CS313D: ADVANCED PROGRAMMING LANGUAGE

Transcription

1 CS313D: ADVANCED PROGRAMMING LANGUAGE Computer Science department Lecture 8 : 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 GetEnumerator. Some collection types offer the GetEnumerator method. This method returns an Enumerator object that can be used to loop through the collection. Complete knowledge of the actual collection type is not needed. interfaces of the.net Framework collections Implementations of these interfaces are provided within the framework.

7 7 collection classes

8 Class Array and Enumerators 8 Use Array static methods to: Copy arrays Sort array elements Search array elements

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10 Line 22 uses static Array method Sort to sort array doublevalues. When this method returns, the array contains its original elements sorted in ascending order. 10

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12 Line 25 uses static Array method Copy to copy elements from array intvalues to array intvaluescopy. 12

13 Lines 32 and 40 invoke static Array method BinarySearch to perform binary searches on array intvalues. Method BinarySearch receives the sorted array in which to search and the key for which to search. The method returns the index in the array at which it finds the key (or a negative number if the key was not found). BinarySearch assumes that it receives a sorted array. 13

14 Enumerator!! 14 GetEnumerator returns an enumerator that can iterate over the collection.

15 Class Array and Enumerators 15 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).

16 16 ArrayList

17 Class ArrayList 17 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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19 Example 19 Use an ArrayList to store string objects Use its methods to add, remove, search elements

20 Code 20 ArrayList constructor accepts a preallocated array!!

21 Code 21 Count vs. Capacity IndexOf returns the position of the element or a negative value if not found

22 code 22 Using Indexers Remove method applies a linear search and deletes only the first occurrence

23 ArrayList & polymorphism 23 elements positions S f l

24 24 Class Stack

25 What is a stack? 25 LIFO data structures

26 Example 26 Using directive Creating a stack of Object references Elements of different types boxing

27 Example 27 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

28 Example 28 Printing elements without modification property Count obtains the number of elements in stack.

29 Output 29

30 30 Class Hashtable

31 What is a Hashtable? 31 When we convert a key into an array index, we literally scramble the bits, making a hash of the number.

32 Why Hashtable? 32 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.

33 33 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 dr. Amal but Khalifa, in reverse. Spr 17

34 Collisions 34 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

35 Hash Function 35 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.

36 Example 36 Creating a HashTable of <string, int>

37 Example 37 unbox and box the int data stored in the Hashtable every time it incremented the count for a particular key inefficient Downcasting boxing & untboxing

38 Example 38 If we need to use any of its stringspecific methods, we need an explicit downcast.

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40 40 Take care!!

41 41 That s all,,,, Chapter 21:

42 42 Case Study

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