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>

using namespace std;

bool get_line( double , double , double , double , double& , double& );

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

	double a1, a2;
	double b1, b2;

	bool result1 , result2;

	int num;

	cin >> num;

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

		result1 = get_line( x1 , x2 , y1 , y2 , a1 , b1 );
		result2 = get_line( x3 , x4 , y3 , y4 , a2 , b2 );

		if( result1 == false && result2 == false ) cout << "YES" << endl;

		else
		{
			if( a1 == a2 ) cout << "YES" << endl;
			else cout << "NO" << endl;
		}
	}

	return 0;
}

bool get_line( double x1 , double x2 , double y1 , double y2 , double& a , double& b )
{
	// if( x1 - x2 >= -0.0000001 && x1 - x2 <= 0.0000001 ) return false;

	if( x1 == x2 ) return false;
	else
	{
		a = ( y1 - y2 ) / ( x1 - x2 );
		b = y1 - a * x1;
		return true;
	}
}

C++: #include <stdio.h>
#include <cctype>
#include <limits.h>
#include <math.h>
#include <complex>
#include <bitset>
#include <vector>
#include <map>
#include <set>
#include <stack>
#include <queue>
#include <cstring>
#include <string>
#include <sstream>
#include <algorithm>
#include <iomanip>
#include <iostream>

#define VARIABLE(x) cerr << #x << "=" << x << endl
#define BINARY(x) static_cast<bitset<16> >(x)
#define rep(i,n) for(int i=0;i<(int)(n);i++)
#define REP(i,m,n) for (int i=m;i<(int)(n);i++)
#define if_range(x, y, w, h) if (0<=(int)(x) && (int)(x)<(int)(w) && 0<=(int)(y) && (int)(y)<(int)(h))
#define ALL(a) (a).begin(),(a).end()
const int INF = 1e9;
const double PI = 3.14159;
int dx[4]={0, 1, 0, -1}, dy[4]={-1, 0, 1, 0};
using namespace std;
typedef long long ll;
//typedef pair<int, int> P;

/* struct P {
	int x, y, n;
	P(int n, int x, int y):n(n), x(x), y(y){}
	P(){}
}; */


/** 幾何ライブラリ **/
// to do : 1183

#include <complex>

#define X real()
#define Y imag()

const double EPS = 1e-11;

typedef double D;
typedef complex<D> P; // Point

// 共役複素数 conj(a);

// 符号
int sig(D a, D b=0) { return a < b - EPS ? -1 : (a > b + EPS ? 1 : 0); }

// 内積
D dot_product(P a, P b) { return (conj(a)*b).real(); }

// 外積
D cross_product(P a, P b) { return (conj(a)*b).imag(); }

/** Problem0021 : Parallelism **/
int main()
{
	int N; cin>>N;
	rep(i, N) {
		P a, b;
		D x1, y1, x2, y2;
		cin>>x1>>y1>>x2>>y2;
		a.X = (x2 - x1);
		a.Y = (y2 - y1);
		
		cin>>x1>>y1>>x2>>y2;
		b.X = (x2 - x1);
		b.Y = (y2 - y1);
		
		if (sig(cross_product(a, b))==0) {
			cout << "YES" << endl;
		} else {
			cout << "NO" << endl;
		}
	}
}

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

#define FOR(i,b,n) for(int i=b;i<n;i++)
#define RFOR(i,b,n) for(int i=n-1;i>=b;i--)
#define CLR(mat) memset(mat, 0, sizeof(mat))
#define NCLR(mat) memset(mat, -1, sizeof(mat))

#define EPS (1e-10)
#define EQ(a,b) (abs((a)-(b)) < EPS)

using namespace std;

typedef complex<double> point;
typedef pair<int, int> paii;
typedef long long ll;

double cross(point va, point vb){
	return real(va) * imag(vb) - imag(va) * real(vb);
}

int is_parallel(point a1, point a2, point b1, point b2) {
  return EQ( cross(a1-a2, b1-b2), 0.0 );
}

int main()
{
	int n;
	cin >> n;
	
	FOR(i, 0, n)
	{
		double x, y;
		point pt[4];
		
		FOR(j, 0, 4)
		{
			cin >> x >> y;
			pt[j] = point(x, y);
		}
		
		if( is_parallel(pt[0], pt[1], pt[2], pt[3]) )
			cout << "YES" << endl;
		else
			cout << "NO" << endl;
	}
	
	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;
		if((y1-y2)/(x1-x2) == (y3-y4)/(x3-x4)) cout << "YES" << endl;
		else cout << "NO" << endl;
	}
    return 0;
}

C++: #include <bits/stdc++.h>
#include <vector>
using namespace std;
double x_1, y_1, x_2, y_2, x_3, y_3, x_4, y_4, a, b, x;
double gradien(double x_1, double y_1, double x_2, double y_2){
	double m = 0;
	m = (y_1 - y_2)/(x_1 - x_2);
	
	return m;
}
int main(){
	cin >> x;
	for(int i = 1; i <= x; i++){
		cin >> x_1 >> y_1 >> x_2 >> y_2 >> x_3 >> y_3 >> x_4 >> y_4;
		a = gradien(x_1, y_1, x_2, y_2);
		b = gradien(x_3, y_3, x_4, y_4);
		if(a == b){
			cout << "YES\n";
		}
		else{
			cout << "NO\n";
		}
	}
	return 0;
}

C++: #include <algorithm>
#include <cmath>
#include <climits>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <fstream>
#include <iostream>
#include <list>
#include <map>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <vector>
#include <cassert>
#include <functional>

using namespace std;

#define LOG(...) printf(__VA_ARGS__)
//#define LOG(...)
#define FOR(i,a,b) for(int i=(int)(a);i<(int)(b);++i)
#define REP(i,n) for(int i=0;i<(int)(n);++i)
#define ALL(a) (a).begin(),(a).end()
#define RALL(a) (a).rbegin(),(a).rend()
#define EXIST(s,e) ((s).find(e)!=(s).end())
#define SORT(c) sort((c).begin(),(c).end())
#define RSORT(c) sort((c).rbegin(),(c).rend())
#define CLR(a) memset((a), 0 ,sizeof(a))

typedef long long ll;
typedef unsigned long long ull;
typedef vector<bool> vb;
typedef vector<int> vi;
typedef vector<ll> vll;
typedef vector<vb> vvb;
typedef vector<vi> vvi;
typedef vector<vll> vvll;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;

const int dx[] = { -1, 0, 1, 0 }; const int dy[] = { 0, 1, 0, -1 };

int main() {
	int n;
	cin>>n;
	while (n--){
		vector<pair<double, double>> pos(4);
		REP(i, 4){
			cin >> pos[i].first >> pos[i].second;
		}

		if (abs((pos[1].first - pos[0].first)*(pos[3].second - pos[2].second) - (pos[1].second - pos[0].second)*(pos[3].first - pos[2].first))<=0.000001)
			cout << "YES"<<endl;
		else
			cout << "NO"<<endl;
		
	}
}

C++: #include <iostream>
#include <complex>
#include <cmath>

using namespace std;

typedef complex<double> xy_t;

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

const double eps = 1e-11;

int main() {
	int n;
	double x[4];
	double y[4];
	cin >> n;
	for (int i = 0; i < n; i++) {
		for (int j = 0; j < 4; j++) {
			cin >> x[j] >> y[j];
		}
		bool result = abs(cross_product(xy_t(x[0], y[0]) - xy_t(x[1], y[1]), 
						xy_t(x[2], y[2]) - xy_t(x[3], y[3]))) < eps;
		cout << (result ? "YES" : "NO") << endl;
	}
}

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

using namespace std;

int main() {
	int n;
	double x1, y1, x2, y2, x3, y3, x4, y4;
	double deg1, deg2;
	cin >> n;
	for (int i = 0; i < n; i++) {
		cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3 >> x4 >> y4;
		deg1 = atan2(x2 - x1, y2 - y1);
		deg2 = atan2(x4 - x3, y4 - y3);
		if (deg1 == deg2)
			cout << "YES" << endl;
		else
			cout << "NO" << endl;
	}

	return 0;
}