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++: import java.util.Scanner;

public class Main{
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        double x[] = new double[4];
        double y[] = new double[4];
      for(;n>0;n--){
            for(int i=0; i<4; i++){
                x[i] = sc.nextDouble();
                y[i] = sc.nextDouble();
            }
            double ab = (y[1]-y[0])/(x[1]-x[0]);
            double cd = (y[3]-y[2])/(x[3]-x[2]);
            System.out.println(ab==cd ? "YES" : "NO");
        }
    }
}

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

#define EPS (1e-10)
#define EQ(a,b) (abs((a)-(b)) < EPS)
#define EQV(a,b) ( EQ( (a).real() , (b).real() ) && EQ( (a).imag(), (b).imag() ) )

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

C++: #include <iostream>
using namespace std;
void judge(double,double,double,double,double,double,double,double);

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

	cin >> n;
	for (int i = 0;i < n;i++) {
		cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3 >> x4 >> y4;
		judge(x1, y1, x2, y2, x3, y3, x4, y4);
	}
	
	return 0;
}

void judge(double x1, double y1, double x2, double y2, double x3, double y3, double x4, double y4) {
	double abx = x2 - x1;
	double aby = y2 - y1;
	double cdx = x4 - x3;
	double cdy = y4 - y3;
	if (abx*cdy - aby*cdx == 0) {
		cout << "YES" << endl;
	}
	else {
		cout << "NO" << endl;
	}

	return;

}

C++: #include <stdio.h>

int main()
{
	int n;
	double x[4],y[4];
	double a,b;
	scanf("%d",&n);
	for(int i = 0; i < n; i++){
		for(int j = 0; j < 4; j++){
			scanf("%lf %lf",&x[j],&y[j]);
		}
		a = (y[1] - y[0]) * (x[3] - x[2]);
		b = (x[1] - x[0]) * (y[3] - y[2]);
		if(a == b){
			printf("YES\n");
		}else{
			printf("NO\n");
		}
	}
	return 0;
}

C++: #include <bits/stdc++.h>
using namespace std;
#define EPS 1e-10
typedef complex<double> P;

double cross(P a, P b) {
  return (a.real() * b.imag() - a.imag() * b.real());
}

int main() {
  int n;
  double x1, y1, x2, y2, x3, y3, x4, y4;
  cin >> n;
  for (int i = 0; i < n; i++) {
    cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3 >> x4 >> y4;
    P a(x1, y1), b(x2, y2), c(x3, y3), d(x4, y4);
    if (fabs(cross(b - a, d - c)) < EPS) {
      cout << "YES\n";
    } else {
      cout << "NO\n";
    }
  }
}

C++: #include <iostream>
#include <vector>
#include <stack>
#include <queue>
#include <map>
#include <set>
#include <string>
#include <math.h>
#include <functional>
#include <algorithm>

typedef long long Int;
#define REP(i,n) for(int i = 0 ; i < n ; ++i)
using namespace std;

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

	REP(i, n){

		double p[8];
		REP(i, 8){
			cin >> p[i];
		}

		double q1 = (p[3] - p[1]) / (p[2] - p[0]);
		double q2 = (p[7] - p[5]) / (p[6] - p[4]);
		string ans = (q1 == q2) ? "YES" : "NO";


		cout << ans << endl;
	}
	return 0;
}

C++: #include <iostream>
#include<cmath>
#include<algorithm>
#include<string>
#include<map>
#include<vector>
using namespace std;
int main (void) {
    int n;
    double x1,x2,x3,x4,y1,y2,y3,y4;
    cin >> n;
    while(n--){
        cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3 >> x4 >> y4;
        if((x1-x2)*(y3-y4)==(x3-x4)*(y1-y2))    cout << "YES\n";
        else    cout << "NO\n";
    }
    return 0;
}

C++: #include <iostream>
#include <complex>
#include <cmath>
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 task = 0; task < n; task++) {
        xy_t p[4];
        double x[4], y[4], eps = 1e-11;
        for (int i = 0; i < 4; i++) cin >> x[i] >> y[i];

        xy_t a[2];
        a[0] = xy_t(x[0], y[0]) - xy_t(x[1], y[1]);
        a[1] = xy_t(x[2], y[2]) - xy_t(x[3], y[3]);

        bool b = abs(cross_product(a[0], a[1])) / 2.0 < eps;
        cout << (b ? "YES" : "NO") << endl;
    }
}

/*
g++ -std=c++14 pr_prog/_main.cpp
 */

C++: #include<iostream>
#include<string>
#include<vector>
#include<cstdio>
#include<sstream>
#include<algorithm>
#include<cmath>
#include<map>
#include<functional>
#include<queue>
#include<stack>
#include<set>
#include<math.h>
using namespace std;
#define SZ(n)  (int)n.size()
#define dump(x) cerr<<#x<<" = "<<(x)<<endl;
#define debug(x)cerr << #x << " = " << (x) << " (L" << __LINE__ << ")" << " " << __FILE__ << endl;
#define MP make_pair
#define PB push_back
	double EPS=0.00000000001;
	bool same(double a,double b){
		if(fabs(a-b)<=EPS)return 1;
		else return 0;
	}
	
	
	int main(){
	int n;
			double x1,y1,x2,y2,x3,y3,x4,y4,a1,a2;
	cin>>n;
	for(int i=0;i<n;i++){

		cin>>x1>>y1>>x2>>y2>>x3>>y3>>x4>>y4;
		a1=(y1-y2)/(x1-x2);
		a2=(y3-y4)/(x3-x4);
		if(same(a1,a2))cout<<"YES"<<endl;
		else cout<<"NO"<<endl;
	}
	
	return 0;
}

C++: #include <iostream>
#include <complex>
using namespace std;
typedef complex<long double> P;
const long double EPS = 1e-15;
int main(){
	int N;
	cin >> N;
	while(N--){
		P a,b,c,d;
		cin >> a.real() >> a.imag() >> b.real() >> b.imag() >> c.real() >> c.imag() >> d.real() >> d.imag();
		P one = b-a;
		P two = d-c;
		bool f = false;
		if( abs(one.real()) < EPS || abs(two.real()) < EPS ){
			f = abs(one.real()) < EPS && abs(two.real()) < EPS;
		}else{
			f = abs(one.imag()/one.real() - two.imag()/two.real()) < EPS;
		}
		cout << (f?"YES":"NO") << endl;
	}
}

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() {
  int n;
  double x1, y1, x2, y2, x3, y3, x4, y4;
  double d1, d2;
  cin >> n;
  for (int i = 0; i < n; i++) {
    cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3 >> x4 >> y4;
    if (x2 == x1 && x4 == x3 ){
      cout << "NO" << endl;
    } else {
      d1 = (y2 - y1) * (x4 - x3);
      d2 = (y4 - y3) * (x2 - x1);
      if (d1 == d2) {
        cout << "YES" << endl;
      }else{
        cout << "NO" << endl;
      }
    }
  }
}

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

using namespace std;

typedef complex<double> Point;

int main() {
    int N;
    cin >> N;
    while(N--) {
        Point ps[4];
        for(int i = 0; i < 4; ++i) cin >> ps[i].real() >> ps[i].imag();
        Point v1 = ps[1]-ps[0], v2 = ps[3]-ps[2];
        double dot = v1.real()*v2.real() + v1.imag()*v2.imag();
        double n = norm(v1) * norm(v2);
        if(abs(dot*dot - n) < 1e-5) cout << "YES" << endl;
        else cout << "NO" << endl;
    }
    return 0;
}