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
//------------------------------------------
#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 <queue>
#include <iterator>
#include <cstdio>
#include <cmath>
#include <cstdlib>
#include <cctype>
#include <string>
#include <cstring>
#include <ctime>

using namespace std;

//conversion
//------------------------------------------
inline int toInt(string s) {int v; istringstream sin(s);sin>>v;return v;}
template<class T> inline string toString(T x) {ostringstream sout;sout<<x;return sout.str();}

//math
//-------------------------------------------
template<class T> inline T sqr(T x) {return x*x;}

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

//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-10;
const double PI  = acos(-1.0);

//clear memory
#define CLR(a) memset((a), 0 ,sizeof(a))

//debug
#define dump(x)  cerr << #x << " = " << (x) << endl;
#define debug(x) cerr << #x << " = " << (x) << " (L" << __LINE__ << ")" << " " << __FILE__ << endl;

int gcd(int a, int b){
   int tmp;
   while(a%b==0){
      tmp=a%b;
      a=b;
      b=tmp;
   }
}

int lcm(int a,int b){
   return a*b/gcd(a,b);
}

bool prime_number(int n){
   if(n < 2){
      return false;
   }
   else if(n == 2){
      return true;
   }
   if(n % 2 == 0){
      return false;
   }
   for(int i=3;i<=n/i;i+=2){
      if(n % i == 0) return false;
      return true;
   }
}

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

C++: #include <iostream>
#include <vector>
#include <string>
#include <stack>
#include <queue>
#include <deque>
#include <set>
#include <map>
#include <algorithm>	// require sort next_permutation count __gcd reverse etc.
#include <cstdlib>	// require abs exit atof atoi 
#include <cstdio>		// require scanf printf
#include <functional>
#include <numeric>	// require accumulate
#include <cmath>		// require fabs
#include <climits>
#include <limits>
#include <cfloat>
#include <iomanip>	// require setw
#include <sstream>	// require stringstream 
#include <cstring>	// require memset
#include <cctype>		// require tolower, toupper
#include <fstream>	// require freopen
#include <ctime>		// require srand
#define rep(i,n) for(int i=0;i<(n);i++)
#define ALL(A) A.begin(), A.end()
#define EPS 1e-9

using namespace std;

typedef long long ll;
typedef pair<int, int> P;

/*
	この解答は、どうやら誤差死。
	長さを求める平方根と
	掛け算とで誤差が出る模様。
	
	double の計算のときには
	如何に計算しないかがカギ。
*/

double dot (double xa, double ya, double xb, double yb )
{
	return (xa*xb + ya*yb );
}

double calc_len (double x, double y )
{
	return (x*x + y*y );
}

int main()
{
//	cut here before submit 
//	freopen ("testcase.Parallelism", "r", stdin );
	int n;
	scanf ("%d", &n );
	rep (i, n ){
		double x1, y1, x2, y2, x3, y3, x4, y4;
		cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3 >> x4 >> y4;
		double d = dot (x2-x1, y2-y1, x4-x3, y4-y3 );
		double len1 = calc_len (x2-x1, y2-y1 );
		double len2 = calc_len (x4-x3, y4-y3 );

		if (fabs (len1*len2 - fabs(d*d ) ) < EPS ){
			cout << "YES" << endl;
		}else{
			cout << "NO" << endl;
		} // end if
	} // end loop
		
	return 0;
}

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

int main()
{
    int n;
    cin>>n;
    for(int i=0;i<n;i++){
        pair<double,double>a[4];
        for(int i=0;i<4;i++){
            cin>>a[i].first>>a[i].second;
        }
        if((a[1].first-a[0].first)==0&&(a[3].first-a[2].first)==0)cout<<"NO"<<endl;
        else if((a[1].second-a[0].second)/(a[1].first-a[0].first)!=(a[3].second-a[2].second)/(a[3].first-a[2].first))
            cout<<"NO"<<endl;
        else cout<<"YES"<<endl;
    }
    return 0;
}

C++: #include<cstdio>

int main(){

    int n;
    scanf("%d",&n);

    for(int i=0;i<n;i++){
        double x1,y1,x2,y2,x3,y3,x4,y4;
        scanf("%lf%lf%lf%lf%lf%lf%lf%lf",&x1,&y1,&x2,&y2,&x3,&y3,&x4,&y4);

        if((y2-y1)/(x2-x1)==(y4-y3)/(x4-x3)) printf("YES\n");
        else printf("NO\n");
    }
}