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>
#include <vector>
#include <string>
#include <cstring>
#include <algorithm>
#include <sstream>
#include <map>
#include <set>
#include <cmath>

#define REP(i,k,n) for(int i=k;i<n;i++)
#define rep(i,n) for(int i=0;i<n;i++)
#define INF 1<<30
#define pb push_back
#define mp make_pair
#define EPS 1e-8
#define equals(a,b) fabs((a) - (b)) < EPS

using namespace std;
typedef long long ll;
typedef pair<int,int> P;

struct Point {
    double x, y;

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

    Point operator+(const Point &o) const { return Point(x+o.x, y+o.y); }

    Point operator-(const Point &o) const { return Point(x-o.x, y-o.y); }

    Point operator*(const double m) const { return Point(x*m, y*m); }

    Point operator/(const double d) const { return Point(x/d, y/d); }

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

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

    double cross(const Point &o) const { return x * o.y - y * o.x; }

    double dot(const Point &o) const { return x * o.x + y * o.y; }

    double atan() const { return atan2(y, x); }

    double norm() const { return sqrt(dot(*this)); }

    double distance(const Point &o) const { return (o - (*this)).norm(); }

    double area(const Point &a,const Point &b) {
        Point p = a - (*this), p2 = b - (*this); 
        return p.cross(p2);
    }

    double area_abs(const Point &a,const Point &b) const {
        Point p = a - (*this), p2 = b - (*this);
        return fabs(p.cross(p2)) / 2.0;
    }	

    //線分abが自身に含まれているのかどうか判断する
    int between(const Point &a,const Point &b) {
        if(area(a,b) != 0) return 0;

        if(a.x != b.x)  return ((a.x <= x) && (x <= b.x) || (a.x >= x) && (x >= b.x));
        else return ((a.y <= y) && (y <= b.y) || (a.y >= y) && (y >= b.y));
    }      

    double distance_seg(const Point& a,const Point& b) {
        if((b-a).dot(*this-a) < EPS) {
            return (*this-a).norm();
        }
        if((a-b).dot(*this-b) < EPS) {
            return (*this-b).norm();
        }
        return abs((b-a).cross(*this-a)) / (b-a).norm();
    }

    bool hitPolygon(const Point& a,const Point& b,const Point& c) {
        double t = (b-a).cross(*this-b);
        double t2 = (c-b).cross(*this-c);
        double t3 = (a-c).cross(*this-a);	

        if((t > 0 && t2 > 0 && t3 > 0) || ( t < 0 && t2 < 0 && t3 < 0)) {
            return true;
        }

        return false;
    }
};

struct Seg {
    Point a,b;

    Seg (Point a, Point b) : a(a),b(b) {}

    bool isOrthogonal(Seg &s) { return equals((b - a).dot(s.b - s.a),0.0); }

    bool isParallel(Seg &s) { return equals((b-a).cross(s.b - s.a),0.0); }

    bool isIntersect(Seg &s) {
        if(s.a.between(a,b) || s.b.between(a,b) || a.between(s.a,s.b) || b.between(s.a,s.b)) {
            return true;
        }
        return ((a-b).cross(s.a-a) * (a-b).cross(s.b-a) < EPS) && ((s.b-s.a).cross(a-s.a)*(s.b-s.a).cross(b-s.a) < EPS);
    }

    bool distance(Seg &s) {
        if((*this).isIntersect(s)) return 0.0;

        return min(min(a.distance_seg(s.a,s.b),b.distance_seg(s.a,s.b)),min(s.a.distance_seg(a,b),s.b.distance_seg(a,b)));
    }

    Point getCrossPoint(Seg &s) {
        Point p = s.b - s.a;
        double d = abs(p.cross(a-s.a));
        double d2 = abs(p.cross(b-s.a));

        double t = d / (d+d2);
        return a + (b-a)*t;
    }        
};

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

    rep(i,n) {
        Point a,b,c,d;

        cin >> a.x >> a.y >> b.x >> b.y >> c.x >> c.y >> d.x >> d.y;

        Seg s(a,b),t(c,d);

        if(s.isParallel(t)) {
            cout << "YES" << endl;
        } else {
            cout << "NO" << endl;
        }

    }

    return 0;
}

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

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((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<cstdio>
#include<iostream>
#include<cmath>
using namespace std;
int main(){
    int n;
    cin>>n;
    while(n--){
        double x1,y1,x2,y2,x3,y3,x4,y4;
        cin>>x1>>y1>>x2>>y2>>x3>>y3>>x4>>y4;
        if(((x1-x2)*(y1-y2))-((x3-x4)*(y3-y4))==0)cout<<"YES"<<endl;
        else cout<<"NO"<<endl;
    }
    return 0;
}

C++: #include <bits/stdc++.h>
using namespace std;
typedef complex<double> xy_t; 
xy_t a[100], b[100], c[100], d[100];
double x, y;
int n;
int main()
{
	cin >> n;
	for(int i=0; i<n; i++)
	{
		cin >> x >> y;
		a[i]=xy_t(x, y);
		cin >> x >> y;
		b[i]=xy_t(x, y);
		cin >> x >> y;
		c[i]=xy_t(x, y);
		cin >> x >> y;
		d[i]=xy_t(x, y);
	}
	for(int i=0; i<n; i++)
	{
		if(abs((conj(b[i]-a[i])*(d[i]-c[i])).imag())<0.000001)
		{
			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;
	cin>>n;
	for(int i=0;i<n;++i){
		cin>>x1>>y1>>x2>>y2>>x3>>y3>>x4>>y4;
		double dydx1=(y2-y1)/(x2-x1),dydx2=(y4-y3)/(x4-x3);
		if(dydx1==dydx2)cout<<"YES\n";
		else cout<<"NO\n";
	}
	return 0;
}

C++: #include <iostream>
using namespace std;
int n;
double xa,xb,xc,xd,ya,yb,yc,yd,ta,tb,tc,td;
int main(){
	int i;
	cin >> n;
	for(i=0;i<n;i++){
		cin >> xa >> ya >> xb >> yb >> xc >> yc >> xd >> yd;
		ta = xa - xb;
		tb = ya - yb;
		tc = xc - xd;
		td = yc - yd;
		if(tc/td == ta/tb) cout << "YES" << endl;
		else if(ta == -tc && tb == -td) cout << "YES" << endl;
		else cout << "NO" << endl;
	}
	return 0;
}

C++: #include<iostream>
#include<vector>
using namespace std;
struct Coordinate {
	double x;
	double y;
};
vector<bool>l;
int main() {
	Coordinate A, B, C, D;
	double Inclination_of_AB;
	double Inclination_of_CD;
	int n;
	cin >> n;
	for (int i = 0; i < n; i++){
		cin >> A.x >> A.y >> B.x >> B.y;
		Inclination_of_AB = (B.x - A.x) / (B.y - A.y);
		cin >> C.x >> C.y >> D.x >> D.y;
		Inclination_of_CD = (D.x - C.x) / (D.y - C.y);
		l.push_back(Inclination_of_AB == Inclination_of_CD);
	}
	for (bool i : l) {
		if (i) cout << "YES";
		else cout << "NO";
		cout << endl;
	}
	return 0;
}

C++: #include <iostream>
#include <stack>
#include <stdio.h>
#include <cmath>
using namespace std;
double x[4],y[4];//p,1,2,3
double make_line(double x1,double y1,double x2,double y2){
	double a,b,c;//ax+by+c=0
	if(x1!=x2&&y1!=y2){
		a=(y1-y2)/(x1-x2);
		b=-1;
		c=-a*x1+y1;
	}else if(x1==x2){
		a=1;
		b=0;
		c=-x1;
	}else if(y1==y2){
		a=0;
		b=1;
		c=-y1;
	}
	return a;
}
int main()
{
	int n;
	cin>>n;
	while(n--){
		cin>>x[0]>>y[0]>>x[1]>>y[1]>>x[2]>>y[2]>>x[3]>>y[3];
		printf(abs(make_line(x[0],y[0],x[1],y[1])-make_line(x[2],y[2],x[3],y[3]))<=0.000001?"YES\n":"NO\n");
	}
    return 0;
}

C++: #include <iostream>
#include <cmath>
using namespace std;
double EPS=1e-5;
double add(double a, double b)
{
	if(abs(a+b)<EPS*(abs(a)+abs(b))) return 0;
	return a+b;
}

int main()
{
	int n; cin >> n ;
	double x[4],y[4];
	while(n--)
	{
		for(int i=0;i<4;i++) cin >> x[i] >> y[i] ;
		double vx[2],vy[2];
		for(int i=0;i<2;i++) vx[i]=add(x[2*i+1],-x[2*i]), vy[i]=add(y[2*i+1],-y[2*i]);
		if(!add(vx[0]*vy[1],-vy[0]*vx[1])) cout << "YES" << '\n' ;
		else cout << "NO" << '\n' ;
	}
}