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-0)
#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]);
			//cout << p[i].x() << " " << p[i].y() << endl;
		}
		S s1(p[0], p[1]);
		S s2(p[2], p[3]);
		//cout << cross(s1.p2 - s1.p1, s2.p2 - s2.p1) << endl;
		cout << (parallel(s1, s2) ? "YES" : "NO") << endl;
	}
}

C++: #include<iostream>

using namespace std;

int main(){
	int n;
	cin >> n;

	double x1, y1, x2, y2, x3, y3, x4, y4;

	while (n--){
		cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3 >> x4 >> y4;
		if ((x2 - x1 == 0) && (x4 - x3 == 0)) cout << "YES\n";
		else
		cout <<(( (y2 - y1)*(x4 - x3) == (x2 - x1)*(y4 - y3) ) ? "YES\n" : "NO\n");
	}

	return 0;
}

C++: #include <iostream>
#include <cstdio>

bool hantei(double x[], double y[]);

int main() {
  int n;
  double x[4], y[4];
  scanf("%d", &n);
  for(int i=0;i<n;i++) {
    for(int j=0;j<4;j++) scanf("%lf %lf", &x[j], &y[j]);
    if(hantei(x, y))
      printf("YES\n");
    else printf("NO\n");
  }
}

bool hantei(double x[], double y[]) {
  if(x[0]-x[2] == x[1]-x[3] && y[0]-y[2] == y[1]-y[3]) return true;
  if(x[0]-x[3] == x[1]-x[2] && y[0]-y[3] == y[1]-y[2]) return true;
  return false;
}

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;
  double x[4],y[4];
  cin>>n;
  for(;n--;){
    for(int j=0;j<4;j++){
      cin>>x[j];
      cin>>y[j];
    }

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

C++: #include <iostream>
#include <cmath>
using namespace std;
int main(){
  int n;
  double x[4],y[4];
  cin >> n;
  while(n--){
    cin >> x[0] >> y[0] >> x[1] >> y[1] >> x[2] >> y[2] >> x[3] >> y[3];
    y[1]-=y[0];
    x[1]-=x[0];
    y[3]-=y[2];
    x[3]-=x[2];
    if(fabs(y[1]*x[3] - y[3]*x[1]) < 10e-10)cout << "YES";
    else cout << "NO";
    cout << endl;
    }
  return 0;
}

C++: #include <stdio.h>

int main()
{
	int n;
	double x1, x2, x3, x4, y1, y2, y3, y4;

	scanf("%d", &n);

	for (int i = 0; i < n; i++){
		scanf("%lf %lf %lf %lf %lf %lf %lf %lf", &x1, &y1, &x2, &y2, &x3, &y3, &x4, &y4);

		if((x2-x1)/(y2-y1) == (x4-x3)/(y4-y3)) printf("YES\n");
		else printf("NO\n");
	}

	return 0;
}

C++: #include<iostream>
using namespace std;
bool EQ(double a,double b){
  return a-0.0000001<b&&b<a+0.0000001;
}
int main(){
  int n;
  double x[4],y[4];
  cin>>n;
  while(n--){
    for(int j=0;j<4;j++) cin>>x[j]>>y[j];
    if(EQ((y[3]-y[2])*(x[1]-x[0]),(y[1]-y[0])*(x[3]-x[2])))cout<<"YES"<<endl;
    else cout<<"NO"<<endl;
  }
  return 0;
}

C++: #include<iostream>
#define dff 1e-19
using namespace std;
int main(){
  int n;
  cin >>n;
  while(n--){
    double p[4][2],x;
    for(int i=0; i<4; i++)
      cin >>p[i][0]>>p[i][1];
    for(int i=0; i<2; i++)
      for(int j=0; j<2; j++)
	p[i*2+1][j]-=p[i*2][j];
    x = p[1][0]*p[3][1]-p[1][1]*p[3][0];
    if(-dff < x && x < dff) cout <<"YES"<<endl;
    else cout <<"NO"<<endl;
  }
  return 0;
}

C++: #include<iostream>
#include<vector>
int main()
{
	int n;
	std::cin >> n;
	std::vector<bool>result(n);
	for (int i = 0; i < n; i++)
	{
		double x1, y1, x2, y2, x3, y3, x4, y4;
		std::cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3 >> x4 >> y4;
		result[i] = x1 == x2 && x3 == x4 || x1 != x2 && x3 != x4 && (y1 - y2) / (x1 - x2) == (y3 - y4) / (x3 - x4);
	}
	for (auto i : result)
	{
		std::cout << (i ? "YES" : "NO") << std::endl;
	}
	return 0;
}