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
//------------------------------------------
#include <vector>
#include <list>
#include <map>
#include <set>
#include <deque>
#include <stack>
#include <bitset>
#include <algorithm>
#include <functional>
#include <numeric>
#include <utility>
#include <sstream>
#include <iostream>
#include <iomanip>
#include <cstdio>
#include <cmath>
#include <cstdlib>
#include <cctype>
#include <string>
#include <cstring>
#include <ctime>
#include <climits>
#include <queue>

using namespace std;

//typedef
//------------------------------------------
typedef vector<int> VI;
typedef vector<VI> VVI;
typedef vector<string> VS;
typedef pair<int, int> PII;
typedef long long LL;

//container util
//------------------------------------------
#define ALL(a)  (a).begin(),(a).end()
#define RALL(a) (a).rbegin(), (a).rend()
#define PB push_back
#define MP make_pair
#define SZ(a) int((a).size())
#define EACH(i,c) for(typeof((c).begin()) i=(c).begin(); i!=(c).end(); ++i)
#define EXIST(s,e) ((s).find(e)!=(s).end())
#define SORT(c) sort((c).begin(),(c).end())

//repetition
//------------------------------------------
#define FOR(i,a,b) for(int i=(a);i<(b);++i)
#define REP(i,n)  FOR(i,0,n)

//constant
//--------------------------------------------
const double EPS = 1e-8;
const double PI  = acos(-1.0);

int main(){
  int N; cin >> N;
  for(;--N;){
	double x[4], y[4];
	REP(i,4)
	  cin >> x[i] >> y[i];
	//x[0] -= x[1], y[0] -= y[1], x[2] -= x[3], y[2] -= y[3];

	if(abs((x[0]-x[1])*(y[2]-y[3])-(x[2]-x[3])*(y[0]-y[1]))<EPS)
	  cout << "YES" << endl;
	else
	  cout << "NO" << endl;
  }
  
  return 0;
}

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

int main(){
  int n, i;
  long double x1, x2, x3, x4, y1, y2, y3, y4, slopeofAB, slopeofCD;

  cin >> n;

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

    slopeofAB = (y2 - y1) / (x2 - x1);
    slopeofCD = (y4 - y3) / (x4 - x3);

    if(slopeofAB == slopeofCD){
      cout << "YES" << endl;
    }else{
      cout << "NO" << endl;
    }
  }

  return 0;
}

C++: #include <cstdio>
#include <cstdlib>
#include <cstring>
#include <cmath>
#include <climits>
#include <cfloat>
#include <ctime>
#include <cassert>
#include <map>
#include <utility>
#include <set>
#include <iostream>
#include <memory>
#include <string>
#include <vector>
#include <algorithm>
#include <functional>
#include <sstream>
#include <complex>
#include <stack>
#include <queue>
#include <numeric>
#include <list>
#include <iomanip>
  
using namespace std;
  
#if __GNUC__
#include <tr1/unordered_map>
#include <tr1/unordered_set>
using namespace tr1;
#else
#include <unordered_map>
#include <unordered_set>
#endif
  
  
  
#ifdef _MSC_VER
#define __typeof__ decltype
template <class T> int popcount(T n) { return n ? 1 + popcount(n & (n - 1)) : 0; }
#endif
#ifdef __GNUC__
template <class T> int popcount(T n);
template <> int popcount(unsigned int n) { return __builtin_popcount(n); }
template <> int popcount(int n) { return __builtin_popcount(n); }
template <> int popcount(unsigned long long n) { return __builtin_popcountll(n); }
template <> int popcount(long long n) { return __builtin_popcountll(n); }
#endif
  
#define rep (i, n) for (int i = 0; i < (int)n; ++i)
#define foreach(it, c) for (__typeof__((c).begin()) it=(c).begin(); it != (c).end(); ++it)
#define rforeach(it, c) for (__typeof__((c).rbegin()) it=(c).rbegin(); it != (c).rend(); ++it)
#define all(c) (c).begin(), (c).end()
#define rall(c) (c).rbegin(), (c).rend()
#define CL(arr, val) memset(arr, val, sizeof(arr))
#define MEMCPY(dest, src) memcpy(dest, src, sizeof(src))
  
  
template <class T> void max_swap(T& a, const T& b) { a = max(a, b); }
template <class T> void min_swap(T& a, const T& b) { a = min(a, b); }
  
typedef long long ll;
typedef pair<int, int> pint;
  
template <class T> string to_s(const T& a) { ostringstream os; os << a; return os.str(); }
  
const double EPS = 1e-8;
const double PI = acos(-1.0);
const int dx[] = { 0, 1, 0, -1 };
const int dy[] = { 1, 0, -1, 0 };
bool valid_pos(int x, int y, int w, int h) { return 0 <= x && x < w && 0 <= y && y < h; }
  
template <class T> void print(T a, int n, int br = 1, const string& deli = ", ") { cout << "{ "; for (int i = 0; i < n; ++i) { cout << a[i]; if (i + 1 != n) cout << deli; } cout << " }"; while (br--) cout << endl; }
template <class T> void print(const vector<T>& v, int br = 1, const string& deli = ", ") { print(v, v.size(), br, deli); }
template <class T> void print2d(T a, int w, int h, int width = -1, int br = 1) { for (int i = 0; i < h; ++i) { for (int j = 0; j < w; ++j) {    if (width != -1) cout.width(width); cout << a[i][j] << ' '; } cout << endl; } while (br--) cout << endl; }
  
template <class T> void input(int n, T& a) { for (int i = 0; i < n; ++i) cin >> a[i]; }
template <class T, class U> void input(int n, T& a, U& b) { for (int i = 0; i < n; ++i) cin >> a[i] >> b[i]; }
template <class T> vector<T> input(int n) { vector<T> res(n); for (int i = 0; i < n; ++i) cin >> res[i]; return res; }
 
 
 
const double EPS_FOR_LIB = 1e-50;
typedef double geo_type;
typedef complex<geo_type> Point;
typedef vector<Point> Poly;
namespace std
{
	bool operator<(const Point& a, const Point& b)
	{
		return a.real() != b.real() ? a.real() < b.real() : a.imag() < b.imag();
	}
	std::istream& operator>>(std::istream& is, Point& p)
	{
		geo_type a, b; is >> a >> b;
		p = Point(a, b);
		return is;
	}
	std::ostream& operator<<(std::ostream& os, Point& p)
	{
		return os << "(" << p.real() << ", " << p.imag() << ")";
	}
};
struct Line
{
	Point a, b;
	Line(Point a, Point b) : a(a), b(b) { }
	Line() { }
};

geo_type dot(const Point& a, const Point& b)
{
	return a.real() * b.real() + a.imag() * b.imag();
}
geo_type cross(const Point& a, const Point& b)
{
	return a.real() * b.imag() - a.imag() * b.real();
}

bool is_parallel(const Line& line1, const Line& line2)
{
	return cross(line1.b - line1.a, line2.b - line2.a) < EPS_FOR_LIB;
}

int main()
{
	int n;
	cin >> n;
	while (n--)
	{
		Point p[4];
		input(4, p);
		cout << (is_parallel(Line(p[0], p[1]), Line(p[2], p[3]))
			? "YES" : "NO") << endl;		
	}
}

C++: #include <algorithm>
#include <cctype>
#include <climits>
#include <cmath>
#include <complex>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <ctime>
#include <deque>
#include <functional>
#include <iostream>
#include <list>
#include <map>
#include <memory>
#include <numeric>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <utility>
#include <vector>
using namespace std;
#define INF 1e+15

int main(){
  double x[4], y[4];
  int n;
  cin >> n;
  while( n-- ){
    cin >> x[0] >> y[0] >> x[1] >> y[1] >> x[2] >> y[2] >> x[3] >> y[3];
    double tan[2];
    for (int i = 0; i < 2; i++) {
      for (int j = 0; j < 2; j++) {
        if ( x[i*2] == x[i*2+1] ) {
          tan[i] = INF;
        } else {
          tan[i] = (y[i*2]-y[i*2+1]) / (x[i*2]-x[i*2+1]);
        }
      }
    }
    if ( abs(tan[0]) == abs(tan[1]) ) {
      cout << "YES" << endl;
    } else {
      cout << "NO" << endl;
    }
  }
}