判断两直线是否平行

 1 /*
 2  *函数名:if_not_parallel
 3  *功能:两条直线不垂直的情况下,判断是否相交
 4  *输入:(x1,y1),(x2,y2)是线段一的两个端点的坐标
 5  *     (x3,y3),(x4,y4)是线段二的两个端点的坐标
 6  *输出:返回整型值判断两条线段是否相交
 7  */
 8 int if_intersect_np(double x1,double y1,double x2,double y2, double x3,double y3,double x4,double y4)
 9 {
10     double x;
11     x=((x1*y2-x2*y1)/(x2-x1)+(x4*y3-x3*y4)/(x4-x3))/((y2-y1)/(x2-x1)-(y4-y3)/(x4-x3));
12     if(((x1-x)*(x-x2)>=0)&&((x3-x)*(x-x4)>=0))
13     return 1;
14     else
15     return 0;
16 }