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 <cmath>

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 {
      double ab = diff_y12 / diff_x12;
      double cd = diff_y34 / diff_x34;
      if (abs(ab - cd) < 0.00001)
        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>
using namespace std;

int main()
{
	int n;
	cin >> n;
	for (int i = 0; i < n; i++)
	{
		double x[4], y[4];
		cin >> x[0] >> y[0] >> x[1] >> y[1] >> x[2] >> y[2] >> x[3] >> y[3];

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




	return 0;
}

C++: #include <cstdio>

using namespace std;

double x1,y1,x2,y2,x3,y3,x4,y4;
    

int main()
{
    int n; scanf( "%d\n", &n );
    while( n > 0 ){
        loop:
        if( n == 0 ) break;
        scanf("%lf %lf %lf %lf %lf %lf %lf %lf %lf\n", &x1, &y1, &y2, &y2, &y3, &y3, &y4, &y4 );
        double num1 = x2 - x1, num2 = x4 - x3, num3 = y2 - y1, num4 = y4 - y3;
        if( (num1 == 0 && num3 == 0) || (num2 == 0 && num4 == 0 ) )
        {
            printf( "YES\n" );
            n--;
            goto loop;
        }
        else
        {
            num2 /= num1;
            num4 /= num3;
            if( num3 == num4 || num3 == -num4 )
            {
                printf( "YES\n" );
                n--;
                goto loop;
            }
        }
        printf( "NO\n" );
        n--;
    }
}

C++: #include <bits/stdc++.h>

//const static double	de_PI	= 3.14159265358979323846;
//const static int	de_MOD	= 1000000007;
const static int	de_MAX	= 999999999;
//const static int	de_MIN = -999999999;

inline double RoundOff(double a, const int n) {

	for (int i = 0; i < n - 1; i++) { a *= 10; }

	if (a > 0) { a += 0.5; }
	else { a -= 0.5; }

	a = static_cast<int>(a);

	for (int i = 0; i < n - 1; i++) { a /= 10; }

	return a;
}

int main(void) {

	//std::ifstream in("123.txt");	std::cin.rdbuf(in.rdbuf());

	int n = 0;
	std::cin >> n;

	double x1 = 0, y1 = 0, x2 = 0, y2 = 0;
	double slope1 = 0, slope2 = 0;
	std::vector<std::string> ans(n);

	for (int i = 0; i < n; i++) {
		std::cin >> x1 >> y1 >> x2 >> y2;
		if (x1 == x2) {
			slope1 = de_MAX;
		}
		else if (y1 == y2) {
			slope1 = 0;
		}
		else {
			slope1 = RoundOff((y2 - y1) / (x2 - x1), 6);
		}

		std::cin >> x1 >> y1 >> x2 >> y2;
		if (x1 == x2) {
			slope1 = de_MAX;
		}
		else if (y1 == y2) {
			slope1 = 0;
		}
		else {
			slope2 = RoundOff((y2 - y1) / (x2 - x1), 6);
		}

		if (slope1 == slope2) {
			ans[i] = "YES";
		}
		else {
			ans[i] = "NO";
		}
	}

	for (int i = 0; i < n; i++) {
		std::cout << ans[i] << std::endl;
	}

}

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

int main()
{
	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;
		if((y2-y1)/(x2-x1) == (y4-y3)/(x4-x3)){
			cout << "YES" <<endl;
		}else{
			cout << "NO" <<endl;
		}
		
	}
	return 0;
}

C++: #include<iostream>
#include<cmath>
using namespace std;
bool EQ(double a,double b){
  return abs(a-b)<0.000000001;
  //  return a-0.0000001<b&&b<a+0.0000001;
}
int main(){
  int n;
  double x[4],y[4];
  cin>>n;
  while(n--){
    for(int j=0;j<4;j++) cin>>x[j]>>y[j];
    if(EQ((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 <iostream>
#include <cstdio>
#include <cmath>
using namespace std;

const double EPS = 1e-9;

signed main() {
  int n;

  cin >> n;
  while (n--) {
    double x[4], y[4];
    for (int i = 0; i < 4; cin >> x[i] >> y[i], i++);
    if (fabs((y[1] - y[0]) / (x[1] - x[0]) - (y[3] - y[2]) / (x[3] - x[2])) < EPS) {
      puts("YES");
    } else {
      puts("NO");
    }
  }

  return 0;
}

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

int main(){

  int num;
  cin >> num;

  long double x[4], y[4];
  int i = 0;

  while( i < num ){
  for( int j = 0; j < 4; j++ ){
    cin >> x[j] >> y[j];
  }
  long double comp1 = (y[1]- y[0]) / (x[1] - x[0]) ;
  long double comp2 = (y[3] - y[2]) / (x[3] - x[2]);
  cout << comp1 << ", "<< comp2 << endl;
  if(comp1 == comp2 || comp1 * (-1) == comp2 ){
    cout <<"YES"<< endl;
  }else{
    cout <<"NO"<< endl;
  }
  i++;
  }

 
  return 0;
}

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


int main(void) {

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