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;
int main(){
  int n;
  double x1,x2,x3,x4,y1,y2,y3,y4;
  cin >> n;
  string ans[n];
  for(int i=0;i<n;i++){
    cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3 >> x4 >> y4;
    if((y1-y2)/(x1-x2)==(y3-y4)/(x3-x4) and (y1-y3)/(x1-x3)!=(y1-y2)/(x1-x2) and x1-x2!=0 and x3-x4!=0 and y1-y2!=0 and y3-y4!=0){
      ans[i]="YES";
    }else{
      ans[i]="NO";
    }
  }
  for(int i=0;i<n;i++){
    cout << ans[i] << endl;
  }
  return 0;
}

C++: #include <iostream>
#include <algorithm>
#include <complex>

using namespace std;

typedef complex<double> P;
#define EPS (1e-10)
#define EQ(a,b) (abs((a)-(b)) < EPS)

// OÏ (cross product) : a~b = |a||b|sinÆ
double cross(P a, P b) {
  return (a.real() * b.imag() - a.imag() * b.real());
}
// 2¼üÌ½s»è : a//b <=> cross(a, b) = 0
int is_parallel(P a1, P a2, P b1, P b2) {
  return EQ( cross(a1-a2, b1-b2), 0.0 );
}
int main(){

	int n;
	cin>>n;
	for(int j = 0; j < n; j++){
		P v[4];
		for(int i = 0; i < 4; i++){
			double x,y;
			cin>>x>>y;
			v[i] = P(x,y);
		}
		if(is_parallel(v[0],v[1],v[2],v[3])){
			cout<<"YES"<<endl;
		}
		else
			cout<<"NO"<<endl;
	}

	return 0;
}

C++: #include <iostream>
using namespace std;
int main(){
  

  int dataSets;
  cin >> dataSets;
  for(int i=0;i<dataSets;i++){
    double x1, y1, x2, y2, x3, y3, x4, y4;
    cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3 >> x4 >> y4;
    if((y2-y1)*(x4-x3) - (x2-x1)*(y4-y3) == 0.0){
      cout << "YES" << endl;
    }else{
      cout << "NO" << endl;
    }
    
  }

  return 0;

}

C++: //include
//------------------------------------------
#include <vector>
#include <list>
#include <map>
#include <set>
#include <deque>
#include <stack>
#include <bitset>
#include <algorithm>
#include <functional>
#include <numeric>
#include <utility>
#include <sstream>
#include <iostream>
#include <iomanip>
#include <cstdio>
#include <cmath>
#include <cstdlib>
#include <cctype>
#include <string>
#include <cstring>
#include <ctime>
#include <climits>
#include <queue>

using namespace std;

//typedef
//------------------------------------------
typedef vector<int> VI;
typedef vector<VI> VVI;
typedef vector<string> VS;
typedef pair<int, int> PII;
typedef long long LL;

//container util
//------------------------------------------
#define ALL(a)  (a).begin(),(a).end()
#define RALL(a) (a).rbegin(), (a).rend()
#define PB push_back
#define MP make_pair
#define SZ(a) int((a).size())
#define EACH(i,c) for(typeof((c).begin()) i=(c).begin(); i!=(c).end(); ++i)
#define EXIST(s,e) ((s).find(e)!=(s).end())
#define SORT(c) sort((c).begin(),(c).end())

//repetition
//------------------------------------------
#define FOR(i,a,b) for(int i=(a);i<(b);++i)
#define REP(i,n)  FOR(i,0,n)

//constant
//--------------------------------------------
const double EPS = 1e-10;
const double PI  = acos(-1.0);

int main(){
  int N; cin >> N;
  for(;N--;){
	double x[4], y[4];
	REP(i,4)
	  cin >> x[i] >> y[i];
	//x[0] -= x[1], y[0] -= y[1], x[2] -= x[3], y[2] -= y[3];

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

C++: #include <bits/stdc++.h>
using namespace std;

double eps = 1e-10;

class Point {
public:
	double x, y;
	
	Point(double x = 0, double y = 0) : x(x), y(y) {}
	
	Point operator + (const Point& p) {
		return Point(x + p.x, y + p.y);
	}
	Point operator - (const Point& p) {
		return Point(x - p.x, y - p.y);
	}
	Point operator * (const double& a) {
		return Point(x * a, y * a);
	}
	Point operator / (const double& a) {
		return Point(x / a, y / a);
	}
	
	bool operator < (const Point& p) const {
		return x != p.x ? x < p.x : y < p.y;
	}
	bool operator == (const Point& p) const {
		return fabs(x - p.x) < eps && fabs(y - p.y) < eps;
	}
	
	double abs() {
		return sqrt(norm());
	}
	double norm() {
		return x * x + y * y;
	}
	
};

typedef Point Vector;

// ????????????
bool equals(const double& a, const double& b) {
	return fabs(a - b) < eps;
}

// ??????
double dot(const Vector& a, const Vector& b){
	return a.x * b.x + a.y * b.y;	
}

// ??????
double cross(const Vector& a, const Vector& b){
	return a.x * b.y - a.y * b.x;
}

// ????????????(????????????)
bool isParallel(const Vector& a, const Vector& b){
	return equals(cross(a, b), 0.0);
}

// ????????????(???4???)
bool isParallel(Point a1, Point a2, Point b1, Point b2){
	return isParallel(a1 - a2, b1 - b2);
}

int main() {
	int n;
	double x1, x2, x3, x4, y1, y2, y3, y4;
	cin >> n;
	for(int i = 0; i < n; ++i){
		cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3 >> x4 >> y4;
		if(isParallel(Point(x1, y1), Point(x2, y2), Point(x3, y3), Point(x4, y4)))
			cout << "YES" << endl;
		else
			cout << "NO" << endl;
	}
	return 0;
}

C++: #include <iostream>
#include <cmath>
#include <vector>
#include <string>
#include <algorithm>
#include <climits>
using namespace std;

int main(){
    double EPS = 1e-12;
    double x[4], y[4];
    double s,t,u,v;
    int n;
    cin >> n;
    for (int t = 0;t < n;++t) {
        for (int i = 0;i < 4;++i){
            cin >> x[i] >> y[i]; 
        }
    }
    s = x[1] - x[0];
    t = y[1] - y[0];
    u = x[3] - x[2];
    v = y[3] - y[2];
    if(s * v - t * u > 0){
        double m = s * v - t * u;
    }
    else{
        double m = t * u - s * v;
    }
    if( m < EPS){
        cout << "YES" << endl;
    }
    else{
        cout << "NO" << endl;
    }
    return 0;
}

C++: #include <iostream>
#include <cmath>
using namespace std;
void judge(double,double,double,double,double,double,double,double);
#define EPS 0.00001

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;
		judge(x1, y1, x2, y2, x3, y3, x4, y4);
	}
	
	return 0;
}

void judge(double x1, double y1, double x2, double y2, double x3, double y3, double x4, double y4) {
	double abx = x2 - x1;
	double aby = y2 - y1;
	double cdx = x4 - x3;
	double cdy = y4 - y3;
	if (abs(abx * cdy - aby * cdx) <= EPS) {
		cout << "YES" << endl;
	}
	else {
		cout << "NO" << endl;
	}

	return;

}

C++: #include<stdio.h>
#include<math.h>

int main(){
	double x1,x2,x3,x4,y1,y2,y3,y4;
	double a1,a2;
	int ans[100]={0};
	int n,i;
	
	scanf("%d",&n);
	
	for(i=0;i<n;i++){
		scanf("%lf %lf %lf %lf %lf %lf %lf %lf",&x1,&x2,&x3,&x4,&y1,&y2,&y3,&y4);
		
		a1=(y1-y2)/(x1-x2);
		a2=(y3-y4)/(x3-x4);
	
	if(a1==a2)ans[i]=1;
	else ans[i]=0;
	}
	
	for(i=0;i<n;i++){
		if(ans[i]==1)printf("YES\n");
		else printf("NO\n");
	}
return 0;
}

C++: #include<iostream>
#include<string>
#include<cstdio>
#include<cstring>
#include<algorithm>
using namespace std;
int main(void){
	int n;
	double x1,x2,x3,x4,y1,y2,y3,y4;
	cin>>n;
	for(int i=0;i<n;i++){
	    cin>>x1>>x2>>x3>>x4>>y1>>y2>>y3>>y4;


    double a,b;
    a=(y2-y1)/(x2-x1);
    b=(y4-y3)/(x4-x3);
    (a==b)? cout<<"YES"<<endl : cout<<"NO"<<endl;
	}
	return 0;
}