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 <string>
#include <vector>
#include <algorithm>
#include <cmath>
#include <cstdio>
#include <functional>
#include <numeric>
#include <stack>
#include <queue>
#include <map>
#include <set>
#include <utility>
#include <sstream>
#include <complex>

using namespace std;

#define FOR(i,a,b) for(long long i=(a);i<(b);i++)
#define REP(i,N) for(long long i=0;i<(N);i++)
#define ALL(s) (s).begin(),(s).end()
#define fi first
#define se second

#define PI acos(-1.0)
#define INF 1000000007

typedef long long ll;
typedef pair<ll, ll> P;
typedef pair<double, double> PD;
typedef pair<string, ll> PS;
typedef vector<ll> V;

int n;
double x[100], y[100];

int main(){
	cin >> n;
	REP(i, n){
		REP(i, 4) cin >> x[i] >> y[i];
		if ((x[1] - x[0])*(y[3] - y[2]) == (x[3] - x[2])*(y[1] - y[0])){
			cout << "YES" << endl;
		}
		else{
			cout << "NO" << endl;
		}
	}
}

C++: #include <iostream>
using namespace std;
int main(){
  int t;
   double x1,x2,x3,x4,y1,y2,y3,y4;
  cin >> t;
  while(t--){
cin>>x1>>y1>>x2>>y2>>x3>>y3>>x4>>y4;
    if( (y2-y1)/(x2-x1) == (y4-y3)/(x4-x3) ) 
      cout<<"YES\n";
    else
      cout<<"NO\n";
  }
  
  return 0;
}

C++: #include<iostream>
using namespace std;
int main()
{
double x;
double x1,y1,x2,y2,x3,y3,xp,yp;
int S=0;
while(cin>>x1>>y1>>x2>>y2>>x3>>y3>>xp>>yp)
{
	if(y1<y2&&y3<y2)
	{
	for(double i=x1;i<=x3;i+=0.0001)
{
x=i;
if((y1-y2)/(x1-x2)*x+y1-(y1-y2)/(x1-x2)*x1>(y1-y2)/(x1-x2)*xp+y1-(y1-y2)/(x1-x2)*x1&&(y2-y3)/(x2-x3)*x+y2-(y2-y3)/(x2-x3)*x2>(y2-y3)/(x2-x3)*xp+y2-(y2-y3)/(x2-x3)*x2&&(y1-y3)/(x1-x3)*x+y1-(y1-y3)/(x1-x3)*x2<(y1-y3)/(x1-x3)*xp+y1-(y1-y3)/(x1-x3)*x1)
{
	S++;
cout<<"YES"<<endl;
break;	
}
}
}
else if(y1>y2&&y1>y1)
{
for(double i=x1;i<=x3;i+=0.0001)
{x=i;
if((y1-y2)/(x1-x2)*x+y1-(y1-y2)/(x1-x2)*x1>(y1-y2)/(x1-x2)*xp+y1-(y1-y2)/(x1-x2)*x1&&(y2-y3)/(x2-x3)*x+y2-(y2-y3)/(x2-x3)*x2<(y2-y3)/(x2-x3)*xp+y2-(y2-y3)/(x2-x3)*x2&&(y1-y3)/(x1-x3)*x+y1-(y1-y3)/(x1-x3)*x2>(y1-y3)/(x1-x3)*xp+y1-(y1-y3)/(x1-x3)*x1)
{
	S++;
cout<<"YES"<<endl;
break;	
	}	
	}	
}
else if(y3>y1&&y3>y2)
{
	for(double i=x1;i<=x3;i+=0.0001)
{x=i;
if((y1-y2)/(x1-x2)*x+y1-(y1-y2)/(x1-x2)*x1<(y1-y2)/(x1-x2)*xp+y1-(y1-y2)/(x1-x2)*x1&&(y2-y3)/(x2-x3)*x+y2-(y2-y3)/(x2-x3)*x2>(y2-y3)/(x2-x3)*xp+y2-(y2-y3)/(x2-x3)*x2&&(y1-y3)/(x1-x3)*x+y1-(y1-y3)/(x1-x3)*x2>(y1-y3)/(x1-x3)*xp+y1-(y1-y3)/(x1-x3)*x1)
{
	S++;
cout<<"YES"<<endl;
break;
}
}
}

{
if(S<1)
{
 	cout<<"NO"<<endl;
 }
}
}
return 0;
}

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

using namespace std;

typedef complex<double> Point;

bool parallel(const Point &v1, const Point &v2) {
    double dot = v1.real()*v2.real() + v1.imag()*v2.imag();
    double n = norm(v1) * norm(v2);
    return abs(dot*dot - n) < 1e-6;
}

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];

        if(parallel(v1, v2) && !parallel(v1, ps[2]-ps[1])) cout << "YES" << endl;
        else cout << "NO" << endl;
    }
    return 0;
}

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

#define EPS 1e-10;

struct POINT_DOUBLE {
  double x;
  double y;
};

struct VECTOR_DOUBLE {
  double x;
  double y;

  VECTOR_DOUBLE(const POINT_DOUBLE& pt1, const POINT_DOUBLE& pt2) {
    x = pt2.x - pt1.x;
    y = pt2.y - pt1.y;
  }

  double Angle() const {
    return atan(y / x);
  }
};

std::istream& operator >> (std::istream& is, POINT_DOUBLE& p)
{
  is >> p.x >> p.y;
  return is;
}

static bool IsParallel(const VECTOR_DOUBLE& vec1, const VECTOR_DOUBLE& vec2)
{
  return std::abs(vec1.Angle() - vec2.Angle()) < EPS;
}

int main(void)
{
  int n;
  std::cin >> n;

  for (int i = 0; i < n; i++) {
    POINT_DOUBLE pt1, pt2, pt3, pt4;
    std::cin >> pt1 >> pt2 >> pt3 >> pt4;

    VECTOR_DOUBLE vec1(pt1, pt2);
    VECTOR_DOUBLE vec2(pt3, pt4);

    std::cout << (IsParallel(vec1, vec2) ? "YES" : "NO") << std::endl;
  }

  return 0;
}