3.3. Variables A variable is an instance of an intrinsic type (such as int ) that can hold a value: int myVariable = 15;
You initialize a variable by writing its type, its identifier , and then assigning a value to that variable. An identifier is just an arbitrary name you assign to a variable, method, class, or other element. In this case, the variable's identifier is myVariable . You can define variables without initializing them: int myVariable;
You can then assign a value to myVariable later in your program: int myVariable; // some other code here myVariable = 15; // assign 15 to myVariable
You can also change the value of a variable later in the program. That is why they're called variables; their values can vary. int myVariable; // some other code here myVariable = 15; // assign 15 to myVariable // some other code here myVariable = 12; // now it is 12
Technically, a variable is a named storage location (that is, stored in memory) with a type. After the final line of code in the previous example, the value 12 is stored in the named location myVariable . Example 3-1 illustrates the use of variables. To test this program, open Visual Studio .NET and create a console application. Type in the code as shown. WriteLine( ) The .NET Framework provides a useful method for displaying output on the screen in console applications: System.Console.WriteLine( ) . How you use this method will become clearer as you progress through the book, but the fundamentals are straightforward. You call the method, passing in a string that you want printed to the console (the screen), as in the Hello World application in Chapter 1. You can also pass in substitution parameters. A substitution parameter is just a placeholder for a value you want to display. For example, you might pass in the substitution parameter {0}, and then when you run the program, you'll substitute the value held in the variable myInt , so that its value is displayed where the parameter {0} appears in the WriteLine( ) statement. Here's how it works. You place a number between braces: System.Console.WriteLine("After assignment, myInt: {0}", myInt);
Notice that you follow the quoted string with a comma and then a variable name. The value of the variable will be substituted into the parameter. Assuming myInt has the value 15, the statement shown previously causes the following to display: After assignment, myInt: 15
If you have more than one parameter, the variable values will be substituted in order, as in the following: System.Console.WriteLine("After assignment, myInt: {0} and myOtherInt: {1}", myInt, myOtherInt);
Assuming myInt has the value 15, and myOtherInt has the value 20, this will cause the following to display: After assignment, myInt: 15 and myOtherInt: 20.
You'll see a great deal more about WriteLine( ) in later chapters. |
Example 3-1. Using variables class Values { static void Main( ) { int myInt = 7; System.Console.WriteLine("Initialized, myInt: {0}", myInt); myInt = 5; System.Console.WriteLine("After assignment, myInt: {0}", myInt); } }
| Press F5 to build and run this application; the output looks like this: Initialized, myInt: 7 After assignment, myInt: 5
Example 3-1 initializes the variable myInt to the value 7, displays that value, reassigns the variable with the value 5, and displays it again. |
