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 <iostream>


using namespace std;

int main()
{
	int i,num;
	float x[4],y[4];
	cin >> num;

	for ( i = 0 ; i< num; i++ ) {
		cin >> x[0] >> y[0] >> x[1] >> y[1] >> x[2] >> y[2] >> x[3] >> y[3];

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

C++: #include <iostream>
#include <complex>
using namespace std;
#define REP(i, n)       REPEAT(i, 0, n)
#define REPEAT(i, k, n) for (int i = (k); i < (n); ++i)

#define X real()
#define Y imag()
typedef double D;
typedef complex<double>P;
#define EPS (1e-9)
 
bool iscross(P &p1, P &p2, P &p3, P &p4){
    P d1 = p2 - p1, d2 = p4 - p3;
    D det = d2.X * d1.Y - d1.X * d2.Y;
    if(abs(det) < EPS) return false;
    return true;
}
 
int main(){
    int n;
    cin >> n;
    REP(i,n){
        double x1, y1, x2, y2, x3, y3, x4, y4;
        cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3 >> x4 >> y4;
        P p1(x1,y1), p2(x2,y2), p3(x3,y3), p4(x4,y4);
        if(!iscross(p1,p2,p3,p4)){
            cout << "YES" << endl;
        } else {
            cout << "NO" << endl;
        }
    }
    return 0;
}

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+10

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 left, right;
      left  = (y[1] - y[0]) * (x[3] - x[2]);
      right = (y[3] - y[2]) * (x[1] - x[0]);

    if ( left == right ) {
      cout << "YES" << endl;
    } else {
      cout << "NO" << endl;
    }
  }
}

C++:  
 #include<iostream>
 #include<stdio.h>
 #include<string>
 #include<math.h>
 #include<iomanip>
 #include<algorithm>
 #include<string.h>
 #include<cctype>
 #include<map>
 #include<set>
 #include<vector>
 #include<sstream>
 #include<stack>
 
 using namespace std;
 
 double abs(double x)
 {
   if(x<0) return -x;
   else return x;       
 }
  
 int main()
 {
   double x1,y1,x2,y2,x3,y3,x4,y4;
   int n;
   cin>>n;
   while(n--)
   {
     cin>>x1>>y1>>x2>>y2>>x3>>y3>>x4>>y4;
     if(x2==x1&&x3==x4) cout<<"YES"<<endl;
     else 
     {
       double k1=(y2-y1)/(x2-x1);
       double k2=(y4-y3)/(x4-x3);
       if(abs(k1-k2)<0.000000000000001) cout<<"YES"<<endl;
       else cout<<"NO"<<endl;     
     }          
   }
   //while(1);      
   return 0;    
 }

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

int main()
{
    int n;
    cin >> n;
    for(int i=0; i<n; i++)
    {
        double x1, x2, x3, x4;
        double y1, y2, y3, y4;
        cin >> x1 >>  y1 >> x2 >> y2 >> x3 >> y3 >> x4 >> y4;
        if(abs((y2-y1)*(x4-x3) - (y4-y3)*(x2-x1)) < 1e-10)
        {
           cout << "YES" << endl; 
        }
        else
        {
            cout << "NO" << endl; 
        }
    }
    return 0;
}

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

#define PRINT(STR) cout << STR << endl;
#define REP(i, n) for (int (i) = 0; (i) < (int)(n); (i)++)
#define REG(i, a, b) for (int (i) = ((int)(a)); (i) < ((int)(b)); i++)
#define ALL(V) (V).begin(), (V).end()

typedef long long ll;

using namespace std;

int nextInt() { int n = -1; cin >> n; return n; }
double nextDouble() { double d = -1; cin >> d; return d; }
ll nextll() { ll n = -1; cin >> n; return n; }
string nextString() { string str; cin >> str; return str; }

int INF = 9999999;

// BEGIN //////////////////////////////////////////////////////////////


int main() {
	int N = nextInt();
	REP(i, N) {
		double x1 = nextDouble();
		double y1 = nextDouble();
		double x2 = nextDouble();
		double y2 = nextDouble();
		double x3 = nextDouble();
		double y3 = nextDouble();
		double x4 = nextDouble();
		double y4 = nextDouble();
		if (x1 > x2) {
			double t = x2;
			x2 = x1; x1 = t;
			t = y2; y2 = y1; y1 = t;
		}
		if (x3 > x4) {
			double t = x4;
			x4 = x3; x3 = t;
			t = y4; y4 = y3; y3 = t;
		}
		if (x2 - x1 == 0 || x4 - x3 == 0) {
			if (x2 - x1 == 0 && x4 - x3 == 0) {
				PRINT("YES");
			} else {
				PRINT("NO");
			}
		} else {
			if ((y2 - y1) / (x2 - x1) - (y4 - y3) / (x4 - x3) < 0.000006) {
				PRINT("YES");
			} else {
				PRINT("NO");
			}
		}
	}

	return 0;
}

// END   //////////////////////////////////////////////////////////////

C++: #include <iostream>
#include <cmath>
 
using namespace std;
 
int main (void) {
  double x1, y1, x2, y2, x3, y3, x4, y4;
  int n;
    
  cin >> n;
   
  for(int i = 0; i < n; i++) {
    cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3 >> x4 >> y4;
    double t1 = (y1-y2)/(x1-x2);
    double t2 = (y3-y4)/(x3-x4);
    //cout << t1 << " " << t2 << endl;
    if ( t1 == t2 ) {
      cout << "YES" << endl;
    } else {
      cout << "NO" << endl;
    }
  }
   
  return 0;
}

C++: #include <iostream>

using namespace std;

int main(){
	int n;
	double x[100],y[100];
	cin>>n;
	while(n>0){
		for(int i=0;i<4;i++)cin>>x[i]>>y[i];
		if((y[0]-y[1])/(x[0]-x[1])==(y[2]-y[3])/(x[2]-x[3])){
			cout<<"YES"<<endl;
			n--;
		}
		else {
			cout<<"NO"<<endl;
			n--;
		}
	}
}

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

int main(){

	typedef struct{
		double x, y;
	}_td;

	int n;
	_td p1, p2, p3, p4;
	double r_ab, r_cd, eps;
	eps = 0.000001;
	
	cin >> n;
	
	for(int i=0; i<n; i++){
		cin >> p1.x >> p1.y >> p2.x >> p2.y >> p3.x >> p3.y >> p4.x >> p4.y;
		
		if(abs(p1.x - p2.x) > eps && abs(p3.x - p4.x) > eps){
			r_ab = (p1.y - p2.y) / (p1.x - p2.x);
			r_cd = (p3.y - p4.y) / (p3.x - p4.x);
			if(abs(r_ab - r_cd) < eps) cout << "YES" << endl;
			else cout << "NO" << endl;
		}
		else if(abs(p1.x - p2.x) <= eps && abs(p3.x - p4.x) <= eps){
			cout << "YES" << endl;
		}
		else cout << "NO" << endl;
	}

	return 0;

}

C++: #include<cstdio>
#include<cmath>
 
int main(){
    int n;
    double x[4], y[4];
    int xi[4], yi[4];
    scanf("%d", &n);
    for(int i = 0; i < n; i++){
        scanf("%lf %lf %lf %lf %lf %lf %lf %lf", &x[0],&y[0],&x[1],&y[1],&x[2],&y[2],&x[3],&y[3]);
 
        for(int i = 0; i < 4; i++){
            xi[i] = (int)(x[i] * 1000000);
            yi[i] = (int)(y[i] * 1000000);
        }
         
        if((yi[0] - yi[1])*(xi[2] - xi[3]) == (xi[0] - xi[1])*(yi[2] - yi[3])){
            printf("YES\n");
        }else{
            printf("NO\n");
        }
    }
    return 0;
}