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() {
  int n;
  double x1, y1, x2, y2, x3, y3, x4, y4;
  double d1, d2;
  cin >> n;
  for (int i = 0; i < n; i++) {
    cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3 >> x4 >> y4;
    if (x2 == x1 && x4 == x3 ){
      cout << "NO" << endl;
    } else {
      if (x2 == x1 || x4 == x3 ){
        cout << "NO" << endl;
      }
      d1 = (y2 - y1) / (x2 - x1);
      d2 = (y4 - y3) / (x4 - x3);
      if (d1 == d2) {
        cout << "YES" << endl;
      }else{
        cout << "NO" << endl;
      }
    }
  }
}

C++: #include <iostream>

using namespace std;

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

	cin >> n;
	for (int i = 0; i < n; i++) {
		cin >> x[0] >> y[0] >> x[1] >> y[1] >> x[2] >> y[2] >> x[3] >> y[3];
		ab = cd = 999999;
		if (x[0] != x[1])
			ab = (y[1] - y[0]) / (x[1] - x[0]);
		if (x[2] != x[3])
			cd = (y[3] - y[2]) / (x[3] - x[2]);
		if (ab == cd)
			cout << "YES" << endl;
		else
			cout << "NO" << endl;
	}
	return 0;
}

C++: #include <iostream>
#include <algorithm>
using namespace std;
typedef long long ll;
const int maxn=10000000;
bool is_prime[maxn];
int solve1(int n){
	for(int i=0;i<=n;i++)is_prime[i]=true;
	int ans=0;
	for(int i=2;i<=n;i++){
		if(is_prime[i]){
			ans++;
			for(int j=i*2;j<=n;j+=i)is_prime[j]=false;
		}
	}
	return ans;
}



int main(){
	int n;
	cin>>n;
	double x1,y1,x2,y2,x3,y3,x4,y4;
	while(n--){
		cin>>x1>>y1>>x2>>y2>>x3>>y3>>x4>>y4;
		double t1=abs(y2-y1);
		double t2=abs(x2-x1);
		double t3=abs(y4-y3);
		double t4=abs(x4-x3);
		if(t1/t2==t3/t4)cout<<"YES"<<endl;
		else cout<<"NO"<<endl;
	}
	return 0;
}

C++: #include <iostream>
#include <cstdio>
#include <algorithm>
#include <map>
#include <vector>
using namespace std;

int main(){
  int n;//データ数
  double x[4];
  double y[4];
  cin >> n ;
  while(n--){
  //入力フェイズ
  for(int i=0;i<4;i++){
    cin >> x[i] >> y[i];
  }
  //傾きの求め方 yの増加量/xの増加量
  if((y[1]-y[0])*(x[3]-x[2])==(x[1]-x[0])*(y[3]-y[2]))
    cout << "YES" << endl;
  else
    cout << "NO" << endl;
}
}

C++: #include<iostream>
#include<cmath>
using namespace std;
#define EPS 1e-10
double x1,y1,x2,y2,x3,y3,x4,y4;
int det(){
	double dx1,dy1,dx2,dy2;
	dx1 = x1-x2;
	dy1 = y1-y2;
	dx2 = x3-x4;
	dy2 = y3-y4;
	return dx1*dy2 - dy1*dx2;
}
int main(){
	int n;
	cin>>n;
	while(n--){
		cin>>x1>>y1>>x2>>y2>>x3>>y3>>x4>>y4;
		cout<<(fabs(det())<EPS?"YES":"NO")<<endl;
	}
	return 0;
}

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

using namespace std;

typedef complex<double> Point;

int main() {
    int N;
    cin >> N;
    while(N--) {
        Point ps[4];
        for(int i = 0; i < 4; ++i) cin >> ps[i].real() >> ps[i].imag();
        Point v1 = ps[1]-ps[0], v2 = ps[3]-ps[2];
        double dot = v1.real()*v2.real() + v1.imag()*v2.imag();
        double n = norm(v1) * norm(v2);
        if(abs(dot*dot - n) < 1e-6) cout << "YES" << endl;
        else cout << "NO" << endl;
    }
    return 0;
}

C++: #include <stdio.h>

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

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

int main(){
	double x1,x2,x3,x4,y1,y2,y3,y4;
	double yoko1,yoko2,tate1,tate2;
	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);
	yoko1=(x1-x3)*(x1-x3);
	yoko2=(x2-x4)*(x2-x4);
	tate1=(y1-y3)*(y1-y3);
	tate2=(y2-y4)*(y2-y4);
	
	if(sqrt(yoko1+tate1)==sqrt(yoko2+tate2))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 <stdio.h>

int main(){
	int num;
	double x1, y1;
	double x2, y2;
	double x3, y3;
	double x4, y4;
	long long ax, ay;
	long long bx, by;
	long long s;
			
	scanf("%d", &num);	
	for(int i = 0; i < num; i++){
		scanf("%lf %lf %lf %lf %lf %lf %lf %lf", &x1, &y1, &x2, &y2, &x3, &y3, &x4, &y4);
		
		ax = x2 - x1;
		ay = y2 - y1;
		bx = x4 - x3;
		by = y4 - y3;
		s = ax*by - ay*bx;
		if(s == 0){
			printf("YES\n");
		}else{
			printf("NO\n");
		}
	}
	
	return 0;
}

C++: #include<stdio.h>
 
int main(void){
    double x[4],y[4];
    int n,i;
     
    scanf("%d",&n);
     
    for(i=0;i<n;i++){
        scanf("%lf %lf %lf %lf %lf %lf %lf %lf",&x[0],&y[0],&x[1],&y[1],&x[2],&y[2],&x[3],&y[3]);
         
        if(((y[1]-y[0])/(x[1]-x[0])) == ((y[3]-y[2])/(x[3]-x[2])))printf("YES\n");
        else printf("NO\n");
    }
     
    return 0;
}

C++: #include <iostream>
#include <cstdio>
#include <vector>
#include <list>
#include <algorithm>
#include <cmath>
#include <stack>
#include <set>
using namespace std;

#define REP(i,n) for(int (i)=0; (i)<(n); (i)++)
#define FOR(i,a,b) for(int (i)=(a); (i)<(b); (i)++)
#define PUSH(n,v) for(int i=0; i<(n); i++) {int j; cin >> j; v.push_back(j);}
#define ALL(v) v.begin(), v.end()

typedef pair<double, double> XY;

bool para(XY p, XY q) {
    double k;
    if (p.first == 0 && q.first == 0) return true;
    if (p.first == 0 || q.first == 0) return false;
    else k = q.first / p.first;
    if ((abs(p.second * k - q.second) * 1000000000000) < 1) return true;
    return false;
}

int main() {
    int n;
    cin >> n;
    REP(i,n) {
        double ax, ay, bx, by, cx, cy, dx, dy;
        cin >> ax >> ay >> bx >> by >> cx >> cy >> dx >> dy;
        XY AB = make_pair(ax-bx, ay-by);
        XY CD = make_pair(cx-dx, cy-dy);
        if (para(AB, CD)) 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 n = 0; n != N; ++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;
        }
    }
    return 0;
}

C++: #include <cassert>
#include <iostream>
#include <vector>

struct Point
{
    double x;
    double y;
};
struct Line
{
    Line(const Point& a, const Point& b)
    : p1(a), p2(b) {}
    Point p1;
    Point p2;
};
double GetSlope(const Line& line)
{
    return
    (line.p2.y - line.p1.y) /
    (line.p2.x - line.p1.x);
}
bool IsParallel(const Line& l1, const Line&l2)
{
    return GetSlope(l1) == GetSlope(l2);
}
bool IsParallel(const std::vector<Point>& points)
{
    assert(points.size() == 4);
    return IsParallel(Line(points[0], points[1]), Line(points[2], points[3]));
}
std::vector<Point> Input(std::istream& is)
{
    std::vector<Point> points(4);
    for(int i = 0; i < points.size(); ++i){
        is >> points[i].x >> points[i].y;
    }
    return points;
}
int main()
{
    int n;
    std::cin >> n;
    for(int i = 0; i < n; ++i) {
        std::vector<Point> points = Input(std::cin);
        std::cout << (IsParallel(points) ? "YES" : "NO") << std::endl; 
    }
    return 0;
}

C++: #include<iostream>
using namespace std;
int main()
{
	double x1,y1,x2,y2,x3,y3,x4,y4;
	int n;
	cin>>n;
	int N[100];
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))
	{
	N[i]=1;
	}
	else
	{
	N[i]=0;
	}
}
for(int j=0;j<n;j++)
{
	if(N[j]=1)
	{
		cout<<"YES"<<endl;
	}
	else if(N[j]=0)
	{
		cout<<"NO"<<endl;
	}
}
return 0;
}