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;

int main(){
  
  double x1,x2,x3,x4;
  double y1,y2,y3,y4;
  
  int n;
  cin >> n;
  while(n--){
    cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3 >> x4 >> y4;
    if( ( y2 - y1 ) / ( x2 - x1 ) == ( y4 - y3 ) / ( x4 - x3 ) ){
      cout << "YES" << endl;
    }
    else cout << "NO" << endl;
  }
}

C++: #include <iostream>

using namespace std;

class Vector2D {
  private:
    double x;
    double y;
  public:
    Vector2D(double, double);
    Vector2D operator - (Vector2D vec2) {
      double newX = this->x - vec2.x;
      double newY = this->y - vec2.y;
      Vector2D newvec(newX, newY);
      return newvec;
    }
    double GetX() { return x; };
    double GetY() { return y; };
    bool isParallel(Vector2D);
};

Vector2D::Vector2D(double _x, double _y) {
  x = _x;
  y = _y;
}

bool Vector2D::isParallel(Vector2D vec2) {
  if (vec2.GetY()/this->y == vec2.GetX()/this->x)
    return true;
  else
    return false;
}

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

  double x1,y1,x2,y2,x3,y3,x4,y4;
  for (int dataset_i = 0; dataset_i < n; dataset_i++) {
    cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3 >> x4 >> y4;
    Vector2D vecAB = Vector2D(x2,y2) - Vector2D(x1,y1);
    Vector2D vecBD = Vector2D(x4,y4) - Vector2D(x3,y3);
    cout << (vecAB.isParallel(vecBD) ? "YES" : "NO") << endl;
  }

  return 0;
}

C++: #include <bits/stdc++.h>

using namespace std;

#define rep(i, n) for(int i = 0; i < (int)n; ++i)
#define repf(i, f, l) for(int i = f; i < (int)l; ++i)

#ifdef ONLINE_JUDGE
#define DEBUG false
#else
#define DEBUG true
#endif

#define pb emplace_back
#define lb lower_bound
#define ul unsigned long
#define ull unsigned long long
#define ll long long
#define INF 1000000007
#define MOD 1000000007
#define fs first
#define sd second

#define ALL(c) (c).begin(),(c).end()

#define DBG0(x)    {if(DEBUG){ cout << #x << ": " << x << "\t"; }}
#define DBG(x)     {if(DEBUG){DBG0(x); cout << endl;}}
#define DBG2(x, y) {if(DEBUG){DBG0(x); DBG(y);}}
#define DBG3(x, y, z) {if(DEBUG){DBG0(x); DBG2(y, z);}}
#define DBG4(w, x, y, z) {if(DEBUG){DBG0(w); DBG3(x, y, z);}}

const double EPS = 1.0e-11;
template <class T>
ostream& operator<<(ostream& os, vector<T> xs){ for(T x: xs) os << x << ' '; return os; }
template <class S, class T>
ostream& operator<<(ostream& os, pair<S,T> st){ os << "(" << st.first << "," << st.second <<")"; return os; }

typedef vector<int> vint;
typedef vector<ll> vll;
typedef vector<ul> vul;
typedef vector<ull> vull;
typedef vector<bool> vbl;
typedef pair<int, int> pii;

typedef complex<double> Point;
istream& operator>>(istream& is, Point &pt){
    double a, b;
    is >> a >> b;
    pt.real(a); pt.imag(b);
    return is;
}

ostream& operator<<(ostream& os, const Point &pt){
    os << make_pair(pt.real(), pt.imag());
    return os;
}

inline double cross(const Point &a, const Point &b){
    return (a.real()*b.imag() - a.imag()*b.real());
}

inline double myarg(Point &a){
    double ret = arg(a);
    return (ret >= 0 ? ret : ret + 2*M_PI);
}

inline double signed_area(const Point &a, const Point &b, const Point &c){
    Point ab = b - a;
    Point ac = c - a;
    return cross(ab, ac) / 2.0;
}

inline bool intersect(const Point &a1, const Point &a2, const Point &b1, const Point &b2) {
    return ((cross(a2-a1, b1-a1) * cross(a2-a1, b2-a1) < EPS) &&
            (cross(b2-b1, a1-b1) * cross(b2-b1, a2-b1) < EPS));
}

void sort_by_arg(vector<Point> &pts){
    sort(pts.begin(), pts.end(), [](Point a, Point b){ return (myarg(a) < myarg(b)); });
}

double area(vector<Point> &vs){
    int n = vs.size();
    vector<Point> pts(n - 1);
    rep(i, n - 1) pts[i] = vs[i + 1] - vs[0];
    sort_by_arg(pts);
    double ans = 0.0;
    rep(i, n - 1) ans += cross(vs[i], vs[i + 1]);
    return ans / 2.0;
}

int main(void){

    int n;
    cin >> n;
    while(n-->0){
        Point a,b,c,d;
        cin >> a >> b >> c >> d;
//        DBG4(a, b, c, d);
//        DBG(cross(b - a, d - c));
        if((cross(b - a, d - c)) == 0.0)
            cout << "YES" << endl;
        else
            cout << "NO" << endl;
    }
    return 0;
}

C++: #include<cstdio>
#include<cmath>
struct Point
{
	double x,y;
};
int main(void)
{
	Point a,b,c,d;
	scanf("%lf %lf",&a.x,&a.y);
	scanf("%lf %lf",&b.x,&b.y);
	scanf("%lf %lf",&c.x,&c.y);
	scanf("%lf %lf",&d.x,&d.y);
	Point vec1,vec2;
	vec1.x=b.x-a.x;vec1.y=b.y-a.y;
	vec2.x=d.x-c.x;vec2.y=d.y-c.y;
	if((float)atan2(vec1.y,vec1.x)==(float)atan2(vec2.y,vec2.x))
	{
		printf("YES\n");
	}
	else
	{
		printf("NO\n");
	}
	return 0;
}