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)
		}
		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>

int main() {
	int n;
	std::cin >> n;
	for (int i = 0; i < n; i++) {
		long double x1, y1, x2, y2, x3, y3, x4, y4, ax, ay, bx, by;
		std::cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3 >> x4 >> y4;
		ax = x1 - x2; ay = y1 - y2;
		bx = x3 - x4; by = y3 - y4;
		if ((ax / ay) == (bx / by)) {
			std::cout << "YES" << std::endl;
		}
		else {
			std::cout << "NO" << std::endl;
		}
	}

	return 0;
}

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


int main(){
	int n;
	pair<double, double> a, b, c, d;
	
	cin >> n;
	
	for(int i = 0;i < n;i++){
		cin >> a.first >> a.second;
		cin >> b.first >> b.second;
		cin >> c.first >> c.second;
		cin >> d.first >> d.second;
		
		double x1 = b.first - a. first;
		double y1 = b.second - a.second;
		double x2 = d.first - c.first;
		double y2 = d.second - c.second;
		
		cout << x1 << ' ' << y1 << ' ' << x2 << ' ' << y2 << endl;
		
		cout << atan2(y1, x1) << ' ' << atan2(y2, x2) << endl;
		
		if(abs((atan2(y1, x1) - atan2(y2, x2))) < 0.0000001) cout << "YES" << endl;
		else cout << "NO" << endl;
	}
	
	return 0;
}

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



int main(){

  int num;
  cin >> num;

  double x[4], y[4];
  double xx, yy;
  int i = 0;

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

 
  return 0;
}

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

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

int main(){
	int n;
	double a1,a2,b1,b2,c1,c2,d1,d2;
	cin>>n;
	for(int i=0;i<n;i++){
		cin>>a1>>a2>>b1>>b2>>c1>>c2>>d1>>d2;
		if(((a2-b2)/(a1-b1))==((c2-d2)/(c1-d1)))cout<<"YES"<<endl;
		else cout<<"NO"<<endl;
	}

}

C++: #include<iostream>
#include<cmath>
#include<string>
#define loop(i,a,b) for(int i=a;i<b;i++)
#define rep(i,a) loop(i,0,a)
using namespace std;

int main(){
	double x[4],y[4],n;
	string ans="";
	cin>>n;
	rep(i,n){
		rep(i,4)cin>>x[i]>>y[i];
		double vx=x[1]-x[0],vy=y[1]-y[0],ux=x[3]-x[2],uy=y[3]-y[2];
		double la=sqrt(vx*vx+vy*vy),lb=sqrt(ux*ux+uy*uy);
		if(abs(abs(vx*ux+vy*uy)-la*lb)<1e-4)ans="YES";
		else ans="NO";
		if(vx==0 && ux==0)ans="YES";
		if(vy==0 && uy==0)ans="YES";
		cout<<ans<<endl;
	}
	return 0;
}

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;
	}
}