Finding points with the shortest distance

Question

This takes in a user specified number of points then finds the two points with the shortest distance.

import java.util.Scanner;
import java.lang.Math;

public class FindNearestPoints
{
    public static void main(String[] args)
    {
        Scanner scan = new Scanner(System.in);

        System.out.print("Enter the number of points: ");
        final int numOfPoints = scan.nextInt();

        double[][] points = new double[numOfPoints][2];
        double shortestDistance=0;
        double distance=0;
        String closestPoint1="";
        String closestPoint2="";

        //enter x,y coords into the ix2 table points[][]
        for (int i=0; i<numOfPoints; i++)
        {
                System.out.print("Enter point x" + i + ": ");
                points[i][0] = scan.nextDouble();
                System.out.print("Enter point y" + i + ": ");
                points[i][1] = scan.nextDouble();
        }

        //get the distance between the point in the ith row and the (m+1)th row
        //and check if it's shorter than the distance between 0th and 1st
        for (int i=0; i<numOfPoints; i++)
        {
            //use m=i rather than 0 to avoid duplicate computations
            for (int m=i; m<numOfPoints-1;m++)
            {
                double dx = points[i][0] - points[m+1][0];
                double dy = points[i][1] - points[m+1][1];
                distance = Math.sqrt(dx*dx + dy*dy);

                //set shortestDistance and closestPoints to the first iteration
                if (m == 0 && i == 0)
                {
                    shortestDistance = distance;
                    closestPoint1 = "(" + points[0][0] + "," + points[0][1] + ")";
                    closestPoint2 = "(" + points[1][0] + "," + points[1][1] + ")";
                }
                //then check if any further iterations have shorter distances
                else if (distance < shortestDistance)
                {
                    shortestDistance = distance;
                    closestPoint1 = "(" + points[i][0] + "," + points[i][1] + ")";
                    closestPoint2 = "(" + points[m+1][0] + "," + points[m+1][1] + ")";
                }
            }
        }
        System.out.println("The shortest distance is: " + shortestDistance);
        System.out.println("The closest points are " + closestPoint1 + " and " + closestPoint2);
    }   
}

Answer 1

Algorithm

This is a classical problem in computational geometry and there are a lot of efficient approaches if you're interested. The longest distance problem (aka. diameter of a polygon) is more interesting though, and will introduce you to a really useful tool in computational geometry: the rotating calipers.

Code

It's easy to read, but three improvements can be made:

1. for (int m=i; m<numOfPoints-1;m++) is indeed an interesting optimization (x2 improvement). It would be more readable to make m go from i+1 to numOfPoints.
2. You can avoid computing Math.sqrt(dx*dx + dy*dy);. dx*dx + dy*dy is enough to compare the distances. When printing the distance, you can use Math.sqrt.
3. First set shortestDistance to Double.MAX_VALUE, this will avoid you the if (m == 0 && i == 0) case and the resulting duplication: you'll always find a distance shorter than infinity.

Answer 2

For starters, this code should use a Point class, consisting of an x and y location. Your variable points would then be Point[] rather than a two dimensional array. The Point class should have a method for reading values, printing values and calculating the distance between two points.

Then, using the Point class, your code would be dramatically simpler and easier to read. It might even be easier to write from scratch rather than try and fix what you have.

Answer 3

import java.util.Scanner;
import java.lang.Math;

public class FindNearestPoints
{
    public static void main(String[] args)
    {
        Scanner scan = new Scanner(System.in);

        System.out.print("Enter the number of points: ");
        final int numOfPoints = scan.nextInt();

        double[][] points = new double[numOfPoints][2];
        double shortestDistance=0;
        double distance=0;
        String closestPoint1="";
        String closestPoint2="";

Declare your variables close to where you use them, not all at the beginning.

        //enter x,y coords into the ix2 table points[][]
        for (int i=0; i<numOfPoints; i++)
        {
                System.out.print("Enter point x" + i + ": ");
                points[i][0] = scan.nextDouble();
                System.out.print("Enter point y" + i + ": ");
                points[i][1] = scan.nextDouble();
        }

        //get the distance between the point in the ith row and the (m+1)th row
        //and check if it's shorter than the distance between 0th and 1st
        for (int i=0; i<numOfPoints; i++)
        {
            //use m=i rather than 0 to avoid duplicate computations
            for (int m=i; m<numOfPoints-1;m++)
            {

Use for(m = i + 1; m < numOfPoints; m++) That way you don't have to m + 1 everywhere

                double dx = points[i][0] - points[m+1][0];
                double dy = points[i][1] - points[m+1][1];
                distance = Math.sqrt(dx*dx + dy*dy);

                //set shortestDistance and closestPoints to the first iteration

I don't understand this comment.

                if (m == 0 && i == 0)
                {
                    shortestDistance = distance;
                    closestPoint1 = "(" + points[0][0] + "," + points[0][1] + ")";
                    closestPoint2 = "(" + points[1][0] + "," + points[1][1] + ")";

I suggest storing the numbers associated with the closest points, not a string.

                }
                //then check if any further iterations have shorter distances
                else if (distance < shortestDistance)

You are doing the same thing you did above. Use || to combine the two conditions

                {
                    shortestDistance = distance;
                    closestPoint1 = "(" + points[i][0] + "," + points[i][1] + ")";
                    closestPoint2 = "(" + points[m+1][0] + "," + points[m+1][1] + ")";
                }
            }
        }
        System.out.println("The shortest distance is: " + shortestDistance);
        System.out.println("The closest points are " + closestPoint1 + " and " + closestPoint2);
    }   
}

The whole thing would do better to split across several functions instead of all in one.