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++: // I haven't implement assertion in two getCrossPoints function.
#include<iostream>
#include<cstdio>
#include<cmath>
#include<vector>
#include<iomanip>
#include<algorithm>
using namespace std;
static const int COUNTER_CLOCKWISE = 1;
static const int CLOCKWISE = -1;
static const int ONLINE_BACK = 2;
static const int ONLINE_FRONT = -2;
static const int ON_SEGMENT = 0;

#define EPS (1e-10)
#define equals(a, b) (fabs((a) - (b)) < EPS)

class Point{
	public:
		double x, y;

		Point(double x = 0, double y = 0): x(x), y(y) {}

		Point operator + (Point p){ return Point(x + p.x, y + p.y);}
		Point operator - (Point p){ return Point(x - p.x, y - p.y);}
		Point operator * (double a){ return Point(a * x, a * y);}
		Point operator / (double a){ return Point(x / a, y / a);}

		double abs(){ return sqrt(norm());}
		double norm(){ return x * x + y * y;}

		bool operator < (const Point &p) const{
			return x != p.x ? x < p.x : y < p.y;
		}

		bool operator == (const Point &p) const{
			return fabs(x - p.x) < EPS && fabs(y - p.y) < EPS;
		}
};

struct Segment {
	Point p1, p2;
};
typedef Segment Line;

class Circle{
	public:
		Point c;
		double r;
		Circle(Point c = Point(), double r = 0.0): c(c), r(r) {}
};

typedef Point Vector;
typedef vector<Point> Polygon;

double dot(Vector a, Vector b){
	return a.x * b.x + a.y * b.y;
}
double cross(Vector a, Vector b){
	return a.x * b.y - a.y * b.x;
}

Point project(Segment s, Point p){
	Vector base = s.p2 - s.p1;
	double r = dot(p - s.p1, base) / base.norm();
	return s.p1 + base * r;
}
Point reflect(Segment s, Point p){
	return p + (project(s, p) - p) * 2.0;
}
int ccw(Point p0, Point p1, Point p2){
	Vector a = p1 - p0;
	Vector b = p2 - p0;
	if(cross(a, b) > EPS) return COUNTER_CLOCKWISE;
	if(cross(a, b) < -EPS) return CLOCKWISE;
	if(dot(a, b) < -EPS) return ONLINE_BACK;
	if(a.norm() < b.norm()) return ONLINE_FRONT;

	return ON_SEGMENT;
}

bool isOrthogonal(Vector a, Vector b){
	return equals(dot(a, b), 0.0);
}
bool isOrthogonal(Point a1, Point a2, Point b1, Point b2){
	return isOrthogonal(a1 - a2, b1 - b2);
}
bool isOrthogonal(Segment s1, Segment s2){
	return equals(dot(s1.p2 - s1.p1, s2.p2 - s2.p1), 0.0);
}

bool isParallel(Vector a, Vector b){
	return equals(cross(a, b), 0.0);
}
bool isParallel(Point a1, Point a2, Point b1, Point b2){
	return isParallel(a1 - a2, b1 - b2);
}
bool isParallel(Segment s1, Segment s2){
	return equals(cross(s1.p2 - s1.p1, s2.p2 - s2.p1), 0.0);
}

bool intersect(Point p1, Point p2, Point p3, Point p4){
	return (ccw(p1, p2, p3) * ccw(p1, p2, p4) <= 0 && 
			ccw(p3, p4, p1) * ccw(p3, p4, p2) <= 0);
}
bool intersect(Segment s1, Segment s2){
	return intersect(s1.p1, s1.p2, s2.p1, s2.p2);
}

Point getCrossPoint(Segment s1, Segment s2){
	Vector base = s2.p2 - s2.p1;
	double d1 = abs(cross(base, s1.p1 - s2.p1));
	double d2 = abs(cross(base, s1.p2 - s2.p1));
	double t = d1 / (d1 + d2);
	return s1.p1 + (s1.p2 - s1.p1) * t;
}

double getDistance(Point a, Point b){
	return (a - b).abs();
}
double getDistanceLP(Line l, Point p){
	return abs(cross(l.p2 - l.p1, p - l.p1) / (l.p2 - l.p1).abs());
}
double getDistanceSP(Segment s, Point p){
	if(dot(s.p2 - s.p1, p - s.p1) < 0.0) return (p - s.p1).abs();
	if(dot(s.p1 - s.p2, p - s.p2) < 0.0) return (p - s.p2).abs();
	return getDistanceLP(s, p);
}
double getDistance(Segment s1, Segment s2){
	if(intersect(s1, s2)) return 0.0;
	return min(min(getDistanceSP(s1, s2.p1), getDistanceSP(s1, s2.p2)),
			min(getDistanceSP(s2, s1.p1), getDistanceSP(s2, s1.p2)));
}

pair<Point, Point> getCrossPoints(Circle c, Line l){
	Vector pr = project(l, c.c);
	Vector e = (l.p2 - l.p1) / (l.p2 - l.p1).abs();
	double base = sqrt(c.r * c.r - (pr - c.c).norm());
	return make_pair(pr + e * base, pr - e * base);
}

double arg(Vector p){return atan2(p.y, p.x);}
Vector polar(double a, double r){return Point(cos(r) * a, sin(r) * a);}

pair<Point, Point> getCrossPoints(Circle c1, Circle c2){
	double d = (c1.c - c2.c).abs();
	double a = acos((c1.r * c1.r + d * d - c2.r * c2.r) / (2 * c1.r * d));
	double t = arg(c2.c - c1.c);
	return make_pair(c1.c + polar(c1.r, t + a), c1.c + polar(c1.r, t - a));
}

/*
   IN 2
   ON 1
   OUT 0
   */
int contains(Polygon g, Point p){
	int n = g.size();
	bool x = false;
	for(int i = 0; i < n; i++){
		Point a = g[i] - p, b = g[(i + 1) % n] - p;
		if(abs(cross(a, b)) < EPS && dot(a, b) < EPS) return 1;
		if(a.y > b.y) swap(a, b);
		if(a.y < EPS && EPS < b.y && cross(a, b) > EPS) x = !x;
	}
	return (x ? 2 : 0);
}

Polygon andrewScan(Polygon s){
	Polygon u, l;
	if(s.size() < 3) return s;
	sort(s.begin(), s.end());
	u.push_back(s[0]);
	u.push_back(s[1]);
	l.push_back(s[s.size() - 1]);
	l.push_back(s[s.size() - 2]);

	for(int i = 2; i < s.size(); i++){
		for(int n = u.size(); n >= 2 && ccw(u[n - 2], u[n - 1], s[i]) != CLOCKWISE; n--){
			u.pop_back();
		}
		u.push_back(s[i]);
	}

	for(int i = s.size() - 3; i >= 0; i--){
		for(int n = l.size(); n >= 2 && ccw(l[n - 2], l[n - 1], s[i]) != CLOCKWISE; n--){
			l.pop_back();
		}
		l.push_back(s[i]);
	}

	reverse(l.begin(), l.end());
	for(int i = u.size() - 2; i >= 1; i--) l.push_back(u[i]);
	
	return l;
}

int main(void){
	int n;
	Point p1, p2, p3, p4;
	cin >> n;
	for(int i = 1; i <= n; i++){
		cin >> p1.x >> p1.y >> p2.x >> p2.y >> p3.x >> p3.y >> p4.x >> p4.y;
		bool ans = isParallel(p1, p2, p3, p4);
		if(ans) cout << "YES" << endl;
		else cout << "NO" << endl;
	}
	return 0;
}

C++: #include <iostream>
using namespace std;
int main(){
	int n;
	double x1,y1,x2,y2,x3,y3,x4,y4;
	double vx1, vy1, vx2, vy2;
	cin >> n;
	for (int i = 0; i < n; i++){
		cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3 >> x4 >> y4;
		vx1 = x2 - x1;
		vy1 = y2 - y1;
		vx2 = x4 - x3;
		vy2 = y4 - y3;
		if (vy1*vx2 == vx1*vy2) cout << "YES\n";
		else cout << "NO\n";
	}
}

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

typedef struct P {
	long double x, y;
}P;
int n;
long double a1, a2, b1, b2, c1, c2;
P p[4];

int main(){
	cin >> n;
	for (int l = 0; l < n; l++) {
		for (int i = 0; i < 4; i++) {
			cin >> p[i].x >> p[i].y;
		}
		a1 = p[0].x - p[1].x;
		a2 = p[0].y - p[1].y;
		b1 = p[2].x - p[3].x;
		b2 = p[3].y - p[3].x;
		c1 = a1*b1;
		c2 = a2*b2;
		c2 -= c1;
		if (c2 < 0.000001 && c2 > -0.000001) {
			cout << "YES" << endl;
		}
		else {
			cout << "NO" << endl;
		}
	}

	return 0;
}

C++: #include <iostream>

int main(){
	
	double a,x1,y1,x2,y2,x3,y3,x4,y4;
	std::cin>>a;
	for(int s=0;s<a;s++){
		std::cin>>x1>>y1>>x2>>y2>>x3>>y3>>x4>>y4;
		if((y2-y1)/(x2-x1)==(y4-y3)/(x4-x3)||(y2-y1)/(x2-x1)==-1/((y4-y3)/(x4-x3)){
			std::cout<<"YES"<<std::endl;
		}else{
			std::cout<<"NO"<<std::endl;
		}
		
    }
    return 0;
}

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

int main(){
    const double eps = 1e-11;
    double x[4], y[4];
    int s,t,u,v;
    int n;
    cin >> n;
    for (int t = 0;t < N;++t) {
        for (int i = 0;i < 4;++i){
            cin >> x[i] >> y[i]; 
        }
    }
    s = x[1] - x[0];
    t = y[1] - y[0];
    u = x[3] - x[2];
    v = y[3] - y[2];
    if(s * v - t * u > 0){
        int m = s * v - t * u;
    }
    else{
        int m = t * u - s * v;
    }
    if( m < eps){
        cout << "YES" << endl;
    }
    else{
        cout << "NO" << endl;
    }
    return 0;
}