Collections Objectives Explore collections in System.Collections - PDF document

Collections

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

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

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

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 5

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 { create ArrayList ArrayList employees = new ArrayList(); to store Employees ... } ArrayList employees object array of object references 6

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

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

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"); Count is 3 int n = a.Count; 9

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 10

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"); remove element a.RemoveAt(1); at index 1 11

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"); remove element a.Remove("aaa"); equal to "aaa" 12

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"); contains? if (a.Contains("aaa")) Console.WriteLine("found"); else Console.WriteLine("not found"); 13

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"); returns 1 int i = a.IndexOf("aaa"); ... 14

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(); use Add to add a.Add("bbb"); new elements a.Add("aaa"); a.Add("ddd"); change existing element a[2] = "ccc"; string s = (string)a[2]; read existing element 15

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 default capacity is 16 ArrayList a = new ArrayList(); ArrayList b = new ArrayList(50); specify capacity of 50 b.Capacity = 100; set capacity to 100 int c = b.Capacity; get capacity 16

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); boxed int y = (int)a[0]; unboxed 17

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 { get enumerator IEnumerator GetEnumerator(); from collection } public interface IEnumerator { access data object Current { get; } traverse bool MoveNext(); void Reset(); } 18

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"); get enumerator IEnumerator e = a.GetEnumerator(); from collection advance and test while (e.MoveNext()) { access and string t = (string)e.Current; cast return ... } 19

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"); handles loop foreach (string s in a) and cast { ... } 20

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 { built-in void Sort() ... compare separate void Sort(IComparer comparer) ... compare void Sort(int index, int count, IComparer comparer) ... ... } 21

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; cast this goes first if (this.id < s.id) return -1; if (this.id > s.id) return 1; parameter goes first equal return 0; } ... } 22

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); sort according students.Sort(); to CompareTo 23

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; cast Student s2 = o2 as Student; compare return s1.name.CompareTo(s2.name); names } } 24

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); comparer StudentNameComparer cmp = new StudentNameComparer(); students.Sort(cmp); sort 25

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

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