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;

typedef long double ld;
typedef complex<ld> P;

const ld eps = 1e-12;

P input() {
	ld x1, y1, x2, y2; cin >> x1 >> y1 >> x2 >> y2;
	return P(x1-x2, y1-y2);
}
ld dot(P p1, P p2) {
	return p1.real()*p2.real() + p1.imag()*p2.imag();
}
int main() {
	int n; cin >> n;
	for (int i = 0; i < n; ++i) {
		P p1 = input();
		P p2 = input();
		ld d = dot( p1 * P(0, 1), p2 );
		if ( abs(d) < eps ) {
			cout << "YES" << endl;
		}
		else {
			cout << "NO" << endl;
		}
	}
}

C++: #include <iostream>
#include <stack>
#include <stdio.h>
#include <cmath>
using namespace std;
double x[4],y[4];//p,1,2,3
double a[2],b[2],c[2];
void make_line(double x1,double y1,double x2,double y2,int num){
	if(x1!=x2&&y1!=y2){
		a[num]=(y1-y2)/(x1-x2);
		b[num]=-1;
		c[num]=-a[num]*x1+y1;
	}else if(x1==x2){
		a[num]=1;
		b[num]=0;
		c[num]=-x1;
	}else if(y1==y2){
		a[num]=0;
		b[num]=1;
		c[num]=-y1;
	}
	return;
}
int main()
{
	int n;
	cin>>n;
	while(n--){
		bool ans=false;
		cin>>x[0]>>y[0]>>x[1]>>y[1]>>x[2]>>y[2]>>x[3]>>y[3];
		make_line(x[0],y[0],x[1],y[1],0);
		make_line(x[2],y[2],x[3],y[3],1);
		if(a[0]==0){
			ans=(a[1]==0);
		}else if((b[0]/a[0])==(b[1]/a[1])){
			ans=true;
		}
		printf(ans?"YES\n":"NO\n");
	}
    return 0;
}

C++: #include <iostream>
#include <cmath>
using namespace std;
void judge(double,double,double,double,double,double,double,double);
#define EPS 0.000001

int main() {
	int n;
	double x1, y1, x2, y2, x3, y3, x4, y4, ab, cd;

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

C++: #ifndef _GLIBCXX_NO_ASSERT
#include <cassert>
#endif
#include <cctype>
#include <cerrno>
#include <cfloat>
#include <ciso646>
#include <climits>
#include <clocale>
#include <cmath>
#include <csetjmp>
#include <csignal>
#include <cstdarg>
#include <cstddef>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <ctime>

//#ifdef __GXX_EXPERIMENTAL_CXX0X__
#include <ccomplex>
#include <cfenv>
#include <cinttypes>
#include <cstdbool>
#include <cstdint>
#include <ctgmath>
#include <cwchar>
#include <cwctype>
//#endif

#include <algorithm>
#include <bitset>
#include <complex>
#include <deque>
#include <exception>
#include <fstream>
#include <functional>
#include <iomanip>
#include <ios>
#include <iosfwd>
#include <iostream>
#include <istream>
#include <iterator>
#include <limits>
#include <list>
#include <locale>
#include <map>
#include <memory>
#include <new>
#include <numeric>
#include <ostream>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <stdexcept>
#include <streambuf>
#include <string>
#include <typeinfo>
#include <utility>
#include <valarray>
#include <vector>

#include <array>
//#include <atomic>
#include <chrono>
//#include <condition_variable>
#include <forward_list>
//#include <future>
#include <initializer_list>
//#include <mutex>
#include <random>
#include <ratio>
#include <regex>
#include <system_error>
//#include <thread>
#include <tuple>
#include <typeindex>
#include <type_traits>
#include <unordered_map>
#include <unordered_set>


using namespace std;


class D3
{
public:
	double x;
	double y;
	double z;

	//?????????????????????
	D3 ( double x_ = 0.0 , double y_ = 0.0 , double z_ = 0.0 )
	{
		x = x_;
		y = y_;
		z = z_;
	}

	//??????
	D3 Cross_product ( D3 partner )
	{
		D3 ret ( y*partner.z - z*partner.y , z*partner.x - x*partner.z , x*partner.y - y*partner.x );
		return ret;
	}

	//??????
	double Inner_product ( D3 partner )
	{
		return x*partner.x + y*partner.y + z*partner.z;
	}

	//?????????????????????????????????
	D3 Make_vector ( D3 partner )
	{
		D3 ret ( partner.x - x , partner.y - y , partner.z - z );
		return ret;
	}

	//???????§????2????????????????????¢???????±???????
	double S_vec ( D3 A , D3 B )
	{
		D3 G;
		G = A.Cross_product ( B );
		return sqrt ( G.Inner_product ( G ) ) / 2;
	}

	//??????????????¢???????±???????
	double S_point ( D3 A , D3 B , D3 C )
	{
		return S_vec ( A.Make_vector ( B ) , A.Make_vector ( C ) );
	}

	//???????§????3????????????????????????????±???????
	double V_vec ( D3 A , D3 B , D3 C )
	{
		D3 G;
		G = A.Cross_product ( B );
		return G.Inner_product ( C ) / 6;
	}

	//4???????????????????±???????
	double V_point ( D3 A , D3 B , D3 C , D3 D )
	{
		return V_vec ( A.Make_vector ( B ) , A.Make_vector ( C ) , A.Make_vector ( D ) );
	}

	D3 operator+( D3 partner )
	{
		return D3 ( x + partner.x , y + partner.y , z + partner.z );
	}

	D3 operator-( D3 partner )
	{
		return D3 ( x - partner.x , y - partner.y , z - partner.z );
	}

	bool operator==( D3 partner )
	{
		return x == partner.x&&y == partner.y&&z == partner.z;
	}

	double length ()
	{
		return sqrt ( x*x + y*y + z*z );
	}

	//?????´????????????
	bool Vertical ( D3 partner )
	{
		return ( *this ).Inner_product ( partner ) == 0;
	}

	//??????????????????
	bool Parallel ( D3 partner )
	{
		return abs(abs ( ( *this ).Inner_product ( partner ) ) - abs ( ( *this ).length ()*partner.length () ))-1e-3<0;
	}
};

int main ()
{
	double a[8];
	int N;
	cin >> N;
	for( size_t k = 0; k < N; k++ )
	{
		for( size_t i = 0; i < 8; i++ )
		{
			cin >> a[i];
		}
		D3 A ( a[0] , a[1] ) , B ( a[2] , a[3] ) , C ( a[4] , a[5] ) , D ( a[6] , a[7] );
		D3 AB = B - A , CD = D - C;
		cout << ( AB.Parallel ( CD ) ? "YES" : "NO" ) << endl;
	}
	return 0;
}

C++: #include <iostream>
#include <cmath>
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 (fabs(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 (fabs(veku1[0]*veku2[1]-veku1[1]*veku2[0]) <= 0.0000001) {
				cout << "NO" << endl;
			} else {*/
				cout << "YES" << endl;
			//}
		} else {
			cout << "NO" << endl;
		}
	}
	return 0;
}