Variable names such as number1, number2 and sum actually correspond to locations in the computer's memory. Every variable has a name, a type, a size and a value.
In the addition application of Fig. 3.18, when the statement (line 16)
number1 = Convert.ToInt32( Console.ReadLine() );
executes, the number typed by the user is placed into a memory location to which the name number1 has been assigned by the compiler. Suppose that the user enters 45. The computer places that integer value into location number1, as shown in Fig. 3.19. Whenever a value is placed in a memory location, the value replaces the previous value in that location and the previous value is lost.
Figure 3.19. Memory location showing the name and value of variable number1.
When the statement (line 20)
number2 = Convert.ToInt32( Console.ReadLine() );
executes, suppose that the user enters 72. The computer places that integer value into location number2. The memory now appears as shown in Fig. 3.20.
Figure 3.20. Memory locations after storing values for number1 and number2.
After the application of Fig. 3.18 obtains values for number1 and number2, it adds the values and places the sum into variable sum. The statement (line 22)
sum = number1 + number2; // add numbers
performs the addition, then replaces sum's previous value. After sum has been calculated, memory appears as shown in Fig. 3.21. Note that the values of number1 and number2 appear exactly as they did before they were used in the calculation of sum. These values were used, but not destroyed, as the computer performed the calculationwhen a value is read from a memory location, the process is nondestructive.
Figure 3.21. Memory locations after calculating and storing the sum of number1 and number2.
Introduction to Computers, the Internet and Visual C#
Introduction to the Visual C# 2005 Express Edition IDE
Introduction to C# Applications
Introduction to Classes and Objects
Control Statements: Part 1
Control Statements: Part 2
Methods: A Deeper Look
Classes and Objects: A Deeper Look
Object-Oriented Programming: Inheritance
Polymorphism, Interfaces & Operator Overloading
Graphical User Interface Concepts: Part 1
Graphical User Interface Concepts: Part 2
Strings, Characters and Regular Expressions
Graphics and Multimedia
Files and Streams
Extensible Markup Language (XML)
Database, SQL and ADO.NET
ASP.NET 2.0, Web Forms and Web Controls
Networking: Streams-Based Sockets and Datagrams
Searching and Sorting
Appendix A. Operator Precedence Chart
Appendix B. Number Systems
Appendix C. Using the Visual Studio 2005 Debugger
Appendix D. ASCII Character Set
Appendix E. Unicode®
Appendix F. Introduction to XHTML: Part 1
Appendix G. Introduction to XHTML: Part 2
Appendix H. HTML/XHTML Special Characters
Appendix I. HTML/XHTML Colors
Appendix J. ATM Case Study Code
Appendix K. UML 2: Additional Diagram Types
Appendix L. Simple Types