Go: package main

import (
        "fmt"
        "math/big"
)

func main() {
        var x1, x2, x3, x4, y1, y2, y3, y4 big.Float
        var n int
        fmt.Scanf("%d", &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)
                var lx, ly, mx, my, mul1, mul2, sub, zz big.Float
                lx.Sub(&x2, &x1)
                ly.Sub(&y2, &y1)
                mx.Sub(&x4, &x3)
                my.Sub(&y4, &y3)
                mul1.Mul(&lx, &my)
                mul2.Mul(&ly, &mx)
                sub.Sub(&mul1, &mul2)
                zz.SetFloat64(0.0)

                if sub.Cmp(&zz) == 0 {
                        fmt.Println("YES")
                } else {
                        fmt.Println("NO")
                }
        }
}

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

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

C++: #include<iostream>
using namespace std;
int main(){
  int n;
  double x[4],y[4];
  
  cin >> n;
  
  while(n--){
    for(int i=0;i<4;i++){
      cin >> x[i] >> y[i];
    }
    
    double a = (y[0]-y[1])/(x[0]-x[1]);
    double b = (y[2]-y[3])/(y[2]-y[3]);
    
    if(a == b) cout << "YES" << endl;
    else cout << "NO" << endl;
  }
  
  return(0);
}

C++: #include<iostream>
#include<cstdio>
#include<algorithm>
#include<cmath>
#define EPS 0.00000000001
using namespace std;

int main()
{
  double x1,x2,x3,x4,y1,y2,y3,y4;
  double dx1,dy1,dx2,dy2;
  int n;
  int i;

  cin>>n;
  for(i=0;i<n;i++){
    cin>>x1>>y1>>x2>>y2>>x3>>y3>>x4>>y4;
    dx1=x2-x1;
    dy1=y2-y1;
    dx2=x4-x3;
    dy2=y4-y3;

    if(abs(dx1)<EPS && abs(dx2)<EPS)
      cout<<"YES"<<endl;
    else if(abs(dx1)<EPS || abs(dx2)<EPS)
      cout<<"NO"<<endl;
    else if(abs(dy1/dx1-dy2/dx2) < EPS)
      cout<<"YES"<<endl;
    else
      cout<<"NO"<<endl;
  }
}

C++: #include <iostream>
#include <fstream>
#include <cstring>
#include <cstdlib>
#include <cstdio>
#include <vector>
#include <algorithm>
#include <queue>
#include <stack>
#include <list>
#include <utility>
#include <cmath>
#include <sstream>
#define F first
#define S second
#define FOR(i,n) for(int i=0;i<(int)n;i++)
#define FORI(i,k,n) for(int i=k;i<(int)n;i++)
using namespace std;
int main() {
	int n;
	cin >> n;
	for (int l = 0; l < n; l++) {
		double x[4], y[4],v[2];
		for (int i = 0; i < 4; i++)cin >> x[i] >> y[i];
		v[0]=(y[1] - y[0]) / (x[1] - x[0]);
		v[1]=(y[3] - y[2]) / (x[3] - x[2]);
		if (x[0] == x[1] && x[2] == x[3]) {
			cout << "YES" << endl;
			continue;
		} 
		else if(x[0]==x[1]||x[2]==x[3])
		{
			cout<<"NO"<<endl;
			continue;
		}
		else if (v[0]==v[1]) {
			cout << "YES" << endl;	
			continue;
		}
		cout << "NO" << endl;
		continue;
	}
	return 0;
}

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

int main(void){
    int n;
    cin >> n;
    rep(itn j = 0,j< n,j++) {

    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(det == 0) 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>
#define INF 10000000
using namespace std;
int main(){
  int n;
  double lI,lII,x1,y1,x2,y2,x3,y3,x4,y4;
  cin >> n;
  while(n--){
    cin>>x1>>y1>>x2>>y2>>x3>>y3>>x4>>y4;
    lI = lII = INF;
    if(x1!=x2) lI = (y2-y1)/(x2-x1);
    if(x3!=x4) lII = (y4-y3)/(x4-x3);
    cout << (lI==lII ?"YES":"NO") << endl;
  }
}

C++: #include <iostream>

using namespace std;

int main(int argc, char **argv)
{
	int n;
	float xa, ya, xb, yb, xc, yc, xd, yd;
	cin >> n;
	while(n--){
		cin >> xa >> ya >> xb >> yb >> xc >> yc >> xd >> yd;
		if(((yb-ya)==0.0&&(xd-xc)==0.0)||((xb-xa)==0.0&&(yd-yc)==0.0)){
			cout << "YES" << endl;
		} else if((xb-xa)==0.0&&(xd-xc)==0.0){
			cout << "NO" << endl;
		} else if(((yb-ya)/(xb-xa))*((yd-yc)/(xd-xc)) == 1.0){
			cout << "YES" << endl;
		} else {
			cout << "NO" << endl;
		}
	}
	
	return 0;
}

C++: #include <cstdio>
#include <algorithm>
#include <complex>
#include <utility>

using namespace std;

static const double EPS=1e-12;
static const double INF=1e24;
static const double PI=3.141592653589793;

using Point=complex<double>;

namespace std {
    bool operator<(const Point &a, const Point &b) {
        return real(a)!=real(b)? real(a)<real(b) : imag(a)<imag(b);
    }
}

double cross_prod(const Point &a, const Point &b) {
    return imag(conj(a)*b);
}

struct Line: public pair<Point, Point> {
    Line() {}
    Line(const Point &a, const Point &b) {
        first = a;
        second = b;
    }
};

bool is_parallel(const Line &l, const Line &m) {
    return abs(cross_prod(l.second-l.first, m.second-m.first)) < EPS;
}

bool testcase_ends() {
    double x, y;
    if (scanf("%lf %lf", &x, &y) == EOF)
        return 1;

    Point A(x, y);
    scanf("%lf %lf", &x, &y);
    Point B(x, y);
    scanf("%lf %lf", &x, &y);
    Point C(x, y);
    scanf("%lf %lf", &x, &y);
    Point D(x, y);

    printf("%s\n", is_parallel(Line(A, B), Line(C, D))? "YES":"NO");
    return 0;
}

int main() {
    size_t n;
    scanf("%zu", &n);

    for (size_t i=0; i<n; ++i)
        if (testcase_ends())
            return 1;

    return 0;
}

C++: #include <stdio.h>

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

	scanf("%d", &n);

	for (int i = 0; i < n; i++){
		scanf("%lf %lf %lf %lf %lf %lf %lf %lf", &x1, &y1, &x2, &y2, &x3, &y3, &x4, &y4);

		if(((y2-y1) != 0 && (y4-y3) != 0 && (x2-x1)/(y2-y1) == (x4-x3)/(y4-y3)) || ((y2-y1) == 0 && (y4-y3) == 0)) printf("YES\n");
		else printf("NO\n");
	}

	return 0;
}

C++: #include <iostream>
using namespace std;
int main()
{
	int n; cin >> n ;
	double x[4],y[4];
	while(n--)
	{
		for(int i=0;i<4;i++) cin >> x[i] >> y[i] ;
		double vx[2],vy[2];
		for(int i=0;i<2;i++) vx[i]=x[2*i+1]-x[2*i], vy[i]=y[2*i+1]-y[2*i];
		if(vx[0]*vy[1]==vy[0]*vx[1]) cout << "YES" << '\n' ;
		else cout << "NO" << '\n' ;
	}
}

C++: #include <iostream>

using namespace std;

int main(){
  double a,b,c,d,e,f,g,h;
  int t;

  cin >> t;
  while(t--){
    cin >> a >> b >> c >> d >> e >> f >> g >> h;
    
    c -= a,d -= b,g -= e,h -= f;
    
    cout << (c*g-d*h ? "NO" : "YES") << endl;
  }
  
  return 0;
}