Collections

2 Objectives Explore collections in System.Collections namespace –memory management –containment testing –sorting –traversal

3 Collection A collection stores a group of objects Provides operations to manipulate contents –add –remove –count –contains –traverse –sort collection of strings "blue" "green" "yellow" "red"

4 Motivation Basic array is too simple for many applications –efficient, easy to use, and typesafe –but fixed size int[] a = new int[10]; a[0] = 17; a[1] = 99;... int x = a[1]; foreach (int i in a)... array size fixed at creation

5 Collection classes Collection library offers many collections –in namespace System.Collections –store object reference to be generic –perform memory management internally Core classes: –ArrayList –Stack –Queue –Hashtable

6 ArrayList ArrayList provides storage for sequence of elements –duplicate values ok –data stored in array –array resized automatically as needed using System.Collections; class Department { ArrayList employees = new ArrayList();... } create ArrayList to store Employees array of object references ArrayList object employees

7 ArrayList services ArrayList is primary class to support concept of list public class ArrayList : IList, ICloneable { int Add (object value)... void Insert(int index, object value)... void Remove (object value)... void RemoveAt(int index)... void Clear ()... bool Contains(object value)... int IndexOf (object value)... object this[int index] { get... set.. } int Capacity { get... set... } void TrimToSize() } control of memory in underlying array add new elements remove containment testing read/write existing element

8 ArrayList Add Use Add method to add new element to ArrayList –added at end ArrayList a = new ArrayList(); a.Add("bbb"); a.Add("aaa"); a.Add("ddd");... element 0 element 1 element 2

9 ArrayList Count Count property records number of elements in ArrayList –read only ArrayList a = new ArrayList(); a.Add("bbb"); a.Add("aaa"); a.Add("ddd"); int n = a.Count; Count is 3

10 ArrayList Insert Use Insert method to insert new element into ArrayList –specify data and index –index must be in range 0 through Count –index equal to Count means add to end –other elements shifted as needed to accommodate new one ArrayList a = new ArrayList(); a.Add("bbb"); a.Add("aaa"); a.Add("ddd"); a.Insert(2, "ccc"); insert at index 2

11 ArrayList RemoveAt Use RemoveAt method to remove element from ArrayList –specify index in range 0 through Count-1 –other elements shifted to fill empty position ArrayList a = new ArrayList(); a.Add("bbb"); a.Add("aaa"); a.Add("ddd"); a.RemoveAt(1); remove element at index 1

12 ArrayList Remove Use Remove method to remove element from ArrayList –pass in element to remove –searches linearly through list –uses Equals method for comparison –removes first occurrence –other elements shifted to fill empty position ArrayList a = new ArrayList(); a.Add("bbb"); a.Add("aaa"); a.Add("ddd"); a.Remove("aaa"); remove element equal to "aaa"

13 ArrayList Contains Use Contains method to do containment test in ArrayList –performs linear search through underlying array –uses Equals method to determine equality ArrayList a = new ArrayList(); a.Add("bbb"); a.Add("aaa"); a.Add("ddd"); if (a.Contains("aaa")) Console.WriteLine("found"); else Console.WriteLine("not found"); contains?

14 ArrayList IndexOf Use IndexOf method search for element index in ArrayList –performs linear search through underlying array –uses Equals method to determine equality –returns index of first occurrence or -1 if not found ArrayList a = new ArrayList(); a.Add("bbb"); a.Add("aaa"); a.Add("ddd"); int i = a.IndexOf("aaa");... returns 1

15 ArrayList indexer Can use indexer to get/set elements –cannot use to add new elements –index must be in range 0 through Count-1 ArrayList a = new ArrayList(); a.Add("bbb"); a.Add("aaa"); a.Add("ddd"); a[2] = "ccc"; string s = (string)a[2]; use Add to add new elements change existing element read existing element

16 ArrayList Capacity Capacity property used to control size of underlying array –can specify initial value during creation –can get or set ArrayList will adjust capacity automatically –defaults to 16 and doubles whenever more space needed –should let list control capacity when size of dataset unknown ArrayList a = new ArrayList(); ArrayList b = new ArrayList(50); b.Capacity = 100; int c = b.Capacity; default capacity is 16 specify capacity of 50 set capacity to 100 get capacity

17 Box/unbox Collections store object reference –value types boxed when added –unboxed using cast when retrieved ArrayList a = new ArrayList(); int x = 7; a.Add(x); int y = (int)a[0]; boxed unboxed

18 Enumeration Collection contents traversed using enumerator –IEnumerable defines how to get an enumerator –IEnumerator used to perform traversal and access data –gives read-only access –available on all collection types public interface IEnumerable { IEnumerator GetEnumerator(); } get enumerator from collection public interface IEnumerator { object Current { get; } bool MoveNext(); void Reset(); } traverse access data

19 Traversal with enumerator Can traverse collection contents with enumerator 1.get IEnumerator from collection with GetEnumerator() 2.call MoveNext() on enumerator to advance to first element 3.use read-only Current property to access element data 4.call MoveNext() to advance to next element 5.repeat while MoveNext() returns true ArrayList a = new ArrayList(); a.Add("bbb"); a.Add("aaa"); a.Add("ddd"); IEnumerator e = a.GetEnumerator(); while (e.MoveNext()) { string t = (string)e.Current;... } access and cast return advance and test get enumerator from collection

20 Traversal with foreach Can traverse enumerable collection with foreach loop –less code than using enumerator –convenient since cast performed implicitly –better than indexer since no chance of index error –gives read-only access ArrayList a = new ArrayList(); a.Add("bbb"); a.Add("aaa"); a.Add("ddd"); foreach (string s in a) {... } handles loop and cast

21 Sort Can sort the contents of Array and ArrayList –use variations on Sort method Must provide comparison method –can build in by implementing IComparable –can supply separate IComparer object –simple types have comparison built in public class ArrayList : IList, ICloneable { void Sort()... void Sort(IComparer comparer)... void Sort(int index, int count, IComparer comparer) } built-in compare separate compare

22 IComparable Build in comparison code with IComparable.CompareTo –return negative, zero, or positive to indicate relative order class Student : IComparable { public string name; public int id; public int CompareTo(object o) { Student s = o as Student; if (this.id < s.id) return -1; if (this.id > s.id) return 1; return 0; }... } cast this goes first parameter goes first equal

23 Using IComparable IComparable objects have CompareTo built in –will be called by no argument Sort method ArrayList students = new ArrayList(); Student ann = new Student("Ann", 8000); Student bob = new Student("Bob", 2000); students.Add(ann); students.Add(bob); students.Sort(); sort according to CompareTo

24 IComparer Separate comparison code with IComparer.Compare –return negative, zero, or positive to indicate relative order –typically implemented in different class –useful if original class does not build in desired comparison class StudentNameComparer : IComparer { public int Compare(object o1, object o2) { Student s1 = o1 as Student; Student s2 = o2 as Student; return s1.name.CompareTo(s2.name); } cast compare names

25 Using IComparer IComparer object created and passed to Sort method –comparison object used in preference to built in comparison ArrayList students = new ArrayList(); Student ann = new Student("Ann", 8000); Student bob = new Student("Bob", 2000); students.Add(ann); students.Add(bob); StudentNameComparer cmp = new StudentNameComparer(); students.Sort(cmp); comparer sort

26 Stack Stack provides last-in-first-out storage –data stored in array –array resized automatically as needed using System.Collections; class Trace { Stack callChain = new Stack();... } create Stack to store sequence of method calls

27 Stack services Stack offers standard last-in-first-out services –Push –Peek –Pop Stack s = new Stack(); s.Push("aaa"); s.Push("bbb"); string t = (string)s.Peek(); string u = (string)s.Pop();... add examine remove

28 Queue Queue provides first-in-first-out storage –data stored in array –array resized automatically as needed using System.Collections; class Watcher { Queue events = new Queue();... } create Queue to store events

29 Queue services Queue offers standard first-in-first-out services –Enqueue –Dequeue –Peek Queue q = new Queue(); q.Enqueue("aaa"); q.Enqueue("bbb"); q.Enqueue("ccc"); string s = (string)q.Peek(); string t = (string)q.Dequeue(); add examine remove

30 Hashtable Hashtable provides collection of key/value pairs –often called a map –data stored in hash table –stores object reference for both key and value –GetHashCode method of key used to determine placement –indexer provides convenient access Hashtable ages = new Hashtable(); ages["Ann"] = 27; ages["Bob"] = 32; ages.Add("Tom", 15); ages["Ann"] = 28; int a = (int)ages["Ann"]; create add update retrieve

31 Hashtable traversal Can traverse Hashtable contents –each element is DictionaryEntry struct –data exposed in Key and Value properties Hashtable ages = new Hashtable(); ages["Ann"] = 27; ages["Bob"] = 32; ages["Tom"] = 15; foreach (DictionaryEntry entry in ages) { string name = (string)entry.Key; int age = (int) entry.Value;... } enumerate entries get key and value

32 Summary Collections provided in System.Collections namespace –primary collections: Array ArrayList Hashtable –specialized collections: Stack Queue Collections store object reference –reference types are compatible –value types are automatically boxed Traversal supported –enumerators –foreach ArrayList can be sorted –must supply comparison method