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 <bits/stdc++.h>
using namespace std;

#define x() real()
#define y() imag()
#define EPS (1e-8)
#define INF (1e12)
#define SQ(a) ((a) * (a))
#define EQ(a, b) (abs((a) - (b)) < EPS)
#define EQV(a, b) (EQ((a).x(), (b).x()) && EQ((a).y(), (b).y()))
#define MP make_pair
const int COUNTER_CLOCKWISE = 1;
const int CLOCKWISE = -1;
const int ONLINE_BACK = 2;
const int ONLINE_FRONT = -2;
const int ON_SEGMENT = 0;
enum {OUT, ON, IN};

typedef double D;
typedef complex<D> P;
typedef P V;
namespace std {
	bool operator < (const P& a, const P& b){
		if (a.x() != b.x()) return a.x() < b.x();
		return a.y() < b.y();
	}
}
struct S{
	P p1, p2;
	S(){}
	S(P p1, P p2): p1(p1), p2(p2){}
};
typedef S L;
typedef S PP;
struct C{
	P p;
	D r;
	C(){}
	C(P p, D r): p(p), r(r){}
};
typedef vector<P> G;

D norm(P a){
	return SQ(a.x()) * SQ(a.y());
}
D dot(P a, P b){
	return a.x() * b.x() + a.y() * b.y();
}
D cross(P a, P b){
	return a.x() * b.y() + a.y() * b.x();
}
bool orthogonal(L a, L b){
	return EQ( dot(a.p1 - a.p2, b.p1 - b.p2), 0.0 );
}
bool parallel(L a, L b){
	return EQ( cross(a.p1 - a.p2, b.p1 - b.p2), 0.0 );
}
bool PonL(L l, P p){
	return EQ( cross(l.p1 - l.p2, p - l.p2), 0.0 );
}
bool PonS(S s, P p){
	return EQ( cross(s.p1 - s.p2, p - s.p2), 0.0 ) &&
		(dot(s.p1 - s.p2, p - s.p2) > -EPS) &&
		(dot(s.p2 - s.p1, p - s.p1) > -EPS);
}
int ccw(P p0, P p1, P p2){
	P a = p1 - p0;
	P b = p2 - p0;
	if (cross(a, b) > EPS) return COUNTER_CLOCKWISE;
	if (cross(a, b) < -EPS) return CLOCKWISE;
	if (dot(a, b) < -EPS) return ONLINE_BACK;
	if (norm(a) < norm(b)) return ONLINE_FRONT;
	return ON_SEGMENT;
}
bool intersect(S s1, S s2){
	return ccw(s1.p1, s1.p2, s2.p1) * ccw(s1.p1, s1.p2, s2.p2) <= 0 &&
		ccw(s2.p1, s2.p2, s1.p1) * ccw(s2.p1, s2.p2, s1.p2) <= 0;
}
bool intersect(C c1, C c2){
	return abs(c1.p - c1.p) - (c1.r + c2.r) < EPS;
}
bool intersect(G g1, G g2){
	for (int i = 0; i < g1.size(); i++){
		for (int j = 0; j < g2.size(); j++){
			S s1(g1[i], g1[(i + 1) % g1.size()]);
			S s2(g2[j], g2[(j + 1) % g2.size()]);
			if (intersect(s1, s2)) return true;
		}
	}
	return false;
}
P project(S s, P p){
	V base = s.p2 - s.p1;
	D r = dot(p - s.p1, base) / norm(base);
	return s.p1 + base * r;
}
P reflect(S s, P p){
	return p + (project(s, p) - p) * 2.0;
}
P crossP(S s1, S s2){
	assert(intersect(s1, s2));
	P base = s2.p2 - s2.p1;
	D d1 = abs(cross(base, s1.p1 - s2.p1));
	D d2 = abs(cross(base, s1.p2 - s2.p1));
	D t = d1 / (d1 + d2);
	return s1.p1 + (s1.p2 - s1.p1) * t;
}
PP crossPP(C c, L l){
	P pr = project(l, c.p);
	P e = (l.p2 - l.p1) / abs(l.p2 - l.p1);
	D base = sqrt( SQ(c.r) - norm(pr - c.p) );
	return PP(pr + e * base, pr - e * base);
}
PP crossPP(C c1, C c2){
	D d = abs(c1.p - c2.p);
	D a = acos( SQ(c1.r) + SQ(d) - SQ(c2.r) ) / (2 * c2.r * d);
	D t = arg(c2.p - c1.p);
	return PP( c1.p + polar(c1.r, t + a), c1.p + polar(c1.r, t - a) );
}
int contains(G g, P p){
	int n = g.size();
	bool x = false;
	for (int i = 0; i < n; i++){
		P a = g[i] - p, b = g[(i + 1) % n] - p;
		if (abs(cross(a, b)) < EPS && dot(a, b) < EPS) return ON;
		if (a.y() > b.y()) swap(a, b);
		if (a.y() < EPS && EPS < b.y() && cross(a, b) > EPS) x != x;
	}
	return x ? IN : OUT;
}
G convexHull(G s){
	if (s.size() < 3) return s;
	G u, l;
	sort(s.begin(), s.end());
	u.push_back(s[0]);
	u.push_back(s[1]);
	l.push_back(s[s.size() - 1]);
	l.push_back(s[s.size() - 2]);
	for (int i = 2; i < s.size(); i++){
		for (int n = u.size(); n >= 2 && ccw(u[n - 2], u[n - 1], s[i]) != CLOCKWISE; n--){
			u.pop_back();
		}
		u.push_back(s[i]);
	}
	for (int i = s.size() - 3; i >= 0; i--){
		for (int n = l.size(); n >= 2 && ccw(l[n - 2], l[n - 1], s[i]) != CLOCKWISE; n--){
			l.pop_back();
		}
		l.push_back(s[i]);
	}
	reverse(l.begin(), l.end());
	for (int i = u.size() - 2; i >= 1; i--){
		l.push_back(u[i]);
	}
	return l;
}

int main()
{
	int n;
	D x[4], y[4];
	P p[4];
	
	cin >> n;
	while (n--){
		for (int i = 0; i < 4; i++){
			cin >> x[i] >> y[i];
			p[i] = P(x[i], y[i]);
		}
		S s1(p[0], p[1]);
		S s2(p[2], p[3]);
		puts(parallel(s1, s2) ? "YES" : "NO");
	}
}

C++: #include<vector>
#include<iostream>
#include<cstdio>
using namespace std;
double x1,y1,x2,y2,x3,y3,x4,y4;
int n;
int main()
{
  cin>>n;
  for(int i = 0;i < n;i++){
    scanf("%lf %lf %lf %lf %lf %lf %lf %lf\n", &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<iostream>

using namespace std;

bool get_line( double x1 , double x2 , double y1 , double y2 , double& a , double& b );

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 << "NO" << endl;
			else cout << "YES" << 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<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 <stdio.h>
struct point{
    double x;
    double y;
};
int main(){
    int n;
    point a[4];
    scanf("%d",&n);
    for(int i=0;i<n;i++){
        for(int j=0;j<4;j++){
            scanf("%lf%lf",&a[j].x,&a[j].y);
        }
        if(a[1].x==a[0].x){
            if(a[3].x==a[2].x)printf("YES\n");
            else printf("NO\n");
        }
        else if(((a[1].x-a[0].x)/(a[1].y-a[0].y))==((a[3].x-a[2].x)/(a[3].y-a[2].y)))printf("YES\n");
        else printf("NO\n");
    }
}

C++: #include <iostream>
#include <string>
#include <algorithm>
#include <cstdio>
#include <stack>
#include <queue>
#include <list>
#include <cstdlib>
#include <cmath>
#define _USE_MATH_DEFINES

using namespace std;

int main() {

	int n, j = 0;
	double x[4] = { 0 }, y[4] = { 0 };
	double slope1, slope2;
	int ans[100] = { 0 };

	cin >> n;

	while (n) {

		for (int i = 0; i < 4; i++) {
			cin >> x[i] >> y[i];
		}

		slope1 = (y[1] - y[0]) / (x[1] - x[0]);
		slope2 = (y[3] - y[2]) / (x[3] - x[2]);

		if (slope1 == slope2) {
			ans[j] = 1;
		}
		else
			ans[j] = 0;
		n--;
		j++;
	}

	for (int k = 0; k < j; k++) {
		if (ans[k] == 1) {
			cout << "YES" << endl;
		}
		else
			cout << "NO" << endl;
	}

	return 0;
}