poj 2653 Pick-up sticks

#include<stdio.h>
#include<math.h>
#include<string.h>
#define eps 1e-8
#define zero(x)(((x)>0 ? (x):(-x))<eps)
struct Point
{
    double x,y;
};
struct Line
{
    Point a,b;
}line[100100],tmp;
int num[100100];
double xmult(Point p1,Point p2,Point p0)//p0p1*p0p2 叉积
{
    return (p1.x-p0.x)*(p2.y-p0.y)-(p2.x-p0.x)*(p1.y-p0.y);
}
double dmult(Point p1,Point p2,Point p0)//p0p1•p0p2点积
{
    return (p1.x-p0.x)*(p2.x-p0.x)+(p1.y-p0.y)*(p2.y-p0.y);
}

int main()
{
    int i,j,n;
    while(scanf("%d",&n)!=EOF&&n)
    {
        memset(num,0,sizeof(num));
        for(i=0;i<n;i++)
        scanf("%lf%lf%lf%lf",&line[i].a.x,&line[i].a.y,&line[i].b.x,&line[i].b.y);
        for(i=0;i<n-1;i++)
        {

            for(j=i+1;j<n;j++)
            {
                if(xmult(line[i].a,line[i].b,line[j].a)*xmult(line[i].a,line[i].b,line[j].b)<-eps&&
                   xmult(line[j].a,line[j].b,line[i].a)*xmult(line[j].a,line[j].b,line[i].b)<-eps)
                   {
                       num[i]=1;
                       break;
                   }
            }

        }
        printf("Top sticks:");
        for(i=0;i<n-1;i++)
        {
            if(!num[i])printf(" %d,",i+1);
        }
        printf(" %d.\n",n);
    }
    return 0;
}