zoj1280

# include <stdio.h>
# define eps 1e-8
struct node
{
    double x,y;
};
node a,b,c,d;

int zero(double x)
{
    if(x<eps&&x>-eps)
        return 1;
    return 0;
}
double cross(node a,node b,node c)
{
    return (b.x-a.x)*(c.y-a.y)-(c.x-a.x)*(b.y-a.y);
}
int main ()
{
    int n;
    scanf("%d",&n);
    printf("INTERSECTING LINES OUTPUT\n");
    while(n--)
    {
        scanf("%lf%lf%lf%lf%lf%lf%lf%lf",&a.x,&a.y,&b.x,&b.y,&c.x,&c.y,&d.x,&d.y);
        if(zero(cross(a,b,c))&&zero(cross(a,b,d)))
        {
            printf("LINE\n"); continue;
        }
        if(zero((b.y-a.y)*(d.x-c.x)-(b.x-a.x)*(d.y-c.y)))
        {
            printf("NONE\n");
            continue;
        }
        node e=a;
        double t=((a.x-c.x)*(c.y-d.y)-(a.y-c.y)*(c.x-d.x))/
                 ((a.x-b.x)*(c.y-d.y)-(a.y-b.y)*(c.x-d.x));
        e.x+=(b.x-a.x)*t;
        e.y+=(b.y-a.y)*t;
        printf("POINT %.2lf %.2lf\n",e.x,e.y);
    }
    printf("END OF OUTPUT\n");
    return 0;
}

關於兩直線相交還有平行 重合以及求交點的例程