Go: package main

import "fmt"

func abs(x float64) float64 {
	if x < 0 {
		return -x
	}
	return x
}

func main() {
	var n int
	var x1, y1, x2, y2, x3, y3, x4, y4 float64
	fmt.Scan(&n)
	for i := 0 ; i < n; i++ {
		fmt.Scanf("%f %f %f %f %f %f %f %f", &x1, &y1, &x2, &y2, &x3, &y3, &x4, &y4)
		if abs((x2-x1)*(y4-y3) - (y2-y1)*(x4-x3)) < 1e-10 {
			fmt.Println("YES")
		} else {
			fmt.Println("NO")
		}
	}
}

C++: #include<iostream>
using namespace std;

typedef struct xyz{
    double x;
    double y;
}XYZ;

int main(void){

    int times;
    cin >> times;

    XYZ a,b,c,d;
    double x1,y1,x2,y2;

    while(times > 0){
        cin >> a.x >> a.y;
        cin >> b.x >> b.y;
        cin >> c.x >> c.y;
        cin >> d.x >> d.y;

        x1 = a.x - b.x;
        y1 = a.y - b.y;
        x2 = c.x - d.x;
        y2 = c.y - d.y;

        if((y1 * x2) == (y2 * x1)){
            cout << "YES" << endl;
        }else{
            cout << "NO" << endl;
        }

        times--;
    }


    return 0;
}

C++: #include<iostream>
#include<string>
#include<algorithm>
#include<map>
#include<vector>
#include<cmath>
#include<cstdio>
#define loop(i,a,b) for(int i=a;i<b;i++) 
#define rep(i,a) loop(i,0,a)
#define pb(in,tmp) in.push_back(tmp)
#define all(in) in.begin(),in.end()
const double PI=acos(-1);
using namespace std;
int main(){
  int n;
  cin>>n;
  rep(i,n){
    double a=543.5,b=629.3;
    double ax,ay,bx,by,cx,cy,dx,dy;
    cin>>ax>>ay>>bx>>by>>cx>>cy>>dx>>dy;

    //if(by-ay)/(bx-ax)=(dy-cy)/(dx-cx)
    if((by-ay)*(dx-cx)==(bx-ax)*(dy-cy))cout<<"YES"<<endl;
    else cout<<"NO"<<endl;
  }
}

C++: #include <iostream>
#include <fstream>
#include <typeinfo>
#include <vector>
#include <cmath>
#include <set>
#include <map>
#include <string>
#include <algorithm>
#include <cstdio>
#include <queue>
#include <iomanip>
#include <cctype>
#define syosu(x) fixed<<setprecision(x)
using namespace std;
typedef long long ll;
typedef pair<int,int> P;
typedef pair<double,double> pdd;
typedef vector<int> vi;
typedef vector<vi> vvi;
typedef vector<char> vc;
typedef vector<vc> vvc;
typedef vector<bool> vb;
typedef vector<vb> vvb;
typedef vector<P> vp;
typedef vector<vp> vvp;
typedef pair<int,P> pip;
typedef vector<pip> vip;
const int inf=1<<25;
const double pi=acos(-1);
const double eps=1e-6;
const vi emp;

double add_double(double a,double b){
	if(abs(a+b)<eps*(abs(a)+abs(b))) return 0;
	return a+b;
}

struct point{
	double x,y;
	point operator+(point p){
		return point{add_double(x,p.x),add_double(y,p.y)};
	}
	point operator-(point p){
		return point{add_double(x,-p.x),add_double(y,-p.y)};
	}
	point operator*(double p){
		return point{x*p,y*p};
	}
	point operator/(double p){
		if(!p) return point{0,0}; 
		return point{x/p,y/p};
	}
	bool operator==(point p){
		return fabs(add_double(x,-p.x))<eps&&fabs(add_double(y,-p.y))<eps;
	}
	bool operator<(point p){
		if(fabs(x-p.x)>eps) return x<p.x;
		return y<p.y;
	}
};

typedef pair<point,point> pp;
typedef vector<point> VP;
const point O{0,0};

class Geom{
	public:
	double Length(point x,point y){
		point z=y-x;
		return sqrt(z.x*z.x+z.y*z.y);
	}
	double IP(point p,point q){
		return p.x*q.x+p.y*q.y;
	}
	double CP(point p,point q){
		return p.x*q.y-q.x*p.y;
	}
	string Counter_Clockwise(pp a,point x){
		point A=a.second-a.first;
		point X=x-a.first;
		double ip=IP(A,X),cp=CP(A,X),Al=Length(O,A),Xl=Length(O,X);
		if(cp>eps) return "Counter_Clockwise";
		if(cp<-eps) return "Clockwise";
		if(ip<-eps) return "Online_Back";
		if(Xl<Al||fabs(Xl-Al)<eps) return "On_Segment";
		return "Online_Front";
	}
	string Parallel_Orthogonal(pp a,pp b){
		point A=a.second-a.first,B=b.second-b.first;
		double ip=IP(A,B),cp=CP(A,B);
		string f=Counter_Clockwise(pp(a.first,a.second),b.first),s=Counter_Clockwise(pp(a.first,a.second),b.second);
		if(f[0]!='C'&&s[0]!='C') return "Agreement";
		if(cp<eps&&cp>-eps) return "Parallel";
		if(ip<eps&&ip>-eps) return "Orthogonal";
		else return "Commonly";
	}
	void Point_in(point& p){
		cin>>p.x>>p.y;
	}
};

int n;
VP p(4);
string s;

int main(){
	Geom geo;
	cin>>n];
	for(int i=0;i<n;i++){
		for(VP::iterator i=p.begin();i!=p.end();i++) geo.Point_in(*i);
		s=geo.Parallel_Orthogonal(pp(p[0],p[1]),pp(p[2],p[3]));
		if(s[0]=='P'||s[0]=='A') cout<<"YES"<<endl;
		else cout<<"NO"<<endl;
	}
}

C++: #include <iostream>
#include <cmath>
#include <vector>
#include <string>
#include <algorithm>
#include <climits>

using namespace std;

double EPS = 1e-10;

//誤差を考慮して足し算を行う
double add(double a, double b) {
	if( abs(a + b) < EPS * (abs(a) + abs(b))) {
		return 0;
	}

	return a + b;
}

//二次元ベクトル構造体
struct P {
	double x, y;

	P() {}
	P(double x, double y) : x(x), y(y) {
	}
	P operator + (P p) {
		return P(add(x, p.x), add(y, p.y));
	}
	P operator - (P p) {
		return P(add(x, -p.x), add(y, -p.y));
	}
	P operator * (double d) {
		return P(x * d, y * d);
	}
	double dot(P p) {	//　内積
		return add(x * p.x, y * p.y);
	}
	double det(P p) {	//　外積
		return add(x * p.x, -y * p.y);
	}
};

//　線分p1-p2上に点qがあるか判定
bool on_seg(P p1, P p2, P q) {
		return (p1 - q).det(p2 - q) == 0 && (p1 - q).dot(p2 - q) <= 0;
}

//　直線p1-p2と直線q1-q2の交点
P intersection(P p1, P p2, P q1, P q2) {
	return p1 + (p2 - p1) * ((q2 - q1).det(q1 - p1) / (q2 - q1).det(p2 - p1));
}


int main() {
	int n;

	cin >> n;

	P p, q, r, s;
	int x1, y1, x2, y2, x3, y3, x4, y4;

	for(int i = 0; i < n; i++) {
		cin >> p.x >> p.y >> q.x >> q.y >> r.x >> r.y >> s.x >> s.y;
		if( (q - p).det(s - r) == 0) {
			cout << "YES" << endl;
		} else {
			cout << "NO" << endl;
		}
	}

}

C++: #include <iostream>

int main(){
	double x1,y1,x2,y2,x3,y3,x4,y4;
	double a,b;
	int n;
	std::cin >> n;
	for(int i=0 ; i<n ; ++i){
		std::cin >>x1>>y1>>x2>>y2>>x3>>y3>>x4>>y4;
		bool parallel=false;
		a=(y2-y1)/(x2-x1);
		b=(y4-y3)/(x4-x3);
		if(x1==x2 && x3==x4) {
			parallel=true;
			goto result;
		}
		else if(x1==x2 || x3==x4) {
			goto result;
		}
		else if (a==b) {
			parallel=true;
		}
		result:
		std::cout << ( parallel ? "YES" : "NO") << std::endl;
	}
	return 0; 
}

C++: #include <iostream>
using namespace std;

int main() {

  int n;
  cin >> n;

  for ( int i = 0; i < n; i++ ) {

    double x[4],y[4];
    cin >> x[0] >> y[0] >> x[1] >> y[1] >> x[2] >> y[2] >> x[3] >> y[3];

    if ( ( x[0] - x[1] ) * ( x[2] - x[3] ) == 0 ) {

      if ( x[0] == x[1] && x[2] == x[3] ) {
	cout << "YES" << endl;
      }else {
	cout << "NO" << endl;
      }

    }else {

      if ( ( y[1] - y[0] ) / ( x[1] - x[0] ) == ( y[3] - y[2] ) / ( x[3] - x[2] ) ) {
	cout << "YES" << endl;
      }else {
	cout << "NO" << endl;
      }

    }

  }

  return 0;

}

C++: #include <iostream>
#include <cmath>
using namespace std;

const double EPS = 1e-10;

double add(double a, double b) {
	if (abs(a + b) < EPS * (abs(a) + abs(b)))
		return 0;
	return a + b;
}

struct P {
	double x, y;
	P() {};
	P(double x, double y) : x(x), y(y) {
	}

	P operator + (P p) {
		return P(add(x, p.x), add(y, p.y));
	}

	P operator - (P p) {
		return P(add(x, -p.x), add(y, -p.y));
	}

	P operator * (double d) {
		return P(x * d, y * d);
	}

	P operator / (double d) {
		return P(x / d, y / d);
	}

	// 内積
	double dot(P p) {
		return add(x * p.x, y * p.y);
	}

	// 外積
	double det(P p) {
		return add(x * p.y, -y * p.x);
	}
};

// 線分上に点があるか
bool on_seg(P p1, P p2, P q) {
	return (p1 - q).det(p2 - q) == 0 && (p1 - q).dot(p2 - q) <= 0;
}

// 直線と直線の交点
P intersection(P p1, P p2, P q1, P q2) {
	return p1 + (p2 - p1) * ((q2 - q1).det(q1 - p1) / (q2 - q1).det(p2 - p1));
}

// 2直線の直行判定
bool line_orth(P a1, P a2, P b1, P b2) {
	return fabs((a1 - a2).dot(b1 - b2)) < EPS;
}

// 2直線の平行判定
bool line_paral(P a1, P a2, P b1, P b2) {
	return fabs((a1 - a2).det(b1 - b2)) < EPS;
}

int main(void) {
	int N;
	P p[4];
	cin >> N;
	for (int i = 0; i < N; i++) {
		for (int j = 0; j < 4; j++) {
			cin >> p[j].x >> p[j].y;
		}
		cout << (line_paral(p[0], p[1], p[2], p[3]) ? "YES" : "NO") << endl;
	}
}