Go: package main

import (
        "fmt"
        "math/big"
)

func main() {
        var x1, x2, x3, x4, y1, y2, y3, y4 big.Float
        var n int
        fmt.Scanf("%d", &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)
                var lx, ly, mx, my, mul1, mul2, sub, zz big.Float
                lx.Sub(&x2, &x1)
                ly.Sub(&y2, &y1)
                mx.Sub(&x4, &x3)
                my.Sub(&y4, &y3)
                mul1.Mul(&lx, &my)
                mul2.Mul(&ly, &mx)
                sub.Sub(&mul1, &mul2)
                zz.SetFloat64(0.0)

                if sub.Cmp(&zz) == 0 {
                        fmt.Println("YES")
                } else {
                        fmt.Println("NO")
                }
        }
}

C++: #define	_USE_MATH_DEFINES
#include <iostream>
#include <algorithm>
#include <functional>
#include <vector>
#include <cstdio>
#include <string>
#include <cmath>
#include <cfloat>
#include <map>
#include <queue>
#include <stack>
#include <list>
using namespace std;

int main(){

	int n;
	cin>>n;
	double x1,y1,x2,y2,x3,y3,x4,y4;
	
	while(n>0){
		double a,b;
	cin>>x1>>y1>>x2>>y2>>x3>>y3>>x4>>y4;
		
		a=(y2-y1)/(x2-x1);
		b=(y4-y3)/(x4-x3);
		
		if(a==b)
			cout<<"YES"<<endl;
		else
			cout<<"NO"<<endl;
	}
}

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

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

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

    cin >> n;
    while(n--){
        cin >>x1 >> y1 >> x2 >> y2 >> x3 >> y3 >> x4 >> y4;

        ab = (y1 - y2) / (x1 - x2);
        cd = (y3 - y4) / (x3 - x4);

        if ((int)ad == (int)cd){
            cout << "YES" << endl;
        }else{
            cout << "NO" << endl;
        }
    }
}

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

typedef complex<double> xy_t;

double cross_product(xy_t a, xy_t b) { return (conj(a)*b).imag(); }

int main(){
  int n;
  cin >> n;
  for(int i=0; i<n; ++i){
    xy_t P[4];
    double x, y;
    for(int j=0; j<4; ++j){
      cin >> x >> y;
      P[j] = xy_t(x, y);
    }
    double area = cross_product(P[1]-P[0], P[3]-P[2]);
    if(area){
      cout << "NO" << endl;
    } else {
      cout << "YES" << endl;
    };

  }
}

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

struct point {
    double x;
    double y;
};

bool isparallel(point p1, point p2)
{
    if(((p1.x * p2.y) - (p1.y * p2.x)) ==0) return true;
    else return false;
}

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(isparallel(p1, p2)) cout << "YES" << endl;
        else cout << "NO" << endl;
    }
    return EXIT_SUCCESS;
}

C++: #include<cstdio>
#include<iostream>
#include<algorithm>
#include<vector>
#include<string>
#include<map>
#include<stack>
#include<cstring>
#include<cstdlib>
#include<cmath>

using namespace std;

#define reps(i,f,n) for(int i=f;i<int(n);i++)
#define rep(i,n) reps(i,0,n)

int main(){
	
	int n;
	cin>>n;
	
	rep(t,n){
		double x[4];
		double y[4];
		
		rep(i,4){
			cin>>x[i]>>y[i];
		}
		
		double dx1 = x[0]-x[1];
		double dx2 = x[2]-x[3];
		
		double dy1 = y[0]-y[1];
		double dy2 = y[2]-y[3];
		
		
		if(dx1==0){
			if(dx2==0)puts("YES");
			else puts("NO");
		}else{
			double a1 = dy1/dx1;
			double a2 = dy2/dx2;
			if(fabs(a1-a2)<0.000000001)puts("YES");
			else puts("NO");
		}
	}
}

C++: #include<iostream>
#include<string>
#include<algorithm>
#include<map>
#include<vector>
#include<cmath>
#include<cstdio>
#define loop(i,a,b) for(int i=a;i<b;i++) 
#define rep(i,a) loop(i,0,a)
#define pb(in,tmp) in.push_back(tmp)
#define all(in) in.begin(),in.end()
const double PI=acos(-1);
using namespace std;
int main(){
  int n;
  cin>>n;
  rep(i,n){
    double a=543.5,b=629.3;
    double ax,ay,bx,by,cx,cy,dx,dy;
    cin>>ax>>ay>>bx>>by>>cx>>cy>>dx>>dy;
    int han=0;
    if(bx==ax)han++;
    else a=(by-ay)/(bx-ax);
    if(dx==cx)han++;
    else b=(dy-cy)/(dx-cx);
    //cout<<han<<" "<<a<<" "<<b<<endl;
    if(a==b||han==2)cout<<"YES"<<endl;
    else cout<<"NO"<<endl;
  }
}

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

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;
        if ((x2-x1)==0 && (x4-x3)==0)
            cout << "YES" << endl;
        else {
            int d = abs((y2-y1)/(x2-x1) - (y4-y3)/(x4-x3));
            cout << ((d <= 1e-10)? "YES" : "NO") << endl;
        }
    }
    return 0;
}

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

using namespace std;

class Vector2D {
  private:
    double x;
    double y;
  public:
    Vector2D(double, double);
    Vector2D operator - (Vector2D vec2) {
      double newX = this->x - vec2.x;
      double newY = this->y - vec2.y;
      Vector2D newvec(newX, newY);
      return newvec;
    }
    double GetX() { return x; };
    double GetY() { return y; };
    bool isParallel(Vector2D);
};

Vector2D::Vector2D(double _x, double _y) {
  x = _x;
  y = _y;
}

bool Vector2D::isParallel(Vector2D vec2) {
  if (this->x == vec2.GetX() || this->y == vec2.GetY())
    return true;
  else if (abs(vec2.GetY()/this->y) == abs(vec2.GetX()/this->x))
    return true;
  else
    return false;
}

int main() {
  int n;
  cin >> n;

  double x1,y1,x2,y2,x3,y3,x4,y4;
  for (int dataset_i = 0; dataset_i < n; dataset_i++) {
    cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3 >> x4 >> y4;
    Vector2D vecAB = Vector2D(x2,y2) - Vector2D(x1,y1);
    Vector2D vecCD = Vector2D(x4,y4) - Vector2D(x3,y3);
    cout << (vecAB.isParallel(vecCD) ? "YES" : "NO") << endl;
  }

  return 0;
}

C++: #include <algorithm>
#include <cmath>
#include <cstdio>

using namespace std;

int main() {
  int n;
  scanf("%d", &n);
  for (int i = 0; i < n; ++i) {
    double x1, y1, x2, y2, x3, y3, x4, y4;
    scanf(
        "%lf %lf %lf %lf %lf %lf %lf %lf",
        &x1, &y1, &x2, &y2, &x3, &y3, &x4, &y4);

    double v1x = x1 - x2, v1y = y1 - y2;
    double v2x = x3 - x4, v2y = y3 - y4;
    double prod = v1x * v2x + v1y * v2y;
    if (prod < 0.0) { prod *= -1; }

    double z1 = sqrt(v1x * v1x + v1y * v1y);
    double z2 = sqrt(v2x * v2x + v2y * v2y);

    puts(abs(prod - z1 * z2) < 0.0000001 ? "YES" : "NO");
  }

  return 0;
}

C++: #include<iostream>
#include<algorithm>
#include<vector>
#include<stack>
#include<queue>
#include<cstdio>
#include<climits>
#include<cmath>
#include<cstring>
#include<string>
#include<complex>
#include<cfloat>
#include<cassert>

#define f first
#define s second
#define mp make_pair

#define REP(i,n) for(int i=0; i<(int)(n); i++)
#define FOR(i,c) for(__typeof((c).begin()) i=(c).begin(); i!=(c).end(); i++)
#define ALL(c) (c).begin(), (c).end()
#define EPS (1e-10)
using namespace std;

typedef unsigned int uint;
typedef long long ll;
typedef complex<double> P;

#define PI (3.1415926535)
#define EPS (1e-10)

int main(){
  int n; cin>>n;
  while(n-->0){
    double x1,x2,x3,x4,y1,y2,y3,y4;
    cin>>x1>>y1>>x2>>y2>>x3>>y3>>x4>>y4;
    double xx1 = x1 - x2;
    double yy1 = y1 - y2;
    double xx2 = x3 - x4;
    double yy2 = y3 - y4;
    if(xx1 == 0 && xx2 == 0){
      cout << "YES" << endl;
      continue;
    }
    double r = xx1 / xx2;
    yy2 *= r;
    if(abs(yy1-yy2) < EPS)
      cout << "YES" << endl;
    else
      cout << "NO" << endl;
  }
  
  return 0;
}