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 <string>
#include <vector>
#include<iostream>
#include<cstdio>
#include<cstdlib>
#include<stack>
#include<queue>
#include<cmath>
#include<algorithm>
#include<functional>
#include<list>
#include<deque>
#include<bitset>
#include<set>
#include<map>
#include<cstring>
#include<sstream>
#include<complex>
#include<iomanip>
#include<numeric>
#define X first
#define Y second
#define pb push_back
#define rep(X,Y) for (int (X) = 0;(X) < (Y);++(X))
#define rrep(X,Y) for (int (X) = (Y-1);(X) >=0;--(X))
#define repe(X,Y) for ((X) = 0;(X) < (Y);++(X))
#define peat(X,Y) for (;(X) < (Y);++(X))
#define all(X) (X).begin(),(X).end()
#define rall(X) (X).rbegin(),(X).rend()

using namespace std;
typedef long long ll;
typedef pair<int,int> pii;
typedef pair<ll,ll> pll;
template<class T> ostream& operator<<(ostream &os, const vector<T> &t) {
os<<"{"; rep(i,t.size()) {os<<t[i]<<",";} os<<"}"<<endl; return os;}
template<class S, class T> ostream& operator<<(ostream &os, const pair<S,T> &t) { return os<<"("<<t.first<<","<<t.second<<")";}

int main(){
  ios_base::sync_with_stdio(false);
  cout<<fixed<<setprecision(0);
  int i,j,k,T;
  cin>>T;
  while(T--){
    double xx,yy;
    ll x[4],y[4];
    rep(i,4){
      cin>>xx>>yy;
      x[i]=xx*1e5;
      y[i]=yy*1e5;
    }
    rep(i,2){
      x[i*2+1]-=x[i*2];
      y[i*2+1]-=y[i*2];
    }
    cout<<(x[1]*y[3]==x[3]*y[1]||x[1]*y[3]==-x[3]*y[1]?"YES":"NO")<<endl;
  }
  return 0;
}

C++: #include<cstdio>
#include<iostream>
 
using namespace std;
 
int main(void){
 
    long 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(y2-y1==0){
            if(y4-y3==0) cout<<"YES"<<endl;
            else cout<<"NO"<<endl;
        }
        else if((x2-x1)/(y2-y1)==(x4-x3)/(y4-y3)) cout<<"YES"<<endl;
        else cout<<"NO"<<endl;
    }
 
    return 0;
}

C++: #include <iostream>
#include <string>
#include <cctype>
#include <cmath>
#include <stdio.h>
#include <algorithm>
#include <vector>
#include <map> // pair
#include <set>
#include <string>

#define MAXSIZE 200000
#define PI 3.141592653589
using namespace std;

template<typename T>
T add(T a,T b){
  return a+b;
}

namespace A
{
  int f(void){
    return 1;
  }
}


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

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

    if(((x1-x2)*(y3-y4)-(x3-x4)*(y1-y2))<0.001){
      std::cout << "YES" << '\n';
    }else{
      std::cout << "NO" << '\n';
    }
  }
}

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

int main()
{
	double x1, y1, x2, y2, x3, y3, x4, y4;
	double slope_ab, slope_cd;
	int loop;
	int i = 0;

	cin >> loop;

	while (cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3 >> x4 >> y4){

		++i;

		slope_ab = (y2 - y1) / (x2 - x1);
		slope_cd = (y4 - y3) / (x4 - x3);

		if (slope_ab == slope_cd){
			cout << "YES" << endl;
		}
		else{
			cout << "NO" << endl;
		}

		if (i == loop) break;
	}
}

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

using namespace std;

int main()
{
	int i,num;
	float x[4],y[4];
	cin >> num;

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

		if( abs( (x[0]-x[1])*(y[2]-y[3]) - (y[0]-y[1])*(x[2]-x[3]) ) < 1e-10 ){
			cout << "YES" << endl;
		}
		else{
			cout << "NO" << 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-10 ? "YES" : "NO") << endl;

	}
}

C++: #include <bits/stdc++.h>
#define rep(i, n) for (int i = 0; i < n; i++)
using namespace std;
using ll = long long;
typedef complex<double> xy_t;

const double eps = 1e-11;
double x[4], y[4];
int n;

double dot_product(xy_t a, xy_t b)
{
    return (conj(a) * b).real();
}

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

int main()
{
    cin >> n;
    rep(t, n)
    {
        rep(i, 4) cin >> x[i] >> y[i];
        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;
    }
    return 0;
}

C++: //#define _USE_MATH_DEFINES
//#include <cmath>
#include <iostream>
//#include <stdio.h>
//#include <stdlib.h>
//#include <iomanip>
//#include <vector>
#include <string>
//#include <algorithm>
//#include <functional>
//#include <time.h>
//#include <sstream>
//#include <queue>

using namespace std;


int main(){

	int n;
	double p[8];
	double k1, k2;

	cin >> n;

	while (n--){
		for (int i = 0; i < 8; i++){
			cin >> p[i];
		}

		if (p[0] == p[2] || p[4] == p[6]){
			if (p[0] == p[2] && p[4] == p[6])cout << "YES" << endl;
			else cout << "NO" << endl;
		}
		else{
			k1 = (p[3] - p[1]) / (p[2] - p[0]);
			k2 = (p[7] - p[5]) / (p[6] - p[4]);
			if ((k1 -k2 )* (k1 -k2) < 0.000000001) cout << "YES" << endl;
			else cout << "NO" << endl;
		}


	}


	return 0;
}

C++: #include<stdio.h>
int main(void) {
	int a, b, c, d;
	double e, f, g, h,x1,x2,x3,x4,y1,y2,y3,y4;
	scanf("%d", &a);
	for (int i = 0; i < a; i++)
	{
		scanf("%lf %lf %lf %lf %lf %lf %lf %lf", &x1, &y1, &x2, &y2, &x3, &y3, &x4, &y4);
		e = (y1 - y2) / (x1 - x2);
		f = (y3 - y4) / (x3 - x4);
		if (e==f)
		{
			printf("YES\n");
		}
		else
		{
			printf("NO\n");
		}
	}
	return 0;
}

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

C++: #include <iostream>
using namespace std;
#define rep(i,n) for(int i=0;i<n;i++)
 
int main(void)
{
    double x1,x2,x3,x4,y1,y2,y3,y4,p1,p2;
    int n;
     
    cin >> n;
     
    rep(i,n)
    {
        cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3 >> x4 >> y4;
        p1 = (y2-y1)/(x2-x1);
        p2 = (y4-y3)/(x4-x3);
        if(p1 == p2)
            cout << "YES" << endl;
        else
            cout << "NO" << endl;
    }
     
    return 0;
}

C++: #include <iostream>
#include <stdio.h>
#define REP(i,n) for(int i = 0; i < (n); ++i)
using namespace std;

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

C++: #include<bits/stdc++.h>
using namespace std;
 
signed main(){
int a;
cin>>a;
double c[8];
for(int b=0;b<a;b++){
    for(int d=0;d<8;d++)scanf("%lf",&c[d]);
    if(abs((c[1]-c[0])*(c[7]-c[6])-(c[3]-c[2])*(c[5]-c[4]))<0.000001)printf("YES\n");
    else printf("NO\n");
    }
}