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 <stdio.h>
#include <iostream>

using namespace std;

int main() {
	int k;
	double x1,x2,x3,x4,y1,y2,y3,y4;
	double veku1[2];
	double veku2[2];
	double veku3[2];
	double t;
	double m;
	cin >> k;

	for (int i = 0; i < k; i++) {
		cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3 >> x4 >> y4;
		veku1[0] = x2 - x1;
		veku1[1] = y2 - y1;
		veku2[0] = x4 - x3;
		veku2[1] = y4 - y3;

		t = veku1[0] * veku2[0] + veku1[1] * veku2[1];

		if (t * t != (veku1[0] * veku1[0] + veku1[1] * veku1[1]) * (veku2[0] * veku2[0] + veku2[1] * veku2[1])) {
			cout << "NO" << endl;
		} else {
			veku3[0] = x3 - x1;
			veku3[1] = y3 - y1;
			m = veku1[0] * veku3[0] + veku1[1] * veku3[1];
			if (m * m != (veku1[0] * veku1[0] + veku1[1] * veku1[1]) * (veku3[0] * veku3[0] + veku3[1] * veku3[1])) {
				cout << "YES" << endl;
			} else {
				cout << "NO" << endl;
			}
		}
	}
	return 0;
}

C++: #include <iostream>
#include <cmath>
using namespace std;
int main(void){
    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;
        if(fabs(x2-x1)<0.0000000001 && fabs(x4-x3)<0.0000000001)   cout << "YES" <<endl;
        else{
            if(fabs((y2-y1)/(x2-x1) - (y4-y3)/(x4-x3))<0.0000000001)  cout << "YES" << endl;
            else cout << "NO" << endl;
        }

    }
    return 0;
}

C++: #include <bits/stdc++.h>
#include <vector>
using namespace std;
double x_1, y_1, x_2, y_2, x_3, y_3, x_4, y_4, a, b, x;
double gradien(double x_1, double y_1, double x_2, double_y2){
	double m = 0;
	m = (y_1 - y_2)/(x_1 - x_2);
	
	return m;
}
int main(){
	cin >> x;
	for(int i = 1; i <= x; i++){
		cin >> x_1 >> y_1 >> x_2 >> y_2 >> x_3 >> y_3 >> x_4 >> y_4;
		a = gradien(x_1, y_1, x_2, y_2);
		b = gradien(x_3, y_3, x_4, y_4);
		if(a == b){
			cout << "YES\n";
		}
		else{
			cout << "NO\n";
		}
	}
	return 0;
}

C++: int main()
{

    float line1;
    float line2;
    float x1,x2,x3,x4;
    float y1,y2,y3,y4;

    cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3 >> x4 >> y4;

    line1 = (y2-y1)/(x2-x1);
    line2 = (y4-y3)/(x4-x3);

    if (line1 == line2)
        {
        cout << "YES" << endl;
        }
    else
        {
        cout << "NO" << endl;
        }



    return 0;
}

C++: #define scanf_s scanf
//#define gets_s gets
#include <stdio.h>
#include <string>
#include <iostream>
#include <math.h>
using namespace std;
#define MAX 201
#define _MAX 100
#define ABC 26
int main(void)
{
	int n;
	double x1, y1, x2, y2, x3, y3, x4, y4, a1, b1, a2, b2;
	scanf_s("%d", &n);
	for (int i = 0; i < n; ++i) {
		scanf_s("%lf %lf %lf %lf %lf %lf %lf %lf", &x1, &y1, &x2, &y2, &x3, &y3, &x4, &y4);
		a1 = (y1 - y2) / (x1 - x2);
		b1 = y1 - (x1 * a1);
		a2 = (y3 - y4) / (x3 - x4);
		b2 = y3 - (x3 * a2);
		if (a1 == a2 && b2 != b1)
			printf("YES\n");
		else
			printf("NO\n");
	}
}

C++: #include<iostream>
using namespace std;
int main() {
	int n;
	cin >> n;
	for (int z = 0; z < n; z++) {
		long float x1, x2, x3, y1, y2, y3, y4, x4;
		cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3 >> x4 >> y4;
		if (y1 > y2) {
			swap(y1, y2);
			swap(x1, x2);
		}
		if (y3 > y4) {
			swap(y3, y4);
			swap(x3, x4);
		}
		if ((x1 > x2 && x3 > x4 && ((y2 - y1) / (x1 - x2) == (y4 - y3) / (x3 - x4))))
			cout << "YES" << endl;
		if (x2 > x1 && x4 > x3 && ((y1 - y2) / (x2 - x1) == (y3 - y4) / (x4 - x3)))
			cout << "YES" << endl;
		else
			cout << "NO" << endl;
	}
}

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

struct point {
    double x;
    double y;
};

double vectorproduct(point p1, point p2)
{
    return p1.x * p2.y + p1.y * p2.x;
    
}

int main()
{
    int n;
    double x1, y1, x2, y2, x3, y3, x4, y4;
    point p1, p2;
    cin >> n;
    for(int c=0; c< n; c++)
    {
        cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3 >> x4 >> y4;
        p1.x = x2 - x1;
        p2.x = x4 - x3;
        p1.y = y2 - y1;
        p2.y = y4 - y3;
        if(vectorproduct(p1, p2)) cout << "YES" << endl;
        else cout << "NO" << endl;
    }
    return EXIT_SUCCESS;
}

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

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

    x[1]-=x[0];
    y[1]-=y[0];
    x[3]-=x[2];
    y[3]-=y[2];

    double xx=x[1]-x[3];
    double yy=y[1]-y[3];

    if(-1e-10 < xx && xx < 1e-10 && -1e-10 < yy && yy < 1e-10 ){
      cout<<"YES"<<endl;
    }
    else{
      cout<<"NO"<<endl;
    }

  }
  return 0;
}

C++: #include <iostream>

using namespace std;

int main() {
  int n, i;
  double x1, y1, x2, y2, x3, y3, x4, y4, a, b;

  cin >> n;

  for (i = 0; i < n; i++) {
    cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3 >> x4 >> y4;

    a = (y2 - y1) / (x2 - x1);
    b = (y3 - y4) / (x3 - x4);

    if (a == b) {
      cout << "YES" << endl;
    } else {
      cout << "NO" << endl;
    }
  }

  return 0;
}

C++: #include <cstdio>
#include <complex>
using namespace std;
typedef complex<double> P;
const double EPS=1e-11;
int main(){
    int n;
    double x1,y1,x2,y2,x3,y3,x4,y4;
    scanf("%d",&n);
    for(int i=0;i<n;i++){
        scanf("%lf%lf%lf%lf%lf%lf%lf%lf",&x1,&y1,&x2,&y2,&x3,&y3,&x4,&y4);
        if(abs((conj(P(x2-x1,y2-y1))*P(x4-x3,y4-y3)).imag()) < EPS) printf("YES\n");
        else printf("NO\n");
    }
}

C++: #include<iostream>
#include<cmath>
using namespace std;
#define EPS 1e-10
double x[4],y[4];
double det(){
  double x1,x2,y1,y2;
  x1 = x[0] - x[1];
  y1 = y[0] - y[1];
  x2 = x[2] - x[3];
  y2 = y[2] - y[3];
  return x1 * y2 - y1 * x2;
}

int main(void){
  int n;
  cin >> n;
  while(n--){
    for(int i = 0 ; i < 4 ; i ++)cin>>x[i]>>y[i];
    cout<<(fabs(det())<EPS?"YES":"NO")<<endl;

  }
  return 0;
}

C++: #include <iostream>

using namespace std;

int main() {
	double x1, y1, x2, y2, x3, y3, x4, y4;
	int n;
	double veku1[2];
	double veku2[2];
	cin >> n;
	for (int i = 0; i < n; i++) {
		cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3 >> x4 >> y4;
		veku1[0] = x2 - x1;
		veku1[1] = y2 - y1;
		veku2[0] = x4 - x3;
		veku2[1] = y4 - y3;
		if (veku1[0]*veku2[1]-veku1[1]*veku2[0] <= 0.0000001) {
			veku1[0] = x3 - x1;
			veku1[1] = y3 - y1;
			veku2[0] = x4 - x1;
			veku2[1] = y4 - y1;
			if (veku1[0]*veku2[1]-veku1[1]*veku2[0] <= 0.0000001) {
				cout << "NO" << endl;
			} else {
				cout << "YES" << endl;
			}
		} else {
			cout << "NO" << endl;
		}
	}
	return 0;
}

C++: #include<stdio.h>
int main(void) {
	int a, b, c, d;
	double e, f, g, h,x1,x2,x3,x4,y1,y2,y3,y4;
	scanf("%d", &a);
	for (size_t i = 0; i < a; i++)
	{
		scanf("%lf %lf %lf %lf %lf %lf %lf %lf", &x1, &y1, &x2, &y2, &x3, &y3, &x4, &y4);
		e = (y1 - y2) / (x1 - x2);
		f = (y3 - y4) / (x3 - x4);
		if (e==f)
		{
			printf("YES");
		}
		else
		{
			printf("NO");
		}
	}
	return 0;
}

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

struct Pos {
	double x, y;
}a, b, c, d;

int main() {
	int n;
	cin >> n;
	for(int i = 0; i < n; i++ ) {
		cin >> a.x >> a.y >> b.x >> b.y >> c.x >> c.y >> d.x >> d.y;
		Pos e;
		e.x = b.x - a.x;
		e.y = b.y - a.y;
		Pos f;
		f.x = d.x - c.x;
		f.y = d.y - c.y;
		
		double men = e.x * f.y - e.y * f.x;
		double eps = 0;
		if(-eps <= men && men <= eps)
			cout << "YES" << endl;
		else
			cout << "NO" << endl;
	}
	return 0;
}