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 <bits/stdc++.h>
using namespace std;

int main() {
	int n; cin >> n;
	while (n--) {
		double x1, y1, x2, y2, x3, y3, x4, y4;
		cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3 >> x4 >> y4;
		// 対角線の中点で判定
		// double cx1 = (x3 - x1) / 2.0;
		// double cy1 = (y3 - y1) / 2.0;
		// double cx2 = (x4 - x2) / 2.0;
		// double cy2 = (y4 - y2) / 2.0;
		// if (cx1 == cx2 && cy1 == cy2) {
		//  cout << "YES" << endl;
		// } else {
		//  cout << "NO" << endl;
		// }
		// 傾きで判定
		if ((y2 - y1) / (x2 - x1) == (y4 - y3) / (x4 - x3)) {
			cout << "YES" << endl;
		} else {
			cout << "NO" << endl;
		}
	}
}

C++: #include<iostream>
#include <complex>
#include <cmath>
#include<cstdio>
using namespace std;
typedef complex<double> xy_t;

xy_t P[110];
double cross_product(xy_t a, xy_t b) { 
    return (conj(a)*b).imag(); 
}

const double eps = 1e-11;
double x[4], y[4];
int N;
int main() {
    cin >> N; // 問題数
    for (int t=0; t<N; ++t) {
        for (int i=0; i<4; ++i) cin >> x[i] >> y[i]; // x0,y0..x3,y3
        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 p = abs(cross_product(a[0],a[1])) < eps;
        cout << (p ? "YES" : "NO") << endl;
    }
}

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

using namespace std;

int main() {
	int k;
	double x1,x2,x3,x4,y1,y2,y3,y4;
	double katamuki1;
	double katamuki2;
	cin >> k;

	for (int i = 0; i < k; i++) {
		cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3 >> x4 >> y4;

		katamuki1 = (y2 - y1) / (x2 - x1);
		katamuki2 = (y4 - y3) / (x4 - x3);

		if (katamuki1 == katamuki2) {
			cout << "YES" << endl;
		} else {
			cout << "NO" << endl;
		}
	}
	return 0;
}

C++: #include <iostream>
#include <complex>
#include <cmath>
#include <algorithm>
#include <string>
#include <utility>
#include <vector>

#define EPS (1e-9)
#define EQ(a,b) (abs((a)-(b)) < EPS)
#define EQV(a,b) ( EQ((a).real(), (b).real()) && EQ((a).imag(), (b).imag()) )

using namespace std;
typedef complex<double> P;

int main(){
  double x1,y1,x2,y2,x3,y3,x4,y4;
  int n;
  P a,b;
  for(cin>>n;n>0;n--){
    cin>>x1>>y1>>x2>>y2>>x3>>y3>>x4>>y4;
    a = P(x2-x1,y2-y1);
    b = P(x4-x3,y4-y3);
    if EQV((a/abs(a)),(b/abs(b)))
      cout << "YES" << endl;
    else
      cout << "NO" << endl;
  }
  return 0;
}

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

#define EPS 1e-7

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 && x3 == x4){
			cout << "YES" << endl;
		}
		else if (x1 == x2 || x3 == x4){
			cout << "NO" << endl;
		}
		else if (abs((y2 - y1) / (x2 - x1) - (y4 - y3) / (x4 - x3)) < EPS){
			cout << "YES" << endl;
		}
		else{
			cout << "NO" << endl;
		}
	}
	return 0;
}

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

int main(){
	int n;
	double x[4], y[4];
	int xi[4], yi[4];
	scanf("%d", &n);
	for(int i = 0; i < n; i++){
		scanf("%lf %lf %lf %lf %lf %lf %lf %lf", &x[0],&y[0],&x[1],&y[1],&x[2],&y[2],&x[3],&y[3]);

		for(int i = 0; i < 4; i++){
			xi[i] = (int)(x[i] * 100000);
			yi[i] = (int)(y[i] * 100000);
		}

		if(xi[0] - xi[1] == 0){
			if(xi[2] - xi[3] == 0){
				printf("YES\n");
			}else{
				printf("NO\n");
			}
		}else{
			if((yi[0] - yi[1])*(xi[2] - xi[3])) == (xi[0] - xi[1])*(yi[2] - yi[3]){
				printf("YES\n");
			}else if((yi[0] - yi[1])*(xi[2] - xi[3])) == - (xi[0] - xi[1])*(yi[2] - yi[3]){
				printf("YES\n");
			}else{
				printf("NO\n");
			}
		}
	}
	return 0;
}

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

C++: #include <bits/stdc++.h>

using namespace std;

int main(){

    int n;
    float p[4][2], ab, cd;

    cin >> n;

    for(int i = 0; i < n; ++i){

        for(int j = 0; j < 4; ++j){
            for(int k = 0; k < 2; ++k){
                cin >> p[j][k];
            }
        }

        ab = (p[0][1] - p[1][1]) / (p[0][0] - p[1][0]);
        cd = (p[2][1] - p[3][1]) / (p[2][0] - p[3][0]);

        if(ab == cd){
            cout << "YES" << endl;
        } else {
            cout << "NO" << endl;
        }

    }

}

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

struct Point2D {
    float x, y;

    Point2D() {}
    Point2D(float ax, float ay) {
        x = ax;
        y = ay;
    }
    Point2D operator -(Point2D &p) {
        return Point2D(x - p.x, y - p.y);
    }
    double slope(Point2D &p) {
        return (p.x != x) ? (p.y - y) / (p.x - x) : numeric_limits<float>::infinity();
    }
    double cross(Point2D &p) { // only z
        return x * p.y - y * p.x;
    }
};

namespace std {
    istream &operator>>(istream &is, Point2D &p) {
        is >> p.x >> p.y;
        return is;
    }

    ostream &operator<<(ostream &os, Point2D &p) {
        os << fixed << p.x << " " << p.y;
        return os;
    }
}

int main()
{
    int n;
    cin >> n;
    for (Point2D a, b, c, d; n-- && cin >> a >> b >> c >> d;)
        cout << ((a.slope(b) == c.slope(d)) ? "YES" : "NO") << endl;

    return 0;
}

C++: #include<iostream>
#include<cmath>
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( fabs((y2-y1)/(x2-x1) -(y4-y3)/(x4-x3))<1e-10 ) 
      cout<<"YES";
    else
      cout<<"NO";
    cout<<endl;
  }
  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 <iostream>

using namespace std;

int main()
{
	int n;
	double x[4], y[4];

	cin >> n;

	for(int i = 0; i < n; i++){
		cin >> x[0] >> y[0] >> x[1] >> y[1] >> x[2] >> y[2] >> x[3] >> y[3];

		if((x[1]-x[0] == 0 && x[3]-x[2] == 0) || (y[1]-y[0] == 0 && y[3]-y[2] == 0))
			cout << "YES" << endl;
		else if(((y[1]-y[0])/(x[1]-x[0])) == ((y[3]-y[2])/(x[3]-x[2])))
			cout << "YES" << endl;
		else 
			cout << "NO" << endl;
	}

	return 0;
}