一个画抛物线的程序

/*这是一个画抛物线的程序*/

#include<graphics.h>

#define BKC 9
#define COLOR 8   /*  抛物线颜色*/
#define G 9.856

main()
{int gd=DETECT,gm,p=1,i;
double Vx,Vy,x,y,x1,y1,t;
char ch;
initgraph(&gd,&gm,"");

while(p==1)        /*主循环体*/
{
 setbkcolor(BKC);
 setcolor(14);
 setlinestyle(0,0,1);
 rectangle(0,0,639,479);       /*画边框*/
 line(20,460,639,460);  /*画横坐标*/
 line(20,0,20,460);   /*画纵坐标*/
 outtextxy(8,467,"0");
 setlinestyle(1,0,1);
 setcolor(15);
 for(i=20;i<640;i+=20)line(i,460,i,0);
 for(i=460;i>0;i-=20)line(20,i,640,i);
 gotoxy(1,1);
 printf("Please Enter Vx:" );
 scanf("%lf",&Vx);
 printf("Please Enter Vy:");
 scanf("%lf",&Vy);

 for(t=0.0;;t+=0.00002)
    {
      x=Vx*t;
      y=Vy*t-0.5*G*t*t;
      x1=2*x+20;      /*画笔定位在用户坐标原点 (20,460)  */
      y1=460-y*2;
      if(y1>460.0||x1>640.0) break;
      putpixel(x1,y1,COLOR);
    }
 setcolor(12);
 outtextxy(250,8,"Finished! Continue(Y/N)?");
 p=-1;
 while(p==-1)
  {
   ch=getch();
   if(ch=='y'||ch=='Y') { cleardevice();p=1;}
   else if(ch=='n'||ch=='N') p=0;
      else outtextxy(220,200,"ERROR! Please Press 'Y' or 'N'_" );
  }
 }
closegraph();
}