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;

/*--------------------template--------------------*/
typedef long double ld;
const ld EPS = 1e-12;
typedef long double ld;
const ld PI = acos(-1.0);
bool eq(ld a, ld b) { return abs(a - b) < EPS; }
typedef complex<ld> Point;
typedef vector<Point> Polygon;

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

struct Line
{
	Point a, b;
	Line(Point p, Point q) :a(p), b(q) {};
	Line(ld x1, ld y1, ld x2, ld y2) :a(Point(x1, y1)), b(Point(x2, y2)) {};
};

struct Circle
{
	Point p; ld r;
	Circle(Point a, ld b) :p(a), r(b) {};
};

ld dot(Point a, Point b)
{
	return real(conj(a)*b);
}

ld cross(Point a, Point b)
{
	return imag(conj(a)*b);
}

int ccw(Point a, Point b, Point c)
{
	b -= a; c -= a;
	if (cross(b, c) > EPS) return 1;    //counter cloclwise
	if (cross(b, c) < -EPS) return -1;  //cloclwise
	if (dot(b, c) < 0) return 2;        //c--a--b on line 
	if (norm(b) < norm(c)) return -2;   //a--b--c on line
	return 0;                           //a--c--b on line
}

bool isis_ll(Line l, Line m)
{
	return abs(cross(l.b - l.a, m.b - m.a)) > EPS;
}

bool isis_ls(Line l, Line s)
{
	return cross(l.b - l.a, s.a - l.a)*cross(l.b - l.a, s.b - l.a) < EPS;
}

bool isis_ss(Line s, Line t)
{
	return (ccw(s.a, s.b, t.a)*ccw(s.a, s.b, t.b) <= 0 &&
		ccw(t.a, t.b, s.a)*ccw(t.a, t.b, s.b) <= 0);
}

bool isis_lp(Line l, Point p)
{
	return (abs(cross(l.b - p, l.a - p)) < EPS);
}

bool isis_sp(Line s, Point p)
{
	return (abs(s.a - p) + abs(s.b - p) - abs(s.b - s.a)) < EPS;
}

Point projection(Line l, Point p)
{
	Point base = l.b - l.a;
	ld r = dot(p - l.a, base) / norm(base);
	return l.a + base*r;
}

Point mirror(Line l, Point p)
{
	return Point(2, 0)*projection(l, p) - p;
}

ld dist_lp(Line l, Point p)
{
	return abs(p - projection(l, p));
}

ld dist_ll(Line l, Line m)
{
	return isis_ll(l, m) ? 0 : dist_lp(l, m.a);
}

ld dist_ls(Line l, Line s)
{
	if (isis_ls(l, s)) return 0;
	return min(dist_lp(l, s.a), dist_lp(l, s.b));
}

ld dist_sp(Line s, Point p)
{
	Point r = projection(s, p);
	if (isis_sp(s, r)) return abs(r - p);
	return min(abs(s.a - p), abs(s.b - p));
}

ld dist_ss(Line s, Line t)
{
	if (isis_ss(s, t)) return 0;
	return min(min(dist_sp(s, t.a), dist_sp(s, t.b)), min(dist_sp(t, s.a), dist_sp(t, s.b)));
}

Point is_ll(Line s, Line t)
{
	ld a = cross(s.b - s.a, t.b - t.a);
	ld b = cross(s.b - s.a, s.b - t.a);
	return t.a + b / a*(t.b - t.a);
}

vector<Point> is_cc(Circle c1, Circle c2)
{
	vector<Point> res;
	ld d = abs(c1.p - c2.p);
	ld rc = (d*d + pow(c1.r, 2) - pow(c2.r, 2)) / (2 * d);
	ld dfr = pow(c1.r, 2) - rc*rc;
	if (abs(dfr) < EPS) dfr = 0;
	if (dfr < 0.0) return res;
	ld rs = sqrt(dfr);
	Point diff = (c2.p - c1.p) / d;
	res.push_back(c1.p + diff*Point(rc, rs));
	if (dfr != 0.0) res.push_back(c1.p + diff*Point(rc, -rs));
	return res;
}

int isis_cc(Circle c1, Circle c2)
{
	ld d = abs(c1.p - c2.p);
	if (d - c1.r - c2.r < -EPS) return -2; //separate
	if (abs(d - c1.r - c2.r) < EPS) return -1; //circumscribe
	if (c1.r < c2.r) swap(c1, c2);
	if (abs(d - c1.r + c2.r) < EPS) return 1;//inscribe
	if (c1.r - d - c2.r > EPS) return 2;//involve
	else return 0;//intersect
}

bool isis_cl(Circle c, Line l)
{
	ld d = dist_lp(l, c.p);
	return d - c.r < -EPS;
}

vector<Point> is_cl(Circle c, Line l)
{
	vector<Point> res;
	ld d = dist_lp(l, c.p);
	if (d > c.r + EPS) return res;
	ld len = (d > c.r) ? 0 : sqrt(c.r*c.r - d*d);
	Point nor = (l.a - l.b) / abs(l.a - l.b);
	res.push_back(projection(l, c.p) + len*nor);
	if (abs(len) > EPS) res.push_back(projection(l, c.p - len*nor));
	return res;
}

vector<Point> is_cs(Circle c, Line s)
{
	vector<Point> v = is_cl(c, s), res;
	for (int i = 0;i < v.size();i++)
	{
		if (ccw(s.a, v[i], s.b) == -2) res.push_back(v[i]);
	}
	return res;
}

vector<Line> tangent_cp(Circle c, Point p)
{
	vector<Line> res;
	Point v = c.p - p;
	ld d = abs(v);
	ld l = sqrt(norm(v) - c.r * c.r);
	if (isnan(l)) { return res; }
	Point v1 = v * Point(l / d, c.r / d);
	Point v2 = v * Point(l / d, -c.r / d);
	res.push_back(Line(p, p + v1));
	if (l < EPS) return res;
	res.push_back(Line(p, p + v2));
	return res;
}

vector<Line> tangent_cc(Circle c1, Circle c2)
{
	vector<Line> res;
	if (abs(c1.p - c2.p) - (c1.r + c2.r) > -EPS)
	{
		Point center = (c1.p * c2.r + c2.p * c1.r) / (c1.r + c2.r);
		res = tangent_cp(c1, center);
	}
	if (abs(c1.r - c2.r) > EPS)
	{
		Point out = (-c1.p * c2.r + c2.p * c1.r) / (c1.r - c2.r);
		vector<Line> nres = tangent_cp(c1, out);
		res.insert(res.end(), nres.begin(), nres.end());
	}
	else
	{
		Point v = c2.p - c1.p; v /= abs(v);
		Point q1 = c1.p + v * Point(0, 1) * c1.r;
		Point q2 = c1.p + v * Point(0, -1) * c1.r;
		res.push_back(Line(q1, q1 + v));
		res.push_back(Line(q2, q2 + v));
	}
	return res;
}

int main() {
	int T; cin >> T;
	for (int t = 0; t < T;t++) {
		Point a[4];
		for (int i = 0; i < 4;i++) {
			ld x, y; cin >> x >> y;
			a[i] = Point(x, y);
		}
		Line l1(a[0], a[1]), l2(a[2], a[3]);
		if (isis_ll(l1, l2)) {
			cout << "NO" << endl;
		}
		else {
			cout << "YES" << endl;
		}
	}
}

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

using namespace std;

int main(void) {
	int ds;
	cin >> ds;
	
	for(int i = 0; i < ds; i++) {
		double X[4], Y[4];
		double a1, a2;
		double b1, b2;
		
		for(int j = 0; j < 4; j++) {
			cin >> X[j] >> Y[j];
		}
		a1 = abs(Y[1] - Y[0]);
		b1 = (X[1] - X[0]);
		
		a2 = abs(Y[3] - Y[2]);
		b2 = (X[3] - X[2]);
		
		a1 /= b1;
		a2 /= b2;
		
		if(a1 == a2) {
			cout << "YES" << endl;
		} else {
			cout << "NO" << endl;
		}
	}
	return 0;
}