compare lists

Question

i'm trying to compare lists i want to know if a values in a list exists more then 1 time in that list. i have an arraylist of coordinates and i want to get to know if there are points that are in the list multiple times. could you help me

Alan · Answer

This problem is easier to solve if we first sort the ArrayList (or a copy thereof) so that any duplicated points are adjacent to each other. As the System.Drawing.Point struct doesn't implement IComparable we need to create a custom IComparer class to do this. Below I've coded a quick console app to illustrate this approach with some sample data which contains duplicates: using System; using System.Collections; using System.Drawing; class Test { static void Main() { ArrayList points = new ArrayList(); points.Add(new Point(1020)); points.Add(new Point(2040)); points.Add(new Point(1020)); points.Add(new Point(3050)); points.Add(new Point(1040)); points.Add(new Point(1020)); points.Add(new Point(3040)); points.Add(new Point(3050)); // clone the ArrayList so that we can sort it without affecting original ArrayList points2 = (ArrayList)points.Clone(); points2.Sort(new PointComparer()); // now check for duplicates bool anyDuplicates = false; int count = 1; for (int i = 1; i { if (points2[i].Equals(points2[i-1])) { count++; if (!anyDuplicates) anyDuplicates = true; if (i == points2.Count - 1) { Console.WriteLine({0} occurred {1} times points2[i] count); } } else { if (count > 1) { Console.WriteLine({0} occurred {1} times points2[i-1] count); count = 1; } } } if (!anyDuplicates) Console.WriteLine(There are no duplicated points); Console.ReadLine(); } } class PointComparer : IComparer { public int Compare (object o1 object o2) { Point p1 = (Point)o1; Point p2 = (Point)o2; if (p1.X return -1; else if (p1 == p2) return 0; else return 1; } }