HDUOJ 1086 You can Solve a Geometry Problem too（计算几何）

solution：

#include 
using namespace std;

struct point{
	double x, y;
	point operator - (point a)
	{
		return point{x - a.x, y - a.y};
	}
	double operator * (point a)
	{
		return x * a.y - y * a.x; 
	}
}; 

struct line{
	point s, t;
}L[105];

bool check(line a, line b)
{
	point A = a.s, B = a.t, C = b.s, D = b.t;
	if (((C - A) * (B - A)) * ((D - A) * (B - A)) > 0.000001)return false;
	if (((A - C) * (D - C)) * ((B - C) * (D - C)) > 0.000001)return false;
	return true;
}

int main()
{
	int n;
	while (cin >> n && n){
		for (int i = 0; i < n; ++i){
			cin >> L[i].s.x >> L[i].s.y >> L[i].t.x >> L[i].t.y;
		}
		int res = 0;
		for (int i = 0; i < n; ++i){
			for (int j = i + 1; j < n; ++j){
				res += check(L[i], L[j]);
			}
		} 
		cout << res << endl;
	}
	return 0;
}