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;

#define EPS 1e-10

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

        double s1 = (y2-y1)/(x2-x1);
        double s2 = (y4-y3)/(x4-x3);
        if ( abs(s1-s2) < EPS ) {
//            if ( abs((y1 - x1 * s1) - (y3 - x3 * s2)) < EPS ) {
//                cout << "NO" << endl;
//            } else {
//                cout << "YES" << endl;
//            }
            cout << "YES" << endl;
        } else {
            cout << "NO" << endl;
        }
    }
    return 0;
}

C++: #include<iostream>
#include<cmath>
using namespace std;
struct point{
  double x,y;
    };
int main(){
  point p[4];
  int n;
  cin >> n;
  for(int i=0;i<n;i++){
    cin >> p[0].x >> p[0].y>>p[1].x>>p[1].y>>p[2].x>>p[2].y>>p[3].x>>p[3].y;
    if(fabs((p[1].y -p[0].y)/(p[3].y-p[2].y) - (p[1].x-p[0].x)/(p[3].x -p[2].x)) < 1e-15){
	cout << "YES" << endl;
      } else {cout << "NO" << endl;}
      
  }
}

C++: #include<bits/stdc++.h>
using namespace std;
 
signed main(){
int a;
cin>>a;
double c[8];
for(int b=0;b<a;b++){
    for(int d=0;d<8;d++)cin>>c[d];
    if(c[0]==c[2]&&c[4]==c[6])printf("YES\n");
    else if((c[6]-c[4])/([7]-c[5])==(c[2]-c[0])/(c[3]-c[1]))printf("YES\n");
    else printf("NO\n");s
    }
}

C++: #include <iostream>
#include <iomanip>
#include <cassert>
#include <algorithm>
#include <functional>
#include <vector>
#include <string>
#include <cstring>
#include <stack>
#include <queue>
#include <map>
#include <bitset>
#include <sstream>
#include <istream>
#include <cmath>
#include <cstdio>

using namespace std;

#define vci vector<int>
#define vcs vector<string>
#define pb push_back
#define sz size()
#define mapci map<char, int>
#define mapsi map<string, int>
#define all(x) x.begin(), x.end()

#define for_(i, a, b) for (int i=(int)a; i<(int)b; i++)
#define for_d(i, a, b) for (int i=(int)a-1; i>=b; i--)
#define for_r(i, a, b, c) for (int i=(int)a; i<(int)b; i += c)
#define for_dr(i, a, b, c) for (int i=(int)a-1; i>=b; i -= c)
#define FOR(i,c) for(__typeof((c).begin())i=(c).begin();i!=(c).end();++i)

#define vcitr vector<int>::iterator
#define vcsitr vector<string>::iterator
#define mapcitr map<char, int>::iterator
#define mapsitr map<string, int>::iterator

typedef long long ll;

const int iINF = 2147483647;
const ll lINF = 9223372036854775807;

template <class T> inline void dbg(T t) { cout << t << endl; }

struct Point2D { 
	double x, y;
	Point2D (double a=0, double b=0) { x=a; y=b; }
};

int main() {
	int n; cin >> n;
	Point2D a, b, c, d;
	
	for (int i=0; i<n; i++) {		
		cin >> a.x >> a.y >> b.x >> b.y >> c.x >> c.y >> d.x >> d.y;
		
		double p = (a.y-b.y)/(a.x-b.x), q = (c.y-d.y)/(c.x-d.x);
		
		if (abs(p-q) < 1e-9) cout << "YES" << endl;
		else cout << "NO" << endl;
	}
	
	return 0;
}

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

int main(){
    const double EPS = 1e-12;
    double x[4];
    double y[4];
    double s;
    double t;
    double u;
    double v;
    double m;
    int n;
    cin >> n;

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

        s = x[1] - x[0];
        t = y[1] - y[0];
        u = x[3] - x[2];
        v = y[3] - y[2];

        if(s * v - t * u > 0){
            m = s * v - t * u;
        }
        else{
            m = t * u - s * v;
        }

        if(m < EPS){
            cout << "YES" << endl;
        }
        else{
            cout << "NO" << endl;
        }
    }
    return 0;
}

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

int main() {
	int n;
	double x1, x2, x3, x4, y1, y2, y3, y4;
	vector<double> X1, Y1, X2, Y2;
	cin >> n;
	for (int i = 0; i < n; i++) {
		cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3 >> x4 >> y4;
		X1.push_back(x2 - x1); Y1.push_back(y2 - y1);
		X2.push_back(x4 - x3); Y2.push_back(y4 - y3);
	}
	for (int i = 0; i < n; i++) {
		if (Y1[i] / X1[i] == Y2[i] / X2[i])cout << "YES" << endl;
		else cout << "NO" << endl;
	}
	return 0;
}

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");
		}
	}
}