hdu1086 You can Solve a Geometry Problem too

#include <stdio.h>//(abXac)(abXad)<0 && (cdXca)(cdXcb)<0struct segment{double x1,y1,x2,y2;}seg[102];int Judge(int i,int j){double a,b,c,d,x,y,a1,b1;a=seg[i].x1-seg[j].x1;b=seg[i].y1-seg[j].y1;a1=seg[i].x2-seg[j].x1;b1=seg[i].y2-seg[j].y1;c=seg[j].x2-seg[j].x1;d=seg[j].y2-seg[j].y1;x=a*d-b*c;y=a1*d-b1*c;if(x*y<=0)return 1;elsereturn 0;}int main(){int n,i,j,count;while(scanf("%d",&n)&&n){count=0;for (i=1;i<=n;i++){scanf("%lf %lf %lf %lf",&seg[i].x1,&seg[i].y1,&seg[i].x2,&seg[i].y2);}for (i=1;i<n;i++){for(j=i+1;j<=n;j++)if(Judge(i,j)&&Judge(j,i))count++;}printf("%d\n",count);}return 0;}