/* poly_area.c  compute area of a closed polygon */
/*              sailors algorithm                */
/*              sum signed areas of trapezoids   */

#include <stdio.h>

int main(int argc, char *argv[])
{
  float x1, y1, x2, y2; /* read points */
  float dx; /* difference x2-x1 */
  float area, da;

  printf("poly_area.c running \n");
  area = 0.0;
  /* read data, closed polygon, last point is first point */
  scanf("%f %f", &x1, &y1);
  printf("data x1=%8.5f,  y1=%8.5f \n", x1, y1);
  y1 = y1+100.0; /* y's must be positive */
  while(1)
  {
    scanf("%f  %f", &x2, &y2);
    if(feof(stdin)) break;
    printf("data x2=%8.5f,  y2=%8.5f \n", x2, y2);
    y2 = y2+100.0; 
    /* compute area */
    dx = x2-x1; /* determines sign of da */
    da = dx*(y1+y2)/2.0; /* area of trapezoid */
    area = area + da;
    printf("dx=%g, da=%g, area=%g \n", dx, da, area);
    x1 = x2;
    y1 = y2;
  }
  printf("\n");
  return 0;
} /* end poly_area */