Data Structures and Collections Principles.NET: –Two libraries: System.Collections System.Collections.Generics FEN 2014UCN Teknologi/act2learn1 Deprecated New one

interface: (e.g. IDictionary) Specification class Appl{ ---- IDictionary dic; dic= new XXX(); application class: Dictionary SortedDictionary ---- ADT Data structure and algorithms Choose and use an adt, e.g. IDictionary Choose and use a data structure, e.g. Dictionary Know about Read and write (use) specifications Data Structures and Collections FEN 2014UCN Teknologi/act2learn2

Overview Abstract data types: –lists/sequences –stack –queue –set –table/map/dictionary.NET-specific: –Collections.Generics –IList<> –ISet<> –IDictionary<> Data structures: –static/dynamic –array –linked list –trees: Search trees –balanced –hashing Algorithms: –search –sweep –sorting –divide & conquer –recursion FEN 2014UCN Teknologi/act2learn3

.NET 2: System.Collections.Generics ICollection IList LinkedList IDictionary List Dictionary SortedDictionary Index able Array-based Balanced search tree Hashtabel (key, value) -pair FEN 2014UCN Teknologi/act2learn4

FEN 2014UCN Teknologi/act2learn5 Demos Lists Dictionaries LinkedList in C#LinkedList

How does they work? Array-based list Linked list FEN 2014UCN Teknologi/act2learn6 used Count Free (waste)

Dynamic vs. Static Data Structures Array-Based Lists: –Fixed (static) size (waste of memory). –May be able to grown and shrink (ArrayList), but this is very expensive in running time (O(n)) –Provides direct access to elements from index (O(1)) Linked List Implementations: –Uses only the necessary space (grows and shrinks as needed). –Overhead to references and memory allocation –Only sequential access: access by index requires searching (expensive: O(n)) FEN 2014UCN Teknologi/act2learn7

Hashing Keys are converted to indices in an array. A hash function, h maps a key to an integer, the hash code. The hash code is divided by the array size and the remainder is used as index If two or more keys gives the same index, we have a collision. FEN 2014UCN Teknologi/act2learn8

Chaining The array doesn’t hold the element itself, but a reference to a collection (a linked list for instance) of all colliding elements. On search that list must be traversed FEN 2014UCN Teknologi/act2learn9

Efficiency of Hashing Worst case (maximum collisions): –retrieve, insert, delete all O(n) Average number of collisions depends on the load factor, λ, not on table size λ = (number of used entries)/(table size) –But not on n. Typically (linear probing): numberOfCollisions avg = 1/(1 - λ) Example: 75% of the table entries in use: –λ = 0.75: 1/(1-0.75) = 4 collisions in average (independent of the table size). FEN 2014UCN Teknologi/act2learn10

When Hashing Is Inefficient Traversing in key order. Find smallest/largest key. Range-search (Find all keys between high and low). Searching on something else than the designated primary key. FEN 2014UCN Teknologi/act2learn11

FEN 2014UCN Teknologi/act2learn12 (Binary) Search Trees Value based container: –The search tree property: For any internal node: the value is greater than the value in the left child For any internal node: the value is less than the value in the right child –Note the recursive nature of this definition: It implies that all sub trees themselves are search trees Every operation must ensure that the search tree property is maintained

FEN 2014UCN Teknologi/act2learn13 Example: A Binary Search Tree Holding Names

FEN 2014UCN Teknologi/act2learn14 InOrder: Traversal Visits Nodes in Sorted Order

FEN 2014UCN Teknologi/act2learn15 Efficiency insert retrieve delete –All operations depend on the depth of the tree –If balanced: O(log n) Most libraries use a balanced version, for instance Red-Black Trees that guarantees O(log n) search, insert and delete. Easy to traverse in key-order. demos\Collections

Collections Library System.Collections Data structures in.NET are normally called Collections Are found in namespace System.Collections Compiled into mscorlib.dll assembly Uses object and polymorphism for generic containers. Deprecated! Classes: –Array –ArrayList –Hashtable –Stack –Queue FEN 2014UCN Teknologi/act2learn16 WARNING: Deprecated!

Collection Interfaces System.Collections implements a range of different interfaces in order to provide standard usage of different containers –Classes that implements the same interface provides the same services –Makes it easier to learn and to use the library –Makes it possible to write generic code towards the interface Interfaces: –ICollection –IEnumerable –IEnumerator –IList –IComparer –IComparable FEN 2014UCN Teknologi/act2learn17

ArrayList ArrayList stores sequences of elements. –duplicate values are ok – position- (index-) based –Elements are stored in an resizable array. –Implements the IList interface public class ArrayList : IList, IEnumerable,... { // IList services... // additional services int Capacity { get... set... } void TrimToSize() int BinarySearch(object value) int IndexOf (object value, int startIndex) int LastIndexOf (object value, int startIndex)... } control of memory in underlying array searching FEN 2014UCN Teknologi/act2learn18

IList Interface IList defineres sequences of elements –Access through index public interface IList : ICollection { 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; } bool IsReadOnly { get; } bool IsFixedSize { get; } } add new elements remove containment testing read/write existing element (see comment) structural properties FEN 2014UCN Teknologi/act2learn19

Hashtable Hashtable supports collections of key/value pairs –keys must be unique, values holds any data –stores object references at key and value –GetHashCode method on key determine position in the table. 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 FEN 2014UCN Teknologi/act2learn20

Hashtable Traversal Traversal of Hashtable –each element is of type DictionaryEntry (struct) –data is accessed using the 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 FEN 2014UCN Teknologi/act2learn21

”Generic” Programming in C#/Java (as it was until Summer 2005 – and you still see it, also in other languages) All classes inherit from Object So we can apply polymorphism and use Object as static type for elements in containers For instance: Object[ ] data –this array may take any object as element –This approach is well known from standard collections as ArrayList, HashTable etc. FEN 2014UCN Teknologi/act2learn22

Pros and Cons Pros –heterogeneous collections –... Cons –many type casts –not type safe type checking is done runtime when casting –int and other native (value) type must be wrapped. (boxing – costs runtime overhead) Is this really an advantage? FEN 2014UCN Teknologi/act2learn23

The Idea: Types as Parameters C# before 2005: ArrayList al = new ArrayList(); Customer c= (Customer)al[i];//cast Instead we want something like: List al = new List (); Customer c= al[i]; – The compiler is able to check that only objects with static type Customer is placed in al – So the compiler knows that everything that may come out from al has static type Customer –So static type checking instead of dynamic type checking is possible –Dynamic casting can be avoided (but is not in all implementations) Type parameter FEN 2014UCN Teknologi/act2learn24

In C#: EmpSeqApplEmpSeqAppl Employee a1 = new Employee("Joe", "Programmer", 10000); Employee a = new Employee("Curt", "Senior Programmer", 20000); Employee b = new Employee("Carl", "Programmer", 10000); Employee c = new Employee("Karen", "System Programmer", 13000); Employee d = new Employee("Lisa", "Programmer", 11000); Employee e = new Employee("John", "System Engineer", 9000); string s = "HELLOOOO!"; ArrayList emps = new ArrayList(); //IList emps = new List (); emps.Add(a1); emps.Add(a); emps.Add(b); emps.Add(c); emps.Add(d); emps.Add(e); emps.Add(s); //no errors //emps.Add(s); //COMPILER ERROR!!!! FEN 2014UCN Teknologi/act2learn25