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 <iostream>
#include <cmath>
using namespace std;
int main(void){
    int n;
    double x1, y1, x2, y2, x3, y3, x4, y4;
    cin >> n;
    for(int i=0; i<n; i++){
    cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3 >> x4 >> y4;
        if(x2==x1 && x4==x3)   cout << "YES" <<endl;
        else{
            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(){
  int n;
  cin >>n;
  while(n--){
    double a[2],b[2],c[2],d[2];
    cin >>a[0]>>a[1]>>b[0]>>b[1]>>c[0]>>c[1]>>d[0]>>d[1];
    b[0]-=a[0];
    b[1]-=a[1];
    d[0]-=c[0];
    d[1]-=c[1];
    if(b[0]*d[1] == b[1]*d[0]){cout <<"YES"<<endl;}
    else{cout <<"NO"<<endl;}
  }
  return 0;
}

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

using namespace std;

#define error 1e-10

int main(){
	double x[5],y[5];
	int n;
	double parallel1,parallel2;

	cin>>n;
	for(int i=0;i<n;i++){
		for(int j=1;j<5;j++)
			cin>> x[j]>> y[j];
		parallel1=(y[2]-y[1])/(x[2]-x[1]);
		parallel2=(y[4]-y[3])/(x[4]=x[3]);
		(fabs(parallel1-parallel2)<error) ? cout<<"YES"<<endl : cout<<"NO"<<endl;
	}
	return 0;
}

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

#include <iostream>
#include <algorithm>

#include <bitset>
#include <vector>
#include <set>
#include <map>
#include <unordered_set>
#include <unordered_map>
#include <stack>
#include <queue>
#include <deque>
#include <cstring>
#include <string>
#include <utility>
#include <array>
#include <complex>
#include <valarray>

#include <cassert>
#include <cmath>
#include <functional>
#include <iomanip>
#include <chrono>
#include <random>
#include <numeric>

#include<memory>


using namespace std;
#define int long long

typedef long long ll;
typedef unsigned long long ull;
//typedef unsigned __int128 HASH;
typedef pair<int,int> pii;
typedef pair<ll, ll> pll;
typedef pair<ull, ull> pullull;
typedef pair<ll,int> plli;
typedef pair<double,int> pdi;
typedef pair<long double, int> pdbi;
typedef pair<int,pii> pipii;
typedef pair<ll,pll> plpll;
typedef vector<int> vi;
typedef vector<ll> vll;
typedef vector<vi> vvi;
typedef vector<vvi> vvvi;
typedef vector<pii> vpii;
typedef vector<vector<int>> mat;

#define rep(i,n) for (int i=0;i<(n);i++)
#define rep2(i,a,b) for (int i=(a);i<(b);i++)
#define rrep(i,n) for (int i=(n);i>0;i--)
#define rrep2(i,a,b) for (int i=(a);i>b;i--)
#define pb push_back
#define fi first
#define se second
#define all(a) (a).begin(),(a).end()
#define rall(a) (a).rbegin(),(a).rend()

const ll hmod1 = 999999937;
const ll hmod2 = 1000000000 + 9;
const int INF = 1<<30;
const ll INFLL = 1LL<<62;
const long double EPS = 1e-12;
const ll mod = 1000000000 + 7;
//const ll mod = 998244353;
const int dx4[4] = {1, 0, -1, 0};
const int dy4[4] = {0, 1, 0, -1};
const int dx8[8] = {1, 1, 1, 0, 0, -1, -1, -1};
const int dy8[8] = {0, 1, -1, 1, -1, 0, 1, -1};
const long double pi = 3.141592653589793;

#define addm(X, Y) (X) = ((X) + ((Y) % mod) + mod) % mod
#define inside(y, x, h, w) (0 <= (y) && (y) < (h) && 0 <= (x) && (x) < (w)) ? true : false

//debug
#define DEBUG

#define DUMPOUT cout

#ifdef DEBUG
#define dump(...) DUMPOUT<<#__VA_ARGS__<<" :["<<__FUNCTION__<<":"<<__LINE__<<"]"<<endl; DUMPOUT<<"    "; dump_func(__VA_ARGS__)
#else
#define dump(...)
#endif

void dump_func() {DUMPOUT << endl;};

template <class Head, class... Tail> void dump_func(Head&& head, Tail&&... tail) {
    DUMPOUT << head;
    if (sizeof...(Tail) == 0) DUMPOUT << " ";
    else DUMPOUT << ", ";
    dump_func(std::move(tail)...);
}

//ostream
template<typename T> ostream& operator << (ostream& os, vector<T>& vec) {
    os << "["; for (int i = 0; i<vec.size(); i++) os << vec[i] << (i + 1 == vec.size() ? "" : ", "); os << "]";
    return os;
}

template<typename T, typename U> ostream& operator << (ostream& os, pair<T, U>& pair_var) {
    os << "(" << pair_var.first << ", " << pair_var.second << ")";
    return os;
}

template<typename T, typename U> ostream& operator << (ostream& os, map<T, U>& map_var) {
    os << "[";
    for (auto itr = map_var.begin(); itr != map_var.end(); itr++) {
        os << "(" << itr->first << ", " << itr->second << ")"; itr++;  if(itr != map_var.end()) os << ", "; itr--;
    }
    os << "]";
    return os;
}

template<typename T> ostream& operator << (ostream& os, set<T>& set_var) {
    os << "[";
    for (auto itr = set_var.begin(); itr != set_var.end(); itr++) {
        os << *itr; ++itr; if(itr != set_var.end()) os << ", "; itr--;
    }
    os << "]";
    return os;
}

int n;

signed main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  cin >> n;
  rep(_, n) {
    double x1, y1, x2, y2, x3, y3, x4, y4;
    cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3 >> x4 >> y4;
    double a = x2 - x1;
    double b = y2 - y1;
    double c = x4 - x3;
    double d = y4 - y3;
    if (abs(a * d - b * c) < EPS) {
      cout << "YES" << endl;
    }
    else {
      cout << "NO" << endl;
    }
  }
}

C++: #include <iostream>
#include <utility>

using namespace std;

struct Point {
    double x, y;
};

int main() {
    int n;
    cin >> n;
    while (n--) {
        Point p[4];
        for (int i = 0; i < 4; i++) {
            cin >> p[i].x >> p[i].y;
        }
        double div1 = (p[1].y - p[0].y) / (p[1].x - p[0].x);
        double div2 = (p[3].y - p[2].y) / (p[3].y - p[2].y);
        if (div1 == div2) {
            cout << "YES\n";
        }
        else {
            cout << "NO\n";
        }
    }
    
    return 0;
}

C++: #include <iostream>
#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;
		double a0 = (y2-y1) / (x2-x1), a1 = (y4-y3) / (x4-x3);
		if(a0 == a1) cout<<"YES"<<endl;
        else cout<<"NO"<<endl;
	}
	return 0;
}

C++: #define _USE_MATH_DEFINES
 
#include <iostream>
#include <cstdio>
#include <algorithm>
#include <string>
#include <vector>
#include <math.h>

using namespace std;

int main(){
	int n;
	cin >> n;
	//ごみ
	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;
		
		double t1 = (y1-y2)*(x3-x4);
		double t2 = (y3-y4)*(x1-x2);

		if( (x1-x2)==0 && (x3-x4)==0 ){
			if( (y1-y2)==0 || (y3-y4)==0 ){
				cout << "NO" << endl;
			}else{
				cout << "YES" << endl;
			}
		}else if( (x1-x2)==0 || (x3-x4)==0 ){
			cout << "NO" << endl;
		}else if( t1==t2 ){
			cout << "YES" << endl;
		}else{
			cout << "NO" << endl;
		}
	}

	return 0;
}

C++: 
#include <iostream>
#include <algorithm>
#include <vector>
#include <math.h>
#include <cmath>
#include <utility>
#include <queue>
#include <stdio.h>
#include <string>
#include <map>
#include <stack>
#include <complex>
#define rep(i,n) for(int i=0;i<n;i++)
#define INF 1000000000007
const double eps = 1e-11;
using namespace std;
typedef long long int ll;
typedef complex<double> xy;

int main(){
    int n;
    cin >> n;
    for (int j=0; j<n; j++) {
        vector<xy> inp;
        double x,y,z,w;
        for (int i=0; i<2; i++) {
            cin >> x >> y >> z >> w;
            inp.push_back(xy(x-z,y-w));
        }
        double ans;
        ans =((conj(inp[0])*inp[1]).imag());
        bool b = abs(ans) < eps;
        cout << (b ? "YES" : "NO")<<endl;
    }
}