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>
#define rep(i,n) for (int i = 0; i < (n); i ++)
using namespace std;
using ll = long long;
using PL = pair<ll,ll>;
using P = pair<int,int>;
constexpr int INF = 1000000000;
constexpr long long HINF = 1000000000000000;
constexpr long long MOD = 1000000007;
constexpr double EPS = 1e-10;
constexpr double PI = 3.14159265358979;

/* geometry libarary */

// a > 0 : +1, a == 0 : 0, a < 0 : -1
int sgn(const double a) {return (a < - EPS ? -1: (a > EPS ? 1: 0));}

/* 2d Point class */
struct Point {
    double x,y;
    constexpr Point(double x = 0.0,double y = 0.0) :x(x),y(y) {}

    Point operator+() const {return *this;}
    Point operator-() const {return Point(-x,-y);}

    constexpr Point &operator+=(const Point &q) {x += q.x;y += q.y;return *this;}
    constexpr Point &operator-=(const Point &q) {x -= q.x;y -= q.y;return *this;}
    constexpr Point &operator*=(double a) {x *= a;y *= a;return *this;}
    constexpr Point &operator/=(double a) {x /= a;y /= a;return *this;}
    
    constexpr Point operator+(const Point &q) const {return Point(*this) += q;}
    constexpr Point operator-(const Point &q) const {return Point(*this) -= q;}
    constexpr Point operator*(double a) const  {return Point(*this) *= a;}
    constexpr Point operator/(double a) const {return Point(*this) /= a;}

    inline bool operator<(const Point&q) const {return (sgn(x - q.x) != 0 ? sgn(x - q.x) < 0 : sgn(y - q.y) < 0);}
    inline bool operator>(const Point&q) const {return (sgn(x - q.x) != 0 ? sgn(x - q.x) > 0 : sgn(y - q.y) > 0);}
    inline bool operator==(const Point&q) const {return (sgn(x - q.x) == 0 && sgn(y - q.y) == 0);}

    friend ostream& operator<<(ostream& os,const Point &p) {return os << '(' << p.x << ", " << p.y << ')';}
    friend istream& operator>>(istream& is,Point &p) {return is >> p.x >> p.y;}
};

constexpr double dot(const Point &p,const Point &q) {return p.x * q.x + p.y * q.y;}
constexpr double cross(const Point &p,const Point &q) {return p.x * q.y - q.x * p.y;}
constexpr double norm(const Point &p) {return dot(p,p);}
inline double length(const Point &p) {return sqrt(norm(p));}
inline double dist(const Point &p,const Point &q) {return length(p - q);}
constexpr double area(const Point &p,const Point &q,const Point &r) {return cross(q - p,r - p)/2.0;}
inline Point normalized(const Point &p) {Point x = p/length(p);return x;}
inline Point rotation(const Point &p,double arg) {return Point(cos(arg) * p.x - sin(arg) * p.y,sin(arg) * p.x + cos(arg) * p.y);}
inline double angle(const Point &p) {return atan2(p.y,p.x);}

/*ab bc __/: +1
  ab bc --\: -1
  abc   ---: +2
  acb   ---: -2
  bac   ---:  0  */
int iSP(const Point &a,const Point &b,const Point &c) { 
    if (sgn(cross(b - a,c - a)) > 0) return 1;
    if (sgn(cross(b - a,c - a)) < 0) return -1;
    if (sgn(dot(b - a,c - b)) > 0) return 2;
    if (sgn(dot(a - b,c - a)) > 0) return -2;
    return 0;
}
//error point: changed depending on the problem
const Point error_point(INF,INF);

/* 2d Line(or Segment or Ray) class */
//2d Line class
struct Line {
    Point P,Q;
    constexpr Line(const Point &P,const Point &Q):P(P),Q(Q) {}
    constexpr Point vec() const {return Q - P;}
};
using Segment = Line;
using Ray = Line;

Point lineIntersection(const Line &l1,const Line &l2) {
    if (sgn(cross(l1.vec(),l2.vec())) == 0) return error_point;
    return l1.P + l1.vec() * abs(cross(l2.Q - l1.P,l2.vec())/cross(l1.vec(),l2.vec()));
}
/*
Point segmenIntersection(const Segment &s1,const Segment &s2) {
}
*/
double distance(const Point &p,const Line &l) {
    return abs(cross(l.vec(),p - l.P)/length(l.vec()));
}

int main() {
    int N; cin >> N;
    rep(i,N) {
        vector<Point> P(4);
        rep(i,4) {
            cin >> P[i];
        }
        if (sgn(cross(P[3] - P[2],P[1] - P[0])) == 0) {
            printf("YES\n");
        }
        else {
            printf("NO\n");
        }
    }
    return 0;
}

C++: #include <stdio.h>

int main(){
    int n,i,j;
    double x[5],y[5];
    scanf("%d",&n);
    for(i=0;i<n;i++){
        for(j=1;j<=4;j++)scanf("%lf %lf",&x[j],&y[j]);
        if((x[2]-x[1])*(y[4]-y[3])-(x[4]-x[3])*(y[2]-y[1])==0){
            if((y[2]-y[1])*(x[3]-x[1])-(x[2]-x[1])*(y[3]-y[1])==0){
                printf("NO\n");
            }else{
                printf("YES\n");
            }
        }else{
            printf("NO\n");
        }
    }
}

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

int main(){
    int n;
    cin >> n;
    while(n--){
        double x[4],y[4],ll,lll;
        for(int i=0;i<4;i++) cin >> x[i] >> y[i];
        ll = (y[0]-y[1])/(x[0]-x[1]);
        lll = (y[2]-y[3])/(x[2]-x[3]);
        cout << ((ll == lll)? "YES" : "NO") << endl;
    }
}

C++: #include <map>
#include <set>
#include <stack>
#include <queue>
#include <cmath>
#include <string>
#include <vector>
#include <iostream>
#include <algorithm>

using namespace std;

#define endl '\n'

typedef long long ll;
typedef long double lb;
typedef vector<int> vi;
typedef vector<vector<int> > vii;
typedef pair<int, int> pii;
typedef pair<int, string> pis;
typedef pair<string, int> psi;

inline bool inside(int y, int x, int H, int W)
{ 
  return (y >= 0 && x >= 0 && y < H && x < W);
}

inline int in() {int x; std::cin >> x; return x;}

const ll MOD = 1e9 + 7;
const ll INF = 1e9 + 9;
const int dx[] = {0, 1, 0, -1, 1, -1, 1, -1};
const int dy[] = {1, 0, -1, 0, 1, -1, -1, 1};

int main()
{
  cin.tie(0);
  ios::sync_with_stdio(false);
  
  double x1, y1, x2, y2, x3, y3, x4, y4;
  int n;
  cin >> n;
  for (int i = 0; i < n; i++){
    cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3 >> x4 >> y4;
    if ((x1 == x2 && x3 == x4) || (((y2 - y1) / (x2 - x1)) == ((y4 - y3) / (x4 - x3)))) cout << "YES" << endl;

    else cout << "NO" << endl;
  }
  return (0);
}

C++: #include <cstdio>
#include <complex>
#include <vector>

using namespace std;

typedef complex<double> P;
struct L : public vector<P> {
    L(const P &a, const P &b) {
        push_back(a);
        push_back(b);
    }
};

const double EPS = 1e-8;

int cross(P a, P b)
{
    return imag(conj(a) * b);
}

bool intersectLL(const L &l, const L &m)
{
    return abs(cross(l[1]-l[0], m[1]-m[0])) > EPS || // non-parallel
        abs(cross(l[1]-l[0], m[0]-l[0])) < EPS;   // same line
}

int main()
{
    int n;
    scanf("%d", &n);
    for (int i = 0; i < n; ++i) {
        P p[4];
        double x, y;
        scanf("%lf %lf", &x, &y);
        p[0] = P(x, y);
        scanf("%lf %lf", &x, &y);
        p[1] = P(x, y);
        scanf("%lf %lf", &x, &y);
        p[2] = P(x, y);
        scanf("%lf %lf", &x, &y);
        p[3] = P(x, y);

        puts(intersectLL(L(p[0], p[1]), L(p[2], p[3])) ? "NO" : "YES");
    }

    return 0;
}

C++: #include <iostream>
#include <string>
#include <vector>
#include <cmath>
#include <algorithm>
#include <cstdlib>
#include <ctime>
#include <cstdio>
#include <functional>
#include <complex>

using namespace std;

const double EPS=(1e-10);

int main()
{
	int n;
	double x[4],y[4];

	cin>>n;

	for(int i=0;i<n;i++){
		for(int j=0;j<4;j++) cin>>x[j]>>y[j];
		
		if( abs((y[1]-y[0])*(x[3]-x[2])-(y[3]-y[2])*(x[1]-x[0]))<EPS  ) cout<<"YES"<<endl;
		else cout<<"NO"<<endl;
	}

	return 0;
}

C++: #include<cstdio>
#include<iostream>
using namespace std;
main(){
  double x1,x2,x3,x4,y1,y2,y3,y4,a1,a2;
  int n,i;
  cin >> n;
  for(i=0;i<n;i++){
    cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3 >> x4 >> y4;
    a1=(y1-y2)/(x1-x2);
    a2=(y3-y4)/(x3-x4);
    if(a1==a2){
      cout << "YES" << endl;
    }
    else {
      cout << "NO" << endl;
    }
  }
}