");
    QRadioButton *radio5 = new QRadioButton("&Cosine");
    QVBoxLayout *vbox = new QVBoxLayout;
    kernelRadioBox->setSizePolicy(QSizePolicy::Fixed, QSizePolicy::Fixed);

    QButtonGroup *kernelGroup = new QButtonGroup;
    kernelGroup->addButton(radio1, 0);
    kernelGroup->addButton(radio2, 1);
    kernelGroup->addButton(radio3, 2);
    kernelGroup->addButton(radio4, 3);
    kernelGroup->addButton(radio5, 4);
    QObject::connect(kernelGroup, SIGNAL(buttonClicked(int)), this, SLOT(getKernel(int)));

    m_svg = new QSvgWidget();
    runRandomDataCmd();         // also calls plot()

    QGroupBox *estimationBox = new QGroupBox("Density estimation bandwidth (scaled by 100)");
    QHBoxLayout *spinners = new QHBoxLayout;
    QVBoxLayout *topright = new QVBoxLayout;
    estimationBox->setSizePolicy(QSizePolicy::Fixed, QSizePolicy::Fixed);
    QHBoxLayout *upperlayout = new QHBoxLayout;

    QHBoxLayout *svglayout = new QHBoxLayout;

    QVBoxLayout *outer = new QVBoxLayout;

void QtDensity::plot(void) {
    const char *kernelstrings[] = { "gaussian", "epanechnikov", "rectangular", "triangular", "cosine" };
    m_R["bw"] = m_bw;
    m_R["kernel"] = kernelstrings[m_kernel]; // that passes the string to R
    std::string cmd1 = "Cairo(width=6,height=6,pointsize=10,surface='svg',filename=tfile); "
                       "plot(density(y, bw=bw/100, kernel=kernel), xlim=range(y)+c(-2,2), main=\"Kernel: ";
    std::string cmd2 = "\"); points(y, rep(0, length(y)), pch=16, col=rgb(0,0,0,1/4));  dev.off()";
    std::string cmd = cmd1 + kernelstrings[m_kernel] + cmd2; // stick the selected kernel in the middle
    filterFile();               // we need to simplify the svg file for display by Qt 

void QtDensity::getBandwidth(int bw) {
    if (bw != m_bw) {
        m_bw = bw;

void QtDensity::getKernel(int kernel) {
    if (kernel != m_kernel) {
        m_kernel = kernel;

void QtDensity::getRandomDataCmd(QString txt) {
    m_cmd = txt;

void QtDensity::runRandomDataCmd(void) {
    std::string cmd = "y <- " + m_cmd.toStdString();
    plot();                     // after each random draw, update plot with estimate

void QtDensity::filterFile() {
    // cairoDevice creates richer SVG than Qt can display
    // but per Michaele Lawrence, a simple trick is to s/symbol/g/ which we do here

    QFile infile(m_tempfile);
    QFile outfile(m_svgfile);
    outfile.open(QFile::WriteOnly | QFile::Truncate);
    QTextStream in(&infile);
    QTextStream out(&outfile);
    QRegExp rx1("<symbol"); 
    QRegExp rx2("</symbol");    
    while (!in.atEnd()) {
        QString line = in.readLine();
        line.replace(rx1, "<g"); // so '<symbol' becomes '<g ...'
        line.replace(rx2, "</g");// and '</symbol becomes '</g'
        out << line << "\n";

What the little application does is actually somewhat neat for the few lines. One key features is that the generated data can be specified directly by an R expression which allows for mixtures (as shown, and as is the default). With that it easy to see how many points are needed in the second hump to make the estimate multi-modal, and how much of a distance between both centers is needed and so on. Obviously, the effect of the chosen kernel and bandwidth can also be visualized. And with the chart the being a support vector graphics display, we can resize and scale at will and it still looks crisp.

The code (for both the simpler png variant and the svg version shown here) is in the SVN repository for RInside and will be in the next release. Special thanks to Michael Lawrence for patiently working through some svg woes with me over a few emails.

Update: Some typos fixed.

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