Go: package main

import (
	"bufio"
	"os"
	"strconv"
	"fmt"
	"strings"
)

var scanner = bufio.NewScanner(os.Stdin)

func nextInt() int {
	scanner.Scan()
	i, e := strconv.Atoi(scanner.Text())
	if e != nil {
		panic(e)
	}
	return i
}

func main() {
	n := nextInt()
	for i := 0; i < n; i++ {
		scanner.Scan()
		points := strings.Split(scanner.Text(), " ")
		fpoints := make([]float64, 0)
		for _, v := range points {
			tmp, _ := strconv.ParseFloat(v, 64)
			fpoints = append(fpoints, tmp)
		}
		if fpoints[0] == fpoints[2] && fpoints[4] == fpoints[6] {
			fmt.Println("YES")
			continue
		}
		a := (fpoints[1]-fpoints[3])/(fpoints[0]-fpoints[2])
		b := (fpoints[5]-fpoints[7])/(fpoints[4]-fpoints[6])
		if a == b {
			fmt.Println("YES")
		} else {
			fmt.Println("NO")
		}
	}
}

C++: #include <cstdio>
#include <cstring>
#include <string>
#include <cmath>
#include <cassert>
#include <iostream>
#include <algorithm>
#include <stack>
#include <queue>
#include <vector>
#include <set>
#include <map>
#include <bitset>
#include <functional>
#include <sstream>
#include <utility>
#include <cstdlib>
#include <cctype>
#include <climits>
#include <fstream>

using namespace std;
typedef long long ll;
typedef pair<int, int> pii;
typedef vector<int> vi;
typedef vector<double> vd;
typedef vector<pair<int, int> > vpii;
#define FOR(i,a,b) for(int i=(a);i<(b);++i)
#define rep(i,n) for(int i=0;i<(n);++i)
#define foreach(itr,c) for(__typeof(c.begin()) itr=c.begin(); itr!=c.end(); itr++)
#define dbg(x) cout << #x"="<< (x) << endl
#define mp(a,b) make_pair((a),(b))
#define pb(a) push_back(a) 
#define in(x) cin >> x;
#define all(x) (x).begin(), (x).end()
#define INF 2147483600
#define fi first
#define se second
using namespace std;

int main(){
	int N;
	vd a(8);
	cin >> N;
	rep(i,N){
		rep(i,8){
			cin>>a[i];
		}
		if(abs((a[1]-a[3])*(a[4]-a[6])-(a[5]-a[7])*(a[0]-a[2]))<0.0000001){
			cout<<"YES"<<endl;
		}else{
			cout<<"NO"<<endl;
		}
	}

}

C++: #include <iostream>
using namespace std;
double d[8];

int main() {
  int n = 0;
  cin >> n;

  for(int i = 0; i < n; i++) {
    for(int j = 0; j < 4*2; j++)
      cin >> d[j];
    double tanAB = (d[3]-d[1])/(d[0]-d[2]);
    double tanCD = (d[7]-d[5])/(d[4]-d[6]);
    if(tanAB == tanCD)
      cout << "YES" << endl;
    else
      cout << "NO" << endl;
  }

  return 0;
}

C++: #include<iostream>

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

	return 0;
}

C++: #include <stdio.h>
#include <math.h>

int main(void){
    int n;
    cin >> n;
    rep(i,n) {

    double x[4],y[4],a,b,c,d,p,q,det,ansx,ansy;
    //入力を多少簡略化
    for(int i=0;i<4;i++) {
        scanf("%lf" ,&x[i]);
        scanf("%lf" ,&y[i]);
    }
    //(x1,y1)(x2,y2)を結ぶax+by=p の係数
    a = y[0]-y[1];  b = x[1]-x[0];
    p = x[1]*y[0]-x[0]*y[1];
    //(x3,y3)(x4,y4)を結ぶcx+dy=q の係数
    c = y[2]-y[3];  d = x[3]-x[2];
    q = x[3]*y[2]-x[2]*y[3];
    //係数についての行列式
    det = a*d-c*b;
    if(a*d == b*c) puts("YES");
    else puts("NO");
    /*
    //det=0で解は存在しない(この問題については２線分は同一直線上にないから)
    if(fabs(det) < 0.00001) puts("YES");
    //逆行列を用いて解を導出
    else {
        /*
        ansx = (d*p - b*q)/det;
        ansy = (-c*p + a*q)/det;
        //ansxがどちらの線分のx座標範囲にも含まれるかを判定
        if((x[0] - ansx)*(x[1] - ansx) <= 0.00001 && (x[2] - ansx)*(x[3] - ansx) <= 0.00001 ) {
            printf("YES (%lf, %lf)\n",ansx,ansy);
        } else puts("NO");
        
       puts("NO");
    }
    */
    
}

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

int main() {
	double x[4], y[4], m[2];
	int i, n;
	cin >> n;
	while (n--) {
		for (i = 0; i < 4; i++)
			cin >> x[i] >> y[i];
		if (x[0] == x[1] && x[2] == x[3])
			cout << "YES" << endl;
		else if (x[0] != x[1] && x[2] != x[3]) {
			m[0] = (y[1]-y[0]) / (x[1]-x[0]);
			m[1] = (y[3]-y[2]) / (x[3]-x[2]);
			if (m[0] - m[1] == 0)
				cout << "YES" << endl;
			else
				cout << "NO" << endl;
		} else
			cout << "NO" << endl;
	}
	return 0;
}

C++: #include <algorithm>
#include <iostream>
#include <vector>
#include <math.h>
#include <set>
#include <map>
#include <string>
#include <stack>
#include <queue>
#include <iomanip>
#include <numeric>
#include <tuple>
#include <bitset>
#include <complex>
#define _USE_MATH_DEFINES
using namespace std;
typedef long long ll;
typedef pair<int, int> pii;
typedef pair<ll, ll> plglg;
typedef tuple<int, int, int> tiii;
typedef tuple<ll, ll, ll> tlglglg;
typedef complex<double> xy_t;
int dx[4] = {1, 0, -1, 0};
int dy[4] = {0, 1, 0, -1};
double pi = 3.141592653589793;
ll mod = 1000000007;
int intmax = 2147483647;
int intmin = -2147483648;
ll llmax = 9223372036854775807;
ll llmin = -9223372036854775807;
ll inf = llmax / 2;
double eps = 1e-11;

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

xy_t P[110];

int main() {
    int n;
    cin >> n;
    double x[4], y[4];
    for (int i = 0; i < n; i++) {
        for (int i = 0; i < 4; i++) {
            cin >> x[i] >> y[i];
        }
        xy_t a[2] = {
            xy_t (x[0], y[0]) - xy_t (x[1], y[1]),
            xy_t (x[2], y[2]) - xy_t (x[3], y[3])
        };
        bool ok = abs(cross_product(a[0], a[1])) < eps;
        cout << (ok ? "YES" : "NO") << endl;
    }
}

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

typedef struct {
  double x;
  double y;
} point;

int main() {
  point a, b, c, d;
  int n;
  cin >> n;
  while (n--) {
    cin >> a.x >> a.y >> b.x >> b.y >> c.x >> c.y >> d.x >> d.y;
    point p, q;
    p.x = b.x - a.x;
    p.y = b.y - a.y;
    q.x = d.x - c.x;
    q.y = d.y - c.y;

    if (-0.000001 < p.x * q.y - p.y * q.x && p.x * q.y - p.y * q.x < 0.000001)
      cout << "YES" << endl;
    else
      cout << "NO" << endl;
  }
  return 0;
}

C++: #include <iostream>

using namespace std;

int main(){
	int n;
	double x[100],y[100];
	while(cin>>n){
		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;
	}
}

C++: #include<iostream>
#include<vector>
using namespace std;
int main(){
	int n;
	cin >> n;
	for(int i=0;i<n;++i){
		vector<double> a(2),b(2),c(2),d(2);
		double tmp;
		cin>>a[0]>>a[1]>>b[0]>>b[1]>>c[0]>>c[1]>>d[0]>>d[1];
		double x1=b[0]-a[0],y1=b[1]-a[1];
		double x2=d[0]-c[0],y2=d[1]-c[1];
		if(x1*y2-x2*y1>0) cout << "NO" << endl;
		else cout << "YES" << endl;
	}
	return 0;
}

C++: #include <iostream>
#include <utility>
#include <complex>
#include <vector>

using namespace std;

typedef complex<double> point;

const double EPS = 1e-7;
double cross(point a,point b){
    return imag(conj(a)*b);
}

bool is_parallel(point a1,point a2,point b1,point b2){
    return (cross(a2-a1,b2-b1) < EPS);
}

int main(){
    int N;
    cin >> N;
    for(int i=0;i<N;i++){
        vector<point> ps(4);
        for(int j=0;j<4;j++){
            cin >> ps[j].real() >> ps[j].imag();
        }
        if(is_parallel(ps[0],ps[1],ps[2],ps[3])){
            cout << "YES" << endl;
        }else{
            cout << "NO" << endl;
        }
    }
    return 0;
}

C++: #include<cstdio>
#include<iostream>
#include<algorithm>
#include<vector>
#include<set>
#include<string>
#include<functional>
#include<queue>
#include<stack>
#include<math.h>
#define INF 999999
#define EPS 1.0e-6

using namespace std;

typedef long long ll;


int main()
{
	int n;
	cin >> n;
	int x1, y1, x2, y2, x3, y3, x4, y4;
	for(int i=0;i<n;i++){
		cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3 >> x4 >> y4;
		int a = (y2 - y1)*(x4 - x3) - (y4 - y3)*(x2 - x1);
		if (a - 1 < EPS )printf("YES");
		else printf("NO");
	}
	
	return 0;
}