CS313D: ADVANCED PROGRAMMING LANGUAGE

Transcription

1 CS313D: ADVANCED PROGRAMMING LANGUAGE Computer Science department Lecture 7 : Collections

2 Lecture Contents 2 Why collections? What is a collection? Non-generic collections: Array & ArrayList Stack HashTable

3 Why collections? 3 Data structures are essential to build apps, but there s no need to build such them from scratch. Instead, you can use the prepackaged collection classes provided by the.net Framework. System.Collections contains collections that store references to objects.

4 What is a collection? 4 Collection classes store collections of data Each instance of one of these classes is a collection of items. Examples: Arrays Stacks Queues HashTables

5 5 interfaces of the.net Framework collections Implementations of these interfaces are provided within the framework.

6 6 collection classes

7 Class Array and Enumerators 7 Use Array static methods to: Copy arrays Sort array elements Search array elements

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10 Enumerator!! 10 GetEnumerator returns an enumerator that can iterate over the collection.

11 Class Array and Enumerators 11 Other static Array methods include Clear (to set a range of elements to 0, false or null, as appropriate) CreateInstance (to create a new array of a specified type) IndexOf (to locate the first occurrence of an object in an array or portion of an array) LastIndexOf (to locate the last occurrence of an object in an array or portion of an array) Reverse (to reverse the contents of an array or portion of an array).

12 Class ArrayList 12 ArrayList collection class mimics the functionality of conventional arrays and provides dynamic resizing of the collection through the class s methods. At any time, an ArrayList contains a certain number of elements less than or equal to its capacity the number of elements currently reserved for the ArrayList. An app can manipulate the capacity with ArrayList property Capacity.

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14 Example 14 Use an ArrayList to store string objects Use its methods to add, remove, search elements

15 Code 15 ArrayList constructor accepts a preallocated array!!

16 Code 16 Count vs. Capacity IndexOf returns the position of the element or a negative value if not found

17 code 17 Using Indexers Remove method applies a linear search and deletes only the first occurrence

18 ArrayList & polymorphism 18 elements positions S f l

19 19 Class Stack

20 What is a stack? 20 LIFO data structures

21 Example 21 Using directive Creating a stack of Object references Elements of different types boxing

22 Example 22 Push adds an object on the top of the stack grows to accommodate new elements Pop removes the object on the top of the stack peek doesn t remove the top element

23 Example 23 Printing elements without modification property Count obtains the number of elements in stack.

24 Output 24

25 25 Class HashTable

26 What is a Hashtable? 26 When we convert a key into an array index, we literally scramble the bits, making a hash of the number.

27 Why Hashtable? 27 Storing Large data, efficient retrieve, saving memory space Example: 100 employees with nine-digit social security numbers you want to store and retrieve employee data by using the social security number as a key nominally require an array with 1,000,000,000 elements. Hashing: a high-speed scheme for converting keys such as social security numbers and inventory part numbers to unique array indices.

28 28 Hashing when an app needs to store something, the scheme could convert the key rapidly to an index and the record of information could be stored at that location in the array Retrieval occurs the same way but in reverse.

29 Collisions 29 two different keys hash into the same cell, or element, in the array we cannot sort two different data records to the same space Solutions: hash again - find an alternative home for all records beyond the first that hash to a particular array index. hash bucket - a linked list of all the key value pairs that hash to that cell

30 Hash Function 30 A hash function performs a calculation that determines where to place data in the hash table. The hash function is applied to the key in a key value pair of objects. Class Hashtable can accept any object as a key. class object defines method GetHashCode, which all objects inherit.

31 31 Example

32 Example 32 unbox and box the int data stored in the Hashtable every time it incremented the count for a particular key ineffecient

33 Example 33 If we need to use any of its stringspecific methods, we need an explicit downcast.

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35 35 Take care!!

36 36 That s all,,,, Chapter 21: