问垂直线使用Bresenham直线算法打印点，而不是直线

EN

#include <graphics.h>
#include <stdlib.h>
#include <stdio.h>
#include <conio.h>

     int main( )
     {
         int x1,y1,x2,y2,dx,dy,ds,dt,d,x,y;
         /* request auto detection */
         int gdriver = DETECT, gmode, errorcode;
      
        /* initialize graphics and local variables */
         initgraph(&gdriver, &gmode, "c:\\tc\\bgi");

         x1=200;
         x2=200;
         y1=200;
         y2=300;

         x=x1;
         y=y1;
         dx=x2-x1;
         dy=y2-y1;
         dt=2*(dy-dx);
         ds=2*dy;
         d=2*dy-dx;

         printf("Using Bresenham's Line Algorithm");
         putpixel(x,y,7);

         while(x<=x2)
            {
            x=x+1;
            if(d<0)
                 d=d+ds;
            else
                 {
                y=y+1;
                d=d+dt;
                 }
            putpixel(x,y,7);
            }

           getch();
       closegraph();
       return 0;
}