Problem:
Projection

For given three points p1, p2, p, find the projection point x of p onto p1p2.

Input

xp1 yp1 xp2 yp2
q
xp0 yp0
xp1 yp1
...
xpq−1 ypq−1

In the first line, integer coordinates of p1 and p2 are given. Then, q queries are given for integer coordinates of p.

Output

For each query, print the coordinate of the projection point x. The output values should be in a decimal fraction with an error less than 0.00000001.

Constraints

1 ≤ q ≤ 1000

-10000 ≤ xi, yi ≤ 10000

p1 and p2 are not identical.

Sample Input 1

0 0 2 0
3
-1 1
0 1
1 1

Sample Output 1

-1.0000000000 0.0000000000
0.0000000000 0.0000000000
1.0000000000 0.0000000000

Sample Input 2

0 0 3 4
1
2 5

Sample Output 2

3.1200000000 4.1600000000

Solution in C# (judge verdict: Accepted)

using System;
//using System.Collections.Generic;
//using System.Linq;
using Vector = CGL.Point;
//using Line = CGL.Segment;
//using Polygon = System.Collections.Generic.List<CGL.Point>;


namespace CGL
{
    class Program
    {
        static void Main(string[] args)
        {
            var p = scan;
            var p1 = new Point(p[0], p[1]);
            var p2 = new Point(p[2], p[3]);
            var s = new Segment(p1, p2);
            var q = scan[0];
            for (int i = 0; i < q; i++)
            {
                p = scan;
                var P = new Point(p[0], p[1]);
                var Q = Projection(s, P);
                Console.WriteLine(Q.x+" "+Q.y);
            }
        }
        static Point Projection(Segment s, Point p)
        {
            Vector b = s.p2 - s.p1;
            double r = b.Dot(p - s.p1) / b.Norm;
            return s.p1 + r * b;

        }
        static int[] scan { get { return Array.ConvertAll(Console.ReadLine().Split(), int.Parse); } }
    }
    
    struct Point
    {
        static readonly double EPS = 1E-10;
        public double x, y;
        public Point(double x, double y)
        {
            this.x = x;
            this.y = y;
        }

        public static Point operator +(Point a, Point b) { return new Point(a.x + b.x, a.y + b.y); }
        public static Point operator -(Point a, Point b) { return new Point(a.x - b.x, a.y - b.y); }
        public static Point operator *(double k, Point a) { return new Point(k * a.x, k * a.y); }
        public static Point operator /(double k, Point a) { return new Point(a.x / k, a.y / k); }
        
        public double Abs { get { return Math.Sqrt(Norm); } }  
        public double Norm { get { return x * x + y * y; } }

        public static bool operator <(Point a, Point b) { return Math.Abs(a.x - b.x) > EPS ? a.x < b.x : a.y < b.y; }
        public static bool operator >(Point a, Point b) { return Math.Abs(a.x - b.x) > EPS ? a.x > b.x : a.y > b.y; }
        public static bool operator ==(Point a, Point b) { return Math.Abs(a.x - b.x) < EPS && Math.Abs(a.y - b.y) < EPS; }
        public static bool operator !=(Point a, Point b) { return Math.Abs(a.x - b.x) >= EPS || Math.Abs(a.y - b.y) >= EPS; }

        public override bool Equals(object p){  return Math.Abs(x - ((Point)p).x) < EPS && Math.Abs(y - ((Point)p).y) < EPS; }
        public override int GetHashCode(){ return x.GetHashCode() ^ y.GetHashCode(); }

        public double Dot(Point p) { return x * p.x + y * p.y; }
        public double Cross(Point p) { return x * p.y - y * p.x; }

    }
    struct Segment
    {
        public Point p1, p2;
        public Segment(Point p1, Point p2)
        {
            this.p1 = p1;
            this.p2 = p2;
        }
    }
    /*struct Circle
    {
        public Point c; double r;
        public Circle(Point c, double r)
        {
            this.c = c;
            this.r = r;
        }

    }*/
}