qfile read from last position

[NathiyaAmose]

i want to plot curve by reading 512 data from text file for each 250ms,but i can plot the curve by reading 512 data once and how can i read the text file from last read position in the file to update my plot.And my code is,

MainWin::MainWin( QWidget *parent): QwtPlot( parent ){
 
    setAxisScale( xBottom, 0.0,512.0,50.0 );    
    setAutoReplot(true);
    setAxisScale( yLeft, 0.0, 255.0,50.0);
    canvas()->setLineWidth( 1 );
    canvas()->setFrameStyle( QFrame::Box | QFrame::Plain );
    canvas()->setBorderRadius( 1 );
    QPalette canvasPalette;
    canvasPalette.setColor( QPalette::Background, QColor( 30, 30, 50 ) );
    canvasPalette.setColor( QPalette::Foreground, QColor( 133, 190, 232 ) );
    canvas()->setPalette( canvasPalette );
 
    QwtPlotCurve *d_curves=new QwtPlotCurve();
    d_curves->setSymbol( new QwtSymbol( QwtSymbol::NoSymbol, Qt::NoBrush,
        QPen( Qt::black ), QSize( 5, 5 ) ) );
    d_curves->setPen( QPen( QColor( 200, 150, 50 ), 1.5 ) );    
    d_curves->setStyle( QwtPlotCurve::Lines );
    d_curves->setRenderHint( QwtPlotCurve::RenderAntialiased );   
    d_curves->setRawSamples( xval, yval, Size );
    d_curves->attach(this);
    (void)startTimer(250);
 
}
void MainWin::timerEvent(QTimerEvent *)
{
    newCurveData();
    replot();   
 
}
 
void MainWin::newCurveData()
{
 
    QFile file("val/measure.txt");
    if(!file.open(QIODevice::ReadOnly | QIODevice::Text)){
         printf("cannot open");
     }
     QTextStream in(&file);
     int j=0;
     for(j=0;j<=512;j++)
     {
      QString line=in.readLine();
      xval[j] = j;
      yval[j]=line.toDouble();
 
   }
 }

MainWin::MainWin( QWidget *parent): QwtPlot( parent ){
   
    setAxisScale( xBottom, 0.0,512.0,50.0 );    
    setAutoReplot(true);
    setAxisScale( yLeft, 0.0, 255.0,50.0);
    canvas()->setLineWidth( 1 );
    canvas()->setFrameStyle( QFrame::Box | QFrame::Plain );
    canvas()->setBorderRadius( 1 );
    QPalette canvasPalette;
    canvasPalette.setColor( QPalette::Background, QColor( 30, 30, 50 ) );
    canvasPalette.setColor( QPalette::Foreground, QColor( 133, 190, 232 ) );
    canvas()->setPalette( canvasPalette );

    QwtPlotCurve *d_curves=new QwtPlotCurve();
    d_curves->setSymbol( new QwtSymbol( QwtSymbol::NoSymbol, Qt::NoBrush,
        QPen( Qt::black ), QSize( 5, 5 ) ) );
    d_curves->setPen( QPen( QColor( 200, 150, 50 ), 1.5 ) );    
    d_curves->setStyle( QwtPlotCurve::Lines );
    d_curves->setRenderHint( QwtPlotCurve::RenderAntialiased );   
    d_curves->setRawSamples( xval, yval, Size );
    d_curves->attach(this);
    (void)startTimer(250);

}
void MainWin::timerEvent(QTimerEvent *)
{
    newCurveData();
    replot();   

}

void MainWin::newCurveData()
{

    QFile file("val/measure.txt");
    if(!file.open(QIODevice::ReadOnly | QIODevice::Text)){
         printf("cannot open");
     }
     QTextStream in(&file);
     int j=0;
     for(j=0;j<=512;j++)
     {
      QString line=in.readLine();
      xval[j] = j;
      yval[j]=line.toDouble();

   }
 }

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