Introduction to Arrays

To declare a class as a member array of another class, you use the same technique as for primitive types. This means that you can declare the array somewhere in the body of the class. Before the variable can be used, make sure you have allocated memory for it, which is done using the new operator. In C# (unlike C++), you can allocate memory either when declaring the array or in a method that would precede its use. A constructor is a prime candidate for this operation.

After allocating memory for the variable, you can use as you see fit. If its values would only be made available to the users, you should make sure the variable has been initialized prior to being used. You can also initialize an array using values requested from the user. You can easily apply the techniques we have used so far on arrays.

1. To declare a class as an array member variable and apply what we have learned so far, change the ListCreator.cs file as follows:
    

   using System; namespace AutoParts { // This class is used to create and manage a list of auto parts // The list will be created as array-based class ListOfParts { // Because we are going to create an array-based list, // we will use this constant as its dimension const int MaxItems = 100; // An object declared as array Part[] Item = new Part[MaxItems]; // This will help us keep track on the number of items in the list int SizeOfList; // This method is used to add a new item to the list of auto parts public void Add(Part P) { // Before adding a new item, first make sure that we still have room if( SizeOfList < MaxItems ) { // If we still have room, add the new item to the array Item[SizeOfList] = P; // Increase the count of items of the list SizeOfList++; } } // This method simply return an item from the array, using a specified index public Part Retrieve(int n) { return Item[n]; } // This method returns the current number of items in the list public int Count() { return SizeOfList; } // Default Constructor: Used to initialize an object public ListOfParts() { // When this class is primarily accessed, we want to indicate that // its list is empty SizeOfList = 0; } } }

2. Save the file
3. Access the Exercise.cs file and change it as follows:
    

   using System; namespace AutoParts { // This class is used to perform a customer's order class OrderProcessing { static void Main() { ListOfParts lstParts = new ListOfParts(); ProcessAnItem(lstParts); Console.WriteLine(); DisplayReceipt(lstParts); } // This method is used to request a part number from the user // Check if that part number exists in the database // If it does, the method adds that part to the list static void ProcessAnItem(ListOfParts LOP) { Part AnItem; string PartID; int Qty; // Ask the user to enter a part number do { Console.Write("Enter the part number (q to stop): "); PartID = Console.ReadLine(); // Scan the list for(int i = 0; i < PartsList.Description.Length; i++) { AnItem = new Part(); // If the part number exists in our database if( PartID == PartsList.ItemNumber[i] ) { // Create a Part object from it AnItem.PartNumber = PartsList.ItemNumber[i]; AnItem.PartName = PartsList.Description[i]; AnItem.UnitPrice = PartsList.Price[i]; // Request the quantity from the user try { Console.Write("How many? "); Qty = int.Parse(Console.ReadLine()); AnItem.Quantity = Qty; } catch(FormatException) { Console.WriteLine("Invalid Quantity!!!"); } // Once the part has been "built", add it to the order LOP.Add(AnItem); // Check no further break; } } } while( PartID != "q" && PartID != "Q" ); } // This method is used to display a receipt // It uses a list, Receipt, passed as argument // It also calculates the price of each item and the total price of the order // They are also part of the receipt static void DisplayReceipt(ListOfParts Receipt) { decimal SubTotal = 0.00M, TotalOrder = 0.00M; Console.WriteLine("========================================================"); Console.WriteLine(" =-= Four-Corner Auto-Parts =-="); Console.WriteLine("------+---+-------------------------+-------+-----------"); Console.WriteLine("Part# Qty Description Price SubTotal"); Console.WriteLine("------+---+-------------------------+-------+-----------"); for(int i = 0; i < Receipt.Count(); i++) { //TPart Item = Receipt.Retrieve(i); SubTotal = Receipt.Retrieve(i).UnitPrice * Receipt.Retrieve(i).Quantity; TotalOrder += SubTotal; Console.WriteLine("{0} {1} {2} {3,6} {4,6}", Receipt.Retrieve(i).PartNumber, Receipt.Retrieve(i).Quantity, Receipt.Retrieve(i).PartName, Receipt.Retrieve(i).UnitPrice, SubTotal); } Console.WriteLine("------+---+-------------------------+-------+-----------"); Console.WriteLine("Total Order: {0:C}", TotalOrder); Console.WriteLine("========================================================\n"); } } }

4. Save the file
5. Access the Command Prompt and change to the above AutoParts1 folder
6. To compile it, type csc /reference:PartCreator.dll /out:"Four-Corner Auto-Parts".exe ListCreator.cs PartsList.cs Exercise.cs and press Enter
7. To execute it, type "Four-Corner Auto-Parts" and press Enter. Here is an example of processing an order:
    

   C:\>CD CSharp Lessons\Libraries1 C:\CSharp Lessons\Libraries1>csc /target:library /out:PartCreator.dll Parts.cs Microsoft (R) Visual C# .NET Compiler version 7.10.3052.4 for Microsoft (R) .NET Framework version 1.1.4322 Copyright (C) Microsoft Corporation 2001-2002. All rights reserved. C:\CSharp Lessons\Libraries1>CD\ C:\>CD CSharp Lessons\AutoParts1 C:\CSharp Lessons\AutoParts1>csc /reference:PartCreator.dll /out:"Four-Corner Au to-Parts".exe ListCreator.cs PartsList.cs Exercise.cs Microsoft (R) Visual C# .NET Compiler version 7.10.3052.4 for Microsoft (R) .NET Framework version 1.1.4322 Copyright (C) Microsoft Corporation 2001-2002. All rights reserved. C:\CSharp Lessons\AutoParts1>"Four-Corner Auto-Parts" Enter the part number (q to stop): LA943 How many? 1 Enter the part number (q to stop): TR944 How many? 2 Enter the part number (q to stop): EQ173 How many? 1 Enter the part number (q to stop): QT847 How many? 1 Enter the part number (q to stop): q ======================================================== =-= Four-Corner Auto-Parts =-= ------+---+-------------------------+-------+----------- Part# Qty Description Price SubTotal ------+---+-------------------------+-------+----------- LA943 1 Clutch Master Clndr 124.55 124.55 TR944 2 Front Wheel Lug Nut 1.75 3.50 EQ173 1 Oil Pump 155.75 155.75 QT847 1 Water Pump 12.95 12.95 ------+---+-------------------------+-------+----------- Total Order: $296.75 ========================================================

8. Return to Notepad

Like an array, a collection is a series of items of the same type. The problem with an array is that you must know in advance the number of items that will make up the list. There are cases you don't know, you can't know, or you can't predict the number of items of the list. The solution is to create a linked list. A linked list is a list in which you don't specify the maximum number of items but you allow the user of the list to add, locate, or remove items at will. Traditionally, it has never been easy to create a linked list, especially for beginning programmers. Aware of this, many programming languages ship with one or more libraries that can be used directly to create a list of almost any kind.

The Microsoft .NET Framework provides various classes that can be used to create different types of lists. Most of the classes are easy to use once you get used to them. The idea is to know what classes are available, what a class does, and what class to use for a particular assignment.