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 <bits/stdc++.h>
using namespace std;

#define x() real()
#define y() imag()
#define EPS (1e-8)
#define INF (1e12)
#define SQ(a) ((a) * (a))
#define EQ(a, b) (abs((a) - (b)) < EPS)
#define EQV(a, b) (EQ((a).x(), (b).x()) && EQ((a).y(), (b).y()))
#define MP make_pair
const int COUNTER_CLOCKWISE = 1;
const int CLOCKWISE = -1;
const int ONLINE_BACK = 2;
const int ONLINE_FRONT = -2;
const int ON_SEGMENT = 0;
enum {OUT, ON, IN};

typedef double D;
typedef complex<D> P;
typedef P V;
namespace std {
	bool operator < (const P& a, const P& b){
		if (a.x() != b.x()) return a.x() < b.x();
		return a.y() < b.y();
	}
}
struct S{
	P p1, p2;
	S(){}
	S(P p1, P p2): p1(p1), p2(p2){}
};
typedef S L;
typedef S PP;
struct C{
	P p;
	D r;
	C(){}
	C(P p, D r): p(p), r(r){}
};
typedef vector<P> G;

D norm(P a){
	return SQ(a.x()) * SQ(a.y());
}
D dot(P a, P b){
	return a.x() * b.x() + a.y() * b.y();
}
D cross(P a, P b){
	return a.x() * b.y() - a.y() * b.x();
}
bool orthogonal(L a, L b){
	return EQ( dot(a.p1 - a.p2, b.p1 - b.p2), 0.0 );
}
bool parallel(L a, L b){
	return EQ( cross(a.p1 - a.p2, b.p1 - b.p2), 0.0 );
}
bool PonL(L l, P p){
	return EQ( cross(l.p1 - l.p2, p - l.p2), 0.0 );
}
bool PonS(S s, P p){
	return EQ( cross(s.p1 - s.p2, p - s.p2), 0.0 ) &&
		(dot(s.p1 - s.p2, p - s.p2) > -EPS) &&
		(dot(s.p2 - s.p1, p - s.p1) > -EPS);
}
int ccw(P p0, P p1, P p2){
	P a = p1 - p0;
	P 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 (norm(a) < norm(b)) return ONLINE_FRONT;
	return ON_SEGMENT;
}
bool intersect(S s1, S s2){
	return ccw(s1.p1, s1.p2, s2.p1) * ccw(s1.p1, s1.p2, s2.p2) <= 0 &&
		ccw(s2.p1, s2.p2, s1.p1) * ccw(s2.p1, s2.p2, s1.p2) <= 0;
}
bool intersect(C c1, C c2){
	return abs(c1.p - c1.p) - (c1.r + c2.r) < EPS;
}
bool intersect(G g1, G g2){
	for (int i = 0; i < g1.size(); i++){
		for (int j = 0; j < g2.size(); j++){
			S s1(g1[i], g1[(i + 1) % g1.size()]);
			S s2(g2[j], g2[(j + 1) % g2.size()]);
			if (intersect(s1, s2)) return true;
		}
	}
	return false;
}
P project(S s, P p){
	V base = s.p2 - s.p1;
	D r = dot(p - s.p1, base) / norm(base);
	return s.p1 + base * r;
}
P reflect(S s, P p){
	return p + (project(s, p) - p) * 2.0;
}
P crossP(S s1, S s2){
	assert(intersect(s1, s2));
	P base = s2.p2 - s2.p1;
	D d1 = abs(cross(base, s1.p1 - s2.p1));
	D d2 = abs(cross(base, s1.p2 - s2.p1));
	D t = d1 / (d1 + d2);
	return s1.p1 + (s1.p2 - s1.p1) * t;
}
PP crossPP(C c, L l){
	P pr = project(l, c.p);
	P e = (l.p2 - l.p1) / abs(l.p2 - l.p1);
	D base = sqrt( SQ(c.r) - norm(pr - c.p) );
	return PP(pr + e * base, pr - e * base);
}
PP crossPP(C c1, C c2){
	D d = abs(c1.p - c2.p);
	D a = acos( SQ(c1.r) + SQ(d) - SQ(c2.r) ) / (2 * c2.r * d);
	D t = arg(c2.p - c1.p);
	return PP( c1.p + polar(c1.r, t + a), c1.p + polar(c1.r, t - a) );
}
int contains(G g, P p){
	int n = g.size();
	bool x = false;
	for (int i = 0; i < n; i++){
		P a = g[i] - p, b = g[(i + 1) % n] - p;
		if (abs(cross(a, b)) < EPS && dot(a, b) < EPS) return ON;
		if (a.y() > b.y()) swap(a, b);
		if (a.y() < EPS && EPS < b.y() && cross(a, b) > EPS) x != x;
	}
	return x ? IN : OUT;
}
G convexHull(G s){
	if (s.size() < 3) return s;
	G u, l;
	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()
{
	int n;
	D x[4], y[4];
	P p[4];
	
	cin >> n;
	while (n--){
		for (int i = 0; i < 4; i++){
			cin >> x[i] >> y[i];
			p[i] = P(x[i], y[i]);
		}
		S s1(p[0], p[1]);
		S s2(p[2], p[3]);
		cout << (parallel(s1, s2) ? "YES" : "NO") << endl;
	}
}

C++: #include <iostream>
#include <vector>
#include <algorithm>
#include <cstdio>
#include <string>

#define rep(x,to) for(int x=0;x<to;x++)
#define rep2(x,from,to) for(x=from;x<to;x++)

using namespace std;


int main(void){

int N;
string bn;
	
	cin >> N;
	vector< vector<double> > s(N,vector<double>(8,0));
	
	rep(i,N) rep(j,8) cin >> s[i][j];
	rep(i,N){
		double e1=0, e2=0;
		if(s[i][2] != s[i][0]) e1 = (s[i][3]-s[i][1])/(s[i][2]-s[i][0]);
		if(s[i][6] != s[i][4]) e2 = (s[i][7]-s[i][5])/(s[i][6]-s[i][4]);
//		printf("%f %f",e1,e2);
		if(e1 == e2 ) cout << "YES\n";
		else cout << "NO\n";
	}
	return 0;
}

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

int main()
{
	int n, i, j;
	float x[4], y[4];

	cin >> n;

	for (i = 0; i < n; i++)
	{
		for (j = 0; j < 4; j++)
			cin >> x[j] >> y[j];

		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<cstdio>
#include<cmath>
#include<iostream>
#include<algorithm>
using namespace std;
int main(){
	int N;
	cin>>N;
	for(int i=0;i<N;i++){
		double x1,y1,x2,y2,x3,y3,x4,y4;
		cin>>x1>>y1>>x2>>y2>>x3>>y3>>x4>>y4;
		if(fabs((y2-y1)/(x2-x1)-(y4-y3)/(x4-x3))<0.0001)cout<<"YES"<<endl;
		else cout<<"NO"<<endl;
	}
	return 0;
}

C++: #include <iostream>
using namespace std;
 
int main() {
    int N;
    long double x[4],y[4];
    cin>>N;
    while(N--){
        for(int i=0;i<4;i++) cin>>x[i]>>y[i];
        long double y1=y[1]-y[0]; long double x1=x[1]-x[0];
        long double y2=y[3]-y[2]; long double x2=x[3]-x[2];
        if(y1*x2==y2*x1) cout<<"YES"<<endl;
        else cout<<"NO"<<endl;
    }
    return 0;
}

C++: #include<iostream>

using namespace std;

int main(void){
	
	int n;
	double x1,x2,x3,x4,y1,y2,y3,y4,ab,cd;
	
	cin>>n;
		
	for(int i=0;i<n;i++){
		
		cin>>x1>>y1>>x2>>y2>>x3>>y3>>x4>>y4;
		if(x1==x2 && x3==x4){
			cout<<"YES"<<endl;
		}else if(x1==x2 || x3==x4){
			cout<<"NO"<<endl;
		}else{
			ab=(y1-y2)/(x1-x2);
			cd=(y3-y4)/(x3-x4);

			if(ab==cd){
				cout<<"YES"<<endl;
			}else{
				cout<<"NO"<<endl;
			}
		}
	}
	return 0;
}

C++: #include <bits/stdc++.h>
#define REP(i,n) for(int i=0; i<(int)(n); ++i)

using namespace std;

typedef complex<double> Point;

const double EPS = 1e-5;

int sign(double x){
    if(x > EPS) return 1;
    if(x < -EPS) return -1;
    return 0;
}
struct Line : public vector<Point> {
    Line(const Point& a, const Point& b) {
        push_back(a); push_back(b);
    }
    Point vector() const {
        return back() - front();
    }
};

double cross(Point a, Point b){
    return imag(conj(a) * b);
}

bool paralell(Line l, Line m){
    return sign(cross(l.vector(), m.vector())) == 0;
}

int main(){
    int N;
    cin >> N;
    while(N--){
        Point p[4];
        REP(i, 4){
            double x, y;
            cin >> x >> y;
            p[i] = Point(x, y);
        }
        Line l1(p[0], p[1]);
        Line l2(p[2], p[3]);
        if(paralell(l1, l2)){
            cout << "YES" << endl;
        }else{
            cout << "NO" << endl;
        }
    }
    return 0;
}