what is the Fix to the bug in plooting simple graph using qwt int qt?

HI i am not able to fix the error .
It gives Errror during compiling as

Attachment 5412

This is the code i wrote
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Code:

#include<qwt.h>
#include<math.h>
#include <qwt_plot.h>
#include <qwt_plot_curve.h>
#include<QApplication>
int main(int argc, char **argv )
{
 
 
QwtPlot *myPlot = new QwtPlot();
 
// add curves
QwtPlotCurve *curve1 = new QwtPlotCurve("Curve 1");
QwtPlotCurve *curve2 = new QwtPlotCurve("Curve 2");
 
// copy the data into the curves
const int points = 500;
    double x[ points ];
    double sn[ points ];
    double sg[ points ];
 
    for( int i=0; i<points; i++ )
    {
      x[i] = (3.0*3.14/double(points))*double(i);
 
      sn[i] = 2.0*sin( x[i] );
      sg[i] = (sn[i]>0)?1:((sn[i]<0)?-1:0);
    }
curve1->setData(x, sn, points );
curve2->setData( x, sg, points);
 
curve1->attach(myPlot);
curve2->attach(myPlot);
 
// finally, refresh the plot
myPlot->replot();
}