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[1] == fpoints[3] {
			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<iostream>
#include<math.h>
#include<string>
#include<vector>
#include<algorithm>
#include<queue>
#include<map>
using namespace std;
#define INF 1<<20

struct edge{
	int to;
	int cost;
	edge() {};
	edge(int a, int b) {
		to = a;
		cost = b;
	}
};

struct Node {
	int pos;
	int cost;
	Node() {};
	Node(int a, int b) {
		pos = a;
		cost = b;
	}
	bool operator <(const Node& rhs) const {
		return cost > rhs.cost;
	}
};

void dijkistra(int st, int TO,int V,vector<vector<edge> > graph) {
	priority_queue<Node> q;
	vector<int> d(V);
	for (int i = 0;i < V;i++) {
		d[i] = INF;
	}
	d[st] = 0;
	q.push(Node(st, 0));
	while (!q.empty()) {
		Node s = q.top(); q.pop();
		int pos = s.pos;
		for (int i = 0;i < graph[pos].size();i++) {
			int to = graph[pos][i].to;
			int cost = graph[pos][i].cost;
			if (d[pos] + cost < d[to]) {
				d[to] = d[pos] + cost;
				q.push(Node(to, d[to]));
			}

		}
	}
	cout << d[TO] << endl;
}
int partition(int A[],int p, int r) {
	int x = A[r];
	int i = p - 1;
	for (int j = p;j < r;j++) {
		if (x >= A[j]) {
			i = i + 1;
			swap(A[i], A[j]);
		}
	}
		swap(A[i + 1], A[r]);
		return i + 1;
}
void quicksort(int A[], int p, int r) {
	if (p < r) {
		int q = partition(A, p, r);
		quicksort(A, p, q-1);
		quicksort(A, q + 1, r);
	}
}

int getNumberOfCoin(int C[],int n,int m) {
	int T[50000];
	for (int j = 0;j <= n;j++) {
		T[j] = INF;
	}
	T[0] = 0;
	for (int i = 0;i < m;i++) {
		for (int j = 0;j <= n;j++) {
			if (j - C[i] >=0  ) {
				T[j] = min(T[j], T[j - C[i]] + 1);
			}
		}
	}
	return T[n];
}
int knapsack(int w, int n,int* v,int* ww) {
	int W[101][10001];
		for (int j = 0;j <= w;j++) {
			W[0][j] = 0;
		}
	for (int i = 0;i < n;i++) {
		for (int k = 0;k < n;k++){
			for (int j = 0;j <= w;j++) {
				if (j - ww[k] < 0) {
					W[i + 1][j] = W[i][j];
				}
				if (j - ww[i] >= 0) {
					W[i + 1][j] = max(W[i][j], W[i][j - ww[k]] + v[k]);

				}
			}
		}
	}
	return W[n][w];
}
int fib(int n) {
	vector<int> v(n+1);
	v[0] = 1;
	v[1] = 1;
	for (int i = 2;i < n+1;i++) {
		v[i] = v[i - 2] + v[i - 1];
	}
	return v[n];
}
int isPrime(int x) {
	if (x <= 1) return false;
	if (x>2 && x % 2 == 0) return false;
	for (int i = 2;i <= sqrt(x);i++) {
		if (x%i == 0) {
			return false;
		}
	}
	if (x == 2) return true;
	return true;
}
int Prime(int x) {
	vector<int> prime(x+1,1);
	int num = 2;
	while (num <= sqrt(x)) {
		for (int i = 2;i <= x / num;i++) {
			prime[num*i] = 0;
		}
		num++;
		while (prime[num] == 0)
			num++;
	}
	int count = 0;
	for (int i = 2;i < x + 1;i++) {
		if (prime[i] == 1) count++;
	}
	return count;
}
int main(){
	double x[4], y[4];
	int n;
	cin >> n;
	for (int i = 0;i < n;i++) {
		for (int j = 0;j < 4;j++) {
			cin >> x[j] >> y[j];
		}
		double X1 = x[1] - x[0];
		double Y1 = y[1] - y[0];
		double X2 = x[3] - x[2];
		double Y2 = y[3] - y[2];
		if (fabs(X1*Y2-X2*Y1)<0.0001) cout << "YES"<<endl;
		else cout << "NO"<<endl;
	}
	

}

C++: #include <iostream>
using namespace std;
int main(){
	int n;
	cin >> n;
	while(n--){
		double x[4], y[4], o;
		for(int i = 0; i < 4) cin >> x[i] >> y[i];
		o = (x[1]-x[0])*(y[3]-y[2]) - (y[1]-y[0])*(x[3]-x[2]);
		if(o < 0.000001) cout << "YES" << endl;
		else cout << "NO" << endl;
	}
	return 0;
}

C++: #include <iostream>
using namespace std;
int main() {
  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;
    cout << (((y2 - y1) / (x2 - x1) == (y4 - y3) / (x4 - x3)) ? "YES" : "NO") << endl;
  }
}

C++: #include <iostream>

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 ( ( (x1==x2) && (y1==y2) ) || ( (x3==x4) && (y3==y4) ) ) {
			cout << "NO" << endl;
			continue;
		}
		
		double LHS = 100 * (y2 - y1) * (x4 - x3);
		double RHS = 100 * (y4 - y3) * (x2 - x1);
		//cout << LHS << " " << RHS << endl;
		//if ( (y2 - y1) * (x4 - x3) == (y4 - y3) * (x2 - x1) ) cout << "YES" << endl;
		if ( LHS == RHS ) cout << "YES" << endl;
		else cout << "NO" << endl;
	}
		
	return 0;
}

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

const double EPS = 1e-12;

double add(double a, double b) {
	if (abs(a + b) < EPS * (abs(a) + abs(b)))
		return 0;
	return a + b;
}

struct P {
	double x, y;
	P() {};
	P(double x, double y) : x(x), y(y) {
	}

	P operator + (P p) {
		return P(add(x, p.x), add(y, p.y));
	}

	P operator - (P p) {
		return P(add(x, -p.x), add(y, -p.y));
	}

	P operator * (double d) {
		return P(x * d, y * d);
	}

	P operator / (double d) {
		return P(x / d, y / d);
	}

	// 内積
	double dot(P p) {
		return add(x * p.x, y * p.y);
	}

	// 外積
	double det(P p) {
		return add(x * p.y, -y * p.x);
	}
};

// 線分上に点があるか
bool on_seg(P p1, P p2, P q) {
	return (p1 - q).det(p2 - q) == 0 && (p1 - q).dot(p2 - q) <= 0;
}

// 直線と直線の交点
P intersection(P p1, P p2, P q1, P q2) {
	return p1 + (p2 - p1) * ((q2 - q1).det(q1 - p1) / (q2 - q1).det(p2 - p1));
}

// 2直線の直行判定
bool line_orth(P a1, P a2, P b1, P b2) {
	return fabs((a1 - a2).dot(b1 - b2)) < EPS;
}

// 2直線の平行判定
bool line_paral(P a1, P a2, P b1, P b2) {
	return fabs((a1 - a2).det(b1 - b2)) < EPS;
}

int main(void) {
	int N;
	P p[4];
	cin >> N;
	for (int i = 0; i < N; i++) {
		for (int j = 0; j < 4; j++) {
			cin >> p[j].x >> p[j].y;
		}
		cout << (line_paral(p[0], p[1], p[2], p[3]) ? "YES" : "NO") << endl;
	}
}

C++: #include <cstdio>

using namespace std;

int main(void){
	int n, i;
	double x1,y1,x2,y2,x3,y3,x4,y4, vabx, vaby, vcdx, vcdy;
	scanf("%d",&n);
	for(i = 0;i < n;i++){
		scanf("%lf%lf%lf%lf%lf%lf%lf%lf",&x1,&y1,&x2,&y2,&x3,&y3,&x4,&y4);
		vabx = x1 - x2, vaby = y1 - y2;
		vcdx = x3 - x4, vcdy = y3 - y4;
		if((vabx == vcdx && vaby == vcdy) || (-vabx == vcdx && -vaby == vcdy)){
			printf("YES\n");
		}else{
			printf("NO\n");
		}
	}
	return 0;
}