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 <cstring>
#include <string>
#include <vector>
#include <algorithm>
#include <tuple>
#include <queue>
#include <set>
#include <cstdio>
#include <climits>
#include <cmath>
#include <array>
#include <functional>
#include <sstream>
#include <list>
#include <iomanip>
#include <set>
const int MOD=1000000007;
const int INF=1000000000;
using namespace std;
typedef long long ll;
typedef vector<int> vi;
const double eps=1e-10;
const int inf=1e9;
typedef pair<int,int> P;
static const int COUNTER_CLOCKWISE=1;
static const int CLOCKWISE=-1;
static const int ONLINE_BACK=2;
static const int ONLINE_FRONT=-2;
static const int ON_SEGMENT=0;

class Point
{
public:
  double x,y;
  Point(double x,double y): x(x),y(y){}
  Point operator + (const Point &p)
  {
    return Point(x+p.x,y+p.y);
  }
  Point operator - (const Point &p)
  {
    return Point(x-p.x,y-p.y);
  }
  Point operator * (double k)
  {
    return Point(x*k,y*k);
  }
  Point operator / (double k)
  {
    return Point(x/k,y/k);
  }
  bool operator < (const Point &p) const
  {
    return x!=p.x ? x<p.x : y<p.y;
  }
  bool operator == (const Point &p) const
  {
    return fabs(x-p.x)<eps && fabs(y-p.y) < eps;
  }
  double abs(){return sqrt(norm());}
  double norm(){return x*x+y*y;}
};
typedef Point Vector;
double dot(Vector a,Vector b)
{
  return a.x*b.x+a.y*b.y;
}
double cross(Vector a,Vector b)
{
  return a.x*b.y - a.y*b.x;
}
double norm(Point a)
{
  return a.x*a.x + a.y*a.y;
}
double abs(Vector a)
{
  return sqrt(norm(a));
}
struct Segment
{
  Point p1,p2;
  Segment(Point p1,Point p2): p1(p1),p2(p2){}
};
typedef Segment Line;

class Circle
{
public:
  Point c;
  double r;
  Circle(Point c,double r):  c(c),r(r){}
};

typedef vector<Point> Polygon;


bool isorthogonal(Vector a,Vector b)
{
  return fabs(dot(a,b)-0.0)<eps;
}

bool isparallel(Vector a,Vector b)
{
  return fabs(cross(a,b)-0.0)<eps;
}
Point project(Segment s,Point p)
{
  Vector base=s.p2-s.p1;
  double r=dot(p-s.p1,base) / norm(base);
  return  s.p1 + base*r;
}
Point reflect(Segment s,Point p)
{
  return p+ (project(s,p)-p)*2.0;
}
double getdistance(Point a,Point b)
{
  return abs(a-b);
}
double getdistanceLP(Line l,Point p)
{
  return abs(cross(l.p2-l.p1,p-l.p1) / abs(l.p2-l.p1));
}
double getdistanceSP(Segment s,Point p)
{
  if(dot(s.p2-s.p1,p-s.p1) <0.0) return abs(p-s.p1);
  if(dot(s.p1-s.p2,p-s.p2) <0.0) return abs(p-s.p2);
  return getdistanceLP(s,p);
}

int ccw(Point p0,Point p1,Point p2)
{
  Vector a=p1-p0;
  Vector b=p2-p0;
  if(cross(a,b)>eps) return COUNTER_CLOCKWISE;
  if(cross(a,b)<-eps) return CLOCKWISE;
  if(dot(a,b)<-eps) return ONLINE_BACK;
  if(a.norm()<b.norm()) return ONLINE_FRONT;
  return ON_SEGMENT;
}
bool intersect(Point p1,Point p2,Point p3,Point p4)
{
  return (ccw(p1,p2,p3)*ccw(p1,p2,p4)<=0 && ccw(p3,p4,p1) * ccw(p3,p4,p2)<=0);
}
bool intersect(Segment s1,Segment s2)
{
  return intersect(s1.p1,s1.p2,s2.p1,s2.p2);
}
Point getcrosspoint(Segment s1,Segment s2)
{
  Vector base =s2.p2-s2.p1;
  double d1=abs(cross(base,s1.p1-s2.p1));
  double d2=abs(cross(base,s1.p2-s2.p1));
  double t=d1/(d1+d2);
  return s1.p1+(s1.p2-s1.p1)*t;
}

double getdistanceSS(Segment s1,Segment s2)
{
  if(intersect(s1,s2)) return 0.0;
  return min(min(getdistanceSP(s1,s2.p1),getdistanceSP(s1,s2.p2)),min(getdistanceSP(s2,s1.p1),getdistanceSP(s2,s1.p2)));
}

pair<Point,Point> getcrosspoints(Circle c,Line l)
{
  //assert(intersect(c,l));
  Vector pr=project(l,c.c);
  Vector e=(l.p2-l.p1)/abs(l.p2-l.p1);
  double base=sqrt(c.r*c.r - norm(pr-c.c));
  return make_pair(pr+e*base,pr-e*base);
}

double arg(Vector p){return atan2(p.y,p.x);}
Vector polar(double a,double r){return Point(cos(r)*a,sin(r)*a);}
pair <Point,Point> getcrosspoints(Circle c1,Circle c2)
{
  double d=abs(c1.c-c2.c);
  double a=acos((c1.r*c1.r+d*d-c2.r*c2.r) / (2*c1.r*d));
  double t=arg(c2.c-c1.c);
  return make_pair(c1.c+polar(c1.r,t+a),c1.c+polar(c1.r,t-a));
}
int contains(Polygon g,Point p)
{
  int n=g.size();
  bool x=false;
  for(int i=0;i<n;i++)
    {
      Point a=g[i]-p,b=g[(i+1)%n]-p;
      if(abs(cross(a,b)) < eps && dot(a,b)<eps) return 1;
      if(a.y>b.y) swap(a,b);
      if(a.y <eps && eps<b.y && cross(a,b)>eps) x=!x;
    }
  return (x ? 2 : 0);
}

Polygon andrewscan(Polygon s)
{
  Polygon u,l;
  if(s.size()<3) return s;
  sort(s.begin(),s.end());
  u.push_back(s[0]);
  u.push_back(s[1]);
  l.push_back(s[s.size()-1]);
  l.push_back(s[s.size()-2]);
  for(int i=2;i<s.size();i++)
    {
      for(int n=u.size();n>= 2 && ccw(u[n-2],u[n-1],s[i])!=CLOCKWISE; n--)
	{
	  u.pop_back();
	}
      u.push_back(s[i]);
    }
  for(int i=s.size()-3;i>=0;i--)
    {
      for(int n=l.size();n>=2&&ccw(l[n-2],l[n-1],s[i]) != CLOCKWISE;n--)
	{
	  l.pop_back();
	}
      l.push_back(s[i]);
    }
  reverse(l.begin(),l.end());
  for(int i=u.size()-2;i>=1;i--) l.push_back(u[i]);
  return l;
}
int main(int argc,char const* argv[])
{
  int n;
  cin >> n;
  for(int i=0;i<n;i++)
    {
      double ax,ay,bx,by,cx,cy,dx,dy;
      cin >> ax >> ay >> bx >> by >> cx >> cy >> dx >> dy;
      Point a=Point(ax,ay);
      Point b=Point(bx,by);
      Point c=Point(cx,cy);	
      Point d=Point(dx,dy);
      Vector ab=Vector(b-a);
      Vector cd=Vector(d-c);
      if(isparallel(ab,cd))
	{
	  cout << "YES" << endl;
	}
      else
	{
	  cout<< "NO" <<endl;
	}
    }
}

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

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

C++: #include <stdio.h>

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

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

using namespace std;
#define EPS 0.0000001
 
 
main(){
  int tcase;   cin >> tcase;
  double x1, y1, x2, y2, x3, y3, x4, y4;
  for ( int i = 0; i < tcase; i++ ){
      cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3 >> x4 >> y4;
 
   
      if ( abs((y2 - y1)*(x4 - x3) - (y4 - y3)*(x2 - x1) ) < EPS ) cout << "YES" << endl;
      else cout << "NO" << endl;
  }
}