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++: /*

Parallelism(https://onlinejudge.u-aizu.ac.jp/problems/0021)
前

*/
#include <iostream>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <climits>
#include <cmath>
#include <complex>
#include <map>
#include <set>
#include <vector>
#include <stack>
#include <queue>
#include <bitset>
#include <algorithm>
#include <numeric>
#include <functional>
#include <cassert>
#include <iomanip>
using namespace std;

#define Rep(b, e, i) for(int i = b; i <= e; i++)
#define Repr(e, b, i) for(int i = e; i >= b; i--)
#define rep(n, i) Rep(0, n-1, i)
#define repr(n, i) Repr(n-1, 0, i)
#define all(v) (v).begin(), (v).end()
#define pb(v) push_back(v)
#define uniq(v) (v).erase(unique(all(v)),(v).end())
#define bitcnt(x) __builtin_popcount(x)
#define fst first
#define snd second
#define Pqaz(T) priority_queue<T,vector<T>,greater<T>>
#define Pqza(T) priority_queue<T>
#define put(x) cout << x;
#define putsp(x) cout << x << ' ';
#define putln(x) cout << x << endl;
#define ENJYU std::ios::sync_with_stdio(false);std::cin.tie(0);

typedef long long ll;
typedef pair<ll, ll> llP;
typedef pair<int, int> intP;
typedef complex<double> comp;
typedef vector <int> vec;
typedef vector <ll> vecll;
typedef vector <double> vecd;
typedef vector <vec> mat;
typedef vector <vecll> matll;
typedef vector <vecd> matd;

//vector の中身を出力
template <class T>ostream &operator<<(ostream &o,const vector<T>&v)
{o<<"{";for(int i=0;i<(int)v.size();i++)o<<(i>0?", ":"")<<v[i];o<<"}";return o;}

const int MAX = 200000;
const double PI = 3.14159265358979323846;
const double EPS = 1e-12;
const int INF = 1<<29;
const ll INFL = 1e18;
const int MOD = 1000000007;

int dx4[4]={1,0,-1,0};
int dy4[4]={0,1,0,-1};
int dx8[8]={1,0,-1,1,-1,1,0,-1};
int dy8[8]={1,1,1,0,0,-1,-1,-1};

//********************************template END****************************************//

//外積を計算する関数
double gaiseki(comp a, comp b)
{
	return (conj(a)*b).imag();
}

void solve(void){

	int N;
	cin >> N;
	while(N--)
	{

		double xs[4], ys[4];
		rep(4, i)
		{
			cin >> xs[i] >> ys[i];
		}

		vector <comp> ps(4);
		rep(4, i)
		{
			ps[i] = comp(xs[i], ys[i]);
		}

		//直線AB,CDに対応するベクトルl,m　回転用のr
		comp l = ps[1]-ps[0], m = ps[3]-ps[2], r = polar(1., PI/180);

		l *= r, m *= r;

		cout << (gaiseki(l, m) == 0 ? "YES" : "NO") << endl;

	}

}

int main(void){
	solve();
	//cout << "yui(*-v・)yui" << endl;
	return 0;
}

C++: #include <bits/stdc++.h>
using namespace std;
#define rep(i,n) for(int (i)=0;(i)<(n);(i)++)

int n;
double xa, ya, xb, yb, xc, yc, xd, yd, d1, d2;

int main() {
	scanf("%d",&n);
	rep(i,n) {
		scanf("%lf %lf %lf %lf %lf %lf %lf %lf", &xa, &ya, &xb, &yb, &xc, &yc, &xd, &yd);
		if (xa==xb) d1 = 1e200;
		else d1 = (yb-ya)/(xb-xa);
		if (xc==xd) d2 = 1e200;
		else d2 = (yd-yc)/(xd-xc);
		if (d1==d2) printf("YES\n");
		else printf("NO\n");
	}
}

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

int main(){
  int num;
  cin >> num;
  for(int i= 0; i<num ; i++){
    float x[4];
    float y[4];
    cin >> x[0] >> y[0] >> x[1] >> y[1] >> x[2] >> y[2] >> x[3] >> y[3];
    float AB=0;
    float CD=0;
    AB =(y[1]-y[0]) / (x[1]-x[0]);
    CD =(y[3]-y[2]) / (x[3]-x[2]);
    if(CD == -0)CD = 0;
    if(AB == -0)AB = 0;
    if(AB == CD)cout << "YES" << endl;
    else cout << "NO" << endl;
  }
  return 0;
}

C++: 
#include<bits/stdc++.h>
using namespace std;
typedef long long ll;

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

C++: #include <bits/stdc++.h>
#include <vector>
using namespace std;
double x_1, y_1, x_2, y_2, x_3, y_3, x_4, y_4, a, b, x;
double gradien(double x_1, double y_1, double x_2, double_y2){
	double m = 0;
	m = (y_1 - y_2)/(x_1 - x_2);
	
	return m;
}
int main(){
	cin >> x;
	for(int i = 1; i <= x; i++){
		cin >> x_1 >> y_1 >> x_2 >> y_2 >> x_3 >> y_3 >> x_4 >> y_4;
		a = gradien(x_1, y_1, x_2, y_2);
		b = gradien(x_3, y_3, x_4, y_4);
		if(a == b){
			cout << "YES\n";
		}
		else{
			cout << "NO\n";
		}
	}
	return 0;
}

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

using namespace std;

int main() {
    int n;
    cin >> n;
    while (n--) {
        double x1,x2,x3,x4,y1,y2,y3,y4,p1,p2;
        cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3 >> x4 >> y4;
        if (x2 - x1  == 0 || x4 - x3 == 0) {
            cout << "No" << endl;
            continue;
        }
        p1 = (y2 - y1) / (x2 - x1);
        p2 = (y4 - y3) / (x4 - x3);
        if(p1 == p2)
            cout << "YES" << endl;
        else
            cout << "NO" << endl;
    }
    
    return 0;
}