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[4] == fpoints[6] {
			fmt.Println("YES")
		} else {
			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 <bits/stdc++.h>

#define REP(i, a, b) for (int i = (int)(a); i < (int)(b); ++i)
#define rep(i, n) REP (i, 0, n)
#define mset(a, n) memset(a, n, sizeof(a))

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

C++: #include<stdio.h>

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

	scanf("%d",&n);
	for(int x=0;x<n;x++)
	{
		for(;;)
		{
			scanf("%lf %lf %lf %lf %lf %lf %lf %lf",&x1,&y1,&x2,&y2,&x3,&y3,&x4,&y4);
			if(x1>=100||y1>=100||x2>=100||y2>=100||x3>=100||y3>=100||x4>=100||y4>=100)
			{
			}
			if(x1<=-100||y1<=-100||x2<=-100||y2<=-100||x3<=-100||y3<=-100||x4<=-100||y4<=-100)
			{
			}
			else
			{
				break;
			}
		}

		x2-=x1;
		y2-=y1;
		x1=0;
		y1=0;
		x4-=x3;
		y4-=y3;
		x3=0;
		y3=0;

		p=x2/x4;
		q=y2/y4;

		if(p==q)
		{
			printf("YES\n");
		}
		else
		{
			printf("NO\n");
		}
	}
	return 0;
}

C++: #include<iostream>

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

C++: #include <vector>
#include <map>
#include <set>
#include <stack>
#include <queue>
#include <algorithm>
#include <numeric>
#include <utility>
#include <sstream>
#include <iostream>
#include <iomanip>
#include <cstdio>
#include <cmath>
#include <string>
#include <cstring>
#include <complex>
#include <ctime>
#include <cstdlib>

using namespace std;

inline int to_int(string s) {int v; istringstream sin(s); sin >> v; return v;}
template<class T> inline string to_str(T x) {ostringstream sout; sout << x; return sout.str();}

typedef long long ll;

int main()
{
	int num;
	double sx1, sy1, sx2, sy2;
	double ex1, ey1, ex2, ey2;

	cin >> num;

	for(int i = 0; i < num; i++)
	{
		cin >> sx1 >> sy1 >> sx2 >> sy2 >> ex1 >> ey1 >> ex2 >> ey2;

		if( (sy2-sy1)/(sx2 - sx1) == (ey2-ey1)/(ex2-ex1))
		{
			cout << "YES" << endl;
		}
		else
		{
			cout << "NO" << endl;
		}
	}
}

C++: #include <iostream>

using namespace std;

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

C++: #include<iostream>
using namespace std;
int main() {
	double zhyoxy[4], line1, line2;
	char hantei[2];
	int time;
	cin >> time;
	for (int i = 0; i < time; ++i) {
		for (int i = 0; i < 4; ++i) {
			cin >> zhyoxy[i];
		}
		if (zhyoxy[3] == zhyoxy[1]) {
			hantei[0] = 'y';
		}
		else if (zhyoxy[2] == zhyoxy[0]) {
			hantei[0] = 'x';
		}
		line1 = (zhyoxy[3] - zhyoxy[1]) / (zhyoxy[2] - zhyoxy[0]);
		for (int i = 0; i < 4; ++i) {
			cin >> zhyoxy[i];
		}
		if (zhyoxy[3] == zhyoxy[1]) {
			hantei[1] = 'y';
		}
		else if (zhyoxy[2] == zhyoxy[0]) {
			hantei[1] = 'x';
		}
		line2 = (zhyoxy[3] - zhyoxy[1]) / (zhyoxy[2] - zhyoxy[0]);
		if (line1 == line2||zhyoxy[0]==zhyoxy[1]) {
			cout << "YES" << endl;
		}
		else {
			cout << "NO" << endl;
		}
	}
	return 0;
}

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

using namespace std;

int n;
double x1, x2, x3, x4, y1, y2, y3, y4, slope1, slope2;

double abs(double s) {
    return s < 0 ? -s : s;
}

int main(void) {
    cin >> n;
    while(n--) {
        cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3 >> x4 >> y4;
        slope1 = (y1 - y2) / (x1 - x2);
        slope2 = (y3 - y4) / (x3 - x4);

        if(slope1 == slope2) {
            cout << "YES" << endl;
        } else {
            cout << "NO" << endl;
        }
    }
    return 0;
}

C++: //0021
#include <bits/stdc++.h>
using namespace std;

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

		AB = (y1-y2)/(x1-x2);
		CD = (y3-y4)/(x3-x4);
		if(AB == CD)
			cout<<"YES"<<endl;

		else
			cout<<"NO"<<endl;
	}
	return 0;
}
//0021
#include <bits/stdc++.h>
using namespace std;

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

		AB = (y1-y2)/(x1-x2);
		CD = (y3-y4)/(x3-x4);
		if(AB == CD)
			cout<<"YES"<<endl;

		else
			cout<<"NO"<<endl;
	}
	return 0;
}

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

int n;

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

		//printf("%lf\n", y[0][0] * y[1][1] - y[0][1] * y[1][0]);

		if(std::abs(y[0][0] * y[1][1] - y[0][1] * y[1][0]) < 1e-9){
			printf("YES\n");
		}
		else{
			printf("NO\n");
		}
	}
	return 0;
}

C++: #include <bits/stdc++.h>
using namespace std;
int main(){
  double n,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;
    if(abs(y1-y2)/abs(x1-x2)==abs(y3-y4)/abs(x3-x4)){
      cout << "YES" << endl;
    }else{
      cout << "NO" << endl;
    }
  }
  return 0;
}

C++: #include<iostream>
#include<cmath>
#include<cassert>
#include<cstdlib>
using namespace std;
#define EPS 0.0000001


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


		if (abs((y2 - y1)*(x4 - x3) - (y4 - y3)*(x2 - x1)) < EPS) cout << "YES" << endl;
		else cout << "NO" << endl;
	}
}

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

#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()) )

using namespace std;

typedef complex<double> P;

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