Go: package main

import (
        "fmt"
        "math/big"
)

func main() {
        var x1, x2, x3, x4, y1, y2, y3, y4 big.Float
        var n int
        fmt.Scanf("%d", &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)
                var lx, ly, mx, my, mul1, mul2, sub, zz big.Float
                lx.Sub(&x2, &x1)
                ly.Sub(&y2, &y1)
                mx.Sub(&x4, &x3)
                my.Sub(&y4, &y3)
                mul1.Mul(&lx, &my)
                mul2.Mul(&ly, &mx)
                sub.Sub(&mul1, &mul2)
                zz.SetFloat64(0.0)

                if sub.Cmp(&zz) == 0 {
                        fmt.Println("YES")
                } else {
                        fmt.Println("NO")
                }
        }
}

C++: #include<iostream>
using namespace std;
bool EQ(double a,double b){
  return a-0.000001<b&&b<a+0.000001;
}
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((x[3]-x[2])==0&&(x[1]-x[0])==0) cout<<"YES"<<endl;
    else if((x[3]-x[2])==0||(x[1]-x[0])==0) cout<<"NO"<<endl;
    else if(EQ((y[3]-y[2])/(x[3]-x[2]),(y[1]-y[0])/(x[1]-x[0]))) cout<<"YES"<<endl;
    else cout<<"NO"<<endl;
  }
  return 0;
}

C++: #include<iostream>
#include<cmath>
using namespace std;
int main() {
  int n;
  cin>>n;
  for(int i=0;i<n;i++){ 
    double x[4],y[4];
    for(int j=0;j<4;j++)
      cin>>x[j]>>y[j];
  
  if( (y[0]-y[1])/(x[0]-x[1]) == (y[2]-y[3])/(x[2]-x[3]) ) 
      cout<<"YES\n";
    else
      cout<<"NO\n";
  }
  return 0;
}

C++: #include <iostream>

using namespace std;

void swap(double &x, double &y, double &xx, double &yy) 
{
  double tmp_x = x;
  double tmp_y = y;
  x = xx;
  y = yy;
  xx = tmp_x;
  yy = tmp_y;
}

void compute(double x1, double y1, double x2, double y2, double x3, double y3, double x4, double y4) 
{
  if (x1 > x2) swap(x1, y1, x2, y2);
  if (x3 > x4) swap(x3, y3, x4, y4);

  double diff_x12 = x2 - x1;
  double diff_y12 = y2 - y1;
  double diff_x34 = x4 - x3;
  double diff_y34 = y4 - y3;

  if (diff_x12 != 0 && diff_x34 != 0) {
    if (diff_y12 == 0.0 && diff_y34 == 0.0)
      cout << "YES" << endl;
    else {
      if (diff_y12 / diff_x12 == diff_y34 / diff_x34)
        cout << "YES" << endl;
      else
        cout << "NO" << endl;
    }
  }
  else if (diff_x12 == 0.0 && diff_x34 == 0.0) {
    cout << "YES" << endl;  
  }
  else
    cout << "NO" << endl;  
}

int main(void) 
{
  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;
    compute(x1, y1, x2, y2, x3, y3, x4, y4);
  }
  
  return 0;
}

C++: #include<iostream>
#define EPS 0.000000000001
using namespace std;

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

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

using namespace std;

int main(){

	int n; cin >> n;
	double x[4],y[4]; //??\????????§?¨?

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

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

	}
	return 0;
}

C++: #include <stdio.h>
#include <iostream>
#include <vector>
#include <string>
#include <math.h>
#include <algorithm>
using namespace std;
#define INF 0x7FFFFFFF
const double EPS = 1e-10;

inline bool eq(double a, double b) { return fabs(b - a) < EPS; }

int main(){
  int n;
  cin>>n;
  while(n){
    n--;
    vector<double> x(4,0);
    vector<double> y(4,0);
    for(int i=0;i<4;i++){
      cin>>x[i]>>y[i];
    }
    double xa,xb,ya,yb;
    xa=x[1]-x[0];
    xb=x[3]-x[2];
    ya=y[1]-y[0];
    yb=y[3]-y[2];
    double a;
    a=xa*yb-xb*ya;
    if(eq(0.0,a)){
      cout<<"YES"<<endl;
    }
    else{
      cout<<"NO"<<endl;
    }
  }
}

C++: #include <iostream>
#include <cfloat>
#include <cmath>
using namespace std;
 
 
//0021
int main(){
    double xA, yA, xB, yB, xC, yC, xD, yD;
    char ch;
	int n;

	cin >> n;
    for(int i=0; i<n; ++i){
		cin >> xA >> yA >> xB >> yB >> xC >> yC >> xD >> yD;
        double x1,y1,x2,y2;
        y1=yB-yA;
        x1=xB-xA;
        y2=yD-yC;
        x2=xD-xC;
 
        if(fabs((x1*y2)-(x2*y1)) <= DBL_EPSILON){
            cout << "YES" << endl;
        }else{
            cout << "NO" << endl;
        }
 
    }
 
}

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)) <= EPS)
            cout << "YES" << endl;
        else
            cout << "NO" << endl;
    }
    return 0;
}

C++: #include <stdio.h>

int main(void)
{
	double x1, x2, x3, x4, y1, y2, y3, y4;
	double m1, m2;
	int n;
	
	scanf("%d", &n);
	while (n > 0){
		scanf("%lf %lf %lf %lf %lf %lf %lf %lf", &x1, &y1, &x2, &y2, &x3, &y3, &x4, &y4);
	
		m1 = (y2 - y1) / (x2 - x1);
		m2 = (y4 - y3) / (x4 - x3);
	
		if (m1 == m2){
			printf("YES\n");
		}
		else {
			printf("NO\n");
		}
		n--;
	}
	return 0;
}