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 <string>
#include <algorithm>
#include <cstdio>
#include <stack>
#include <queue>
#include <list>
#include <cstdlib>
#include <cmath>
#define _USE_MATH_DEFINES

using namespace std;

int main() {

	int n, j = 0;
	double x[4] = { 0 }, y[4] = { 0 };
	double slope1, slope2;
	int ans[100] = { 0 };

	cin >> n;

	while (n) {

		for (int i = 0; i < 4; i++) {
			cin >> x[i] >> y[i];
		}

		slope1 = (y[1] - y[0]) / (x[1] - x[0]);
		slope2 = (y[3] - y[2]) / (x[3] - x[2]);

		if (slope1 == slope2) {
			ans[j] = 1;
		}
		else
			ans[j] = 0;
		n--;
		j++;
	}

	for (int k = 0; k < j; k++) {
		if (ans[k] == 1) {
			cout << "YES" << endl;
		}
		else
			cout << "NO" << endl;
	}

	return 0;
}

C++: #include <cassert>
#include <cmath>
#include <iostream>
#include <vector>
#include <iomanip>

struct Point
{
    float x;
    float y;
};
struct Line
{
    Line(const Point& a, const Point& b)
    : p1(a), p2(b) {}
    Point p1;
    Point p2;
};
double GetSlope(const Line& line)
{
    return
    (line.p2.y - line.p1.y) /
    (line.p2.x - line.p1.x);
}
bool IsParallel(const Line& l1, const Line&l2)
{
    // std::cerr << std::fixed << std::setprecision(100) << GetSlope(l1) << std::endl;
    // std::cerr << std::fixed << std::setprecision(100) << GetSlope(l2) << std::endl;
    // return GetSlope(l1) == GetSlope(l2);
    const double gosa = 0;
    double Slope1 = GetSlope(l1);
    double Slope2 = GetSlope(l2);
    return std::fabs(Slope1 - Slope2) < gosa;
}
bool IsParallel(const std::vector<Point>& points)
{
    assert(points.size() == 4);
    return IsParallel(Line(points[0], points[1]), Line(points[2], points[3]));
}
std::vector<Point> Input(std::istream& is)
{
    std::vector<Point> points(4);
    for(int i = 0; i < points.size(); ++i){
        is >> points[i].x >> points[i].y;
    }
    return points;
}
int main()
{
    int n;
    std::cin >> n;
    for(int i = 0; i < n; ++i) {
        std::vector<Point> p = Input(std::cin);
        std::cout << (IsParallel(p) ? "YES" : "NO") << std::endl; 
        // if((p[1].y-p[0].y)/(p[1].x-p[0].x) == (p[3].y-p[2].y)/(p[3].x-p[2].x)){
        //     std::cout << "YES";
        // }else{
        //     std::cout << "NO";
        // }
        // std::cout << std::endl;
    }
    return 0;
}

C++: #include <iostream>
#include <cmath>
using namespace std;
int main(){
  double x1,x2,x3,x4;
  double y1,y2,y3,y4;
  int n;
  cin >> n;
  while(n--){
    cin >> x1>> y1 >> x2 >> y2 >> x3 >> y3 >> x4 >> y4;
  if(fabs((x2 - x1)*(x4 - x3) - (y2 - y1)*(y4 - y3)) < 10e-10) cout << "YES"<<endl;
  else cout << "NO" << endl;
}
  return 0;
}

C++: #include <cstdio>
#include <cstring>
#include <algorithm>
#include <string>
#include <cmath>
#include <stack>
#include <vector>
#include <iostream>


using namespace std;

int main()
{
	int n;
	scanf("%d", &n);
	for (int i = 0; i < n; ++i)
	{
		double x[4], y[4];
		for (int j = 0; j < 4; ++j)
			scanf("%lf %lf", x+j, y+j);
		if (abs(atan((y[1]-y[0])/(x[1]-x[0])) - atan((y[3]-y[2])/(x[3]-x[2]))) < 3.14/100000)
			puts("YES");
		else
			puts("NO");
	}

	return 0;
}

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

const double EPS = 1E-9;

inline bool cmp(double x, double y){
  return abs(x-y) < EPS;
}

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;
    if(cmp(((y2-y1)*(x4-x3)), ((y4-y3)*(x2-x1)))) cout << "YES" << endl;
    else cout<<"NO"<<endl;
  }
  return 0;
}

C++: #include <iostream>

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

    while (n--) {
        double x1, y1, x2, y2, x3, y3, x4, y4;
        std::cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3 >> x4 >> y4;

        std::cout << (((x1 - x2) * (y3 - y4) == (y1 - y2) * (x3 - x4))?
                     "YES": "NO") << std::endl;
    }

    return 0;
}

C++: #include <stdio.h>
int main() {int N;double x1,x2,x3,x4,y1,y2,y3,y4;scanf("%d",&N);while (N--) {scanf("%lf %lf %lf %lf %lf %lf %lf %lf",&x1,&y1,&x2,&y2,&x3,&y3,&x4,&y4);double A1=y2-y1,A2=y4-y3,B1=x1-x2,B2=x3-x4;if (A1*B2==A2*B1) puts("YES"); else puts("NO");}return 0;}

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

int main()
{
	int N; cin >> N;
	while(N--)
	{
		double x1,x2,x3,x4,y1,y2,y3,y4;
		cin >> x1 >> y1 >> x2 >> y2;
		cin >> x3 >> y3 >> x4 >> y4;

		x2 -= x1; y2 -= y1;
		x4 -= x3; y4 -= y3;

		cout << ( fabs(x2*y4-x4*y2)<1e-5 ? "YES" : "NO") << endl;

	}
}

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

struct vec{
	double x;
	double y;
};

int main(){
	int n;
	cin>>n;
	double x[4],y[4];//0:A 1:B 2:C 3:D
	rep(i,n){
		rep(j,4)cin>>x[j]>>y[j];
		vec ABvec;
		vec CDvec;
		ABvec.x=x[1]-x[0];
		ABvec.y=y[1]-y[0];
		CDvec.x=x[3]-x[2];
		CDvec.y=y[3]-y[2];
		double elmeet=ABvec.x*CDvec.x+ABvec.y*CDvec.y;
		double ABnolme=sqrt(pow(ABvec.x,2)+pow(ABvec.y,2));
		double CDnolme=sqrt(pow(CDvec.x,2)+pow(CDvec.y,2));
		if(abs(ABnolme*CDnolme-elmeet)<10e-7)cout<<"YES"<<endl;
		else cout<<"NO"<<endl;
	}
	return 0;
}

C++: #include<iostream>
#include<cmath>
//#define e 0.00000000001
using namespace std;
int main(){
  double a[2],b[2],c[2],d[2];
  double e = 0.000001;
  int n;
  cin >> n;
  for(int i=0;i<n;i++){
    cin >> a[0] >> a[1] >> b[0] >> b[1] >> c[0] >> c[1] >> d[0] >> d[1];
    if(abs((b[1]-a[1])*(d[0]-c[0])-(d[1]-c[1])*(b[0]-a[0]))<e) cout << "YES" << endl;
    else cout << "NO" << endl;
  }   
  return 0;
}

C++: #include <iostream>
using namespace std;
int main(){
    int n;
    cin >> n;
    double x1,x2,x3,x4,y1,y2,y3,y4;
    for(int i=0;i<n;i++){
        cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3 >> x4 >> y4;
        if(x4-x3==0 || x2-x1==0){
            if(x4-x3==0 && x2-x1==0)
                cout << "YES" << endl;
            else
                cout << "NO" << endl;
        }
        else{
            double p=(y2-y1)/(x2-x1);
            double q=(y4-y3)/(x4-x3);
            if(p==q)
                cout << "YES" << endl;
            else
                cout << "NO" << endl;
        }
    }

}

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

typedef complex<double> P;

// e·éë·Ã
#define EPS (1e-10)
// 2ÂÌXJ[ªµ¢©Ç¤©
#define EQ(a,b) (abs((a)-(b)) < EPS)

// OÏ (cross product) : a~b = |a||b|sin¦
double cross(P a, P b) {
  return (a.real() * b.imag() - a.imag() * b.real());
}

// 2¼üÌ½s»è : a//b <=> cross(a, b) = 0
int is_parallel(P a1, P a2, P b1, P b2) {
  return EQ( cross(a1-a2, b1-b2), 0.0 );
}

int main(){
	int n;
	double x1,x2,x3,x4,y1,y2,y3,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( is_parallel(a,b,c,d) ){
			cout << "YES" << endl;
		}else{
			cout << "NO" << endl;
		}
	}

	return 0;
}

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

#define PI 3.14159265358979323

using namespace std;

int main() {
	int n;
	double x1, y1, x2, y2, x3, y3, x4, y4;
	double deg1, deg2;
	double rad1, rad2;
	cin >> n;
	for (int i = 0; i < n; i++) {
		cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3 >> x4 >> y4;
		if (x1 > x2) {
			swap(x1, x2);
			swap(y1, y2);
		}
		if (x3 > x4) {
			swap(x3, x4);
			swap(y3, y4);
		}
		rad1 = atan2(y2 - y1, x2 - x1);
		rad2 = atan2(y4 - y3, x4 - x3);
		deg1 = rad1 * 180 / PI;
		deg2 = rad2 * 180 / PI;
		if (deg1 == deg2)
			cout << "YES" << endl;
		else
			cout << "NO" << endl;
	}

	return 0;
}