5.2 Variables

A variable is an object 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. The previous section explained types. In this example, the variable's type is int (which is, as you've seen, a type of integer).

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 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 (i.e., 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.

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 2. You can also pass in substitution parameters. A substitution parameter is just a place holder 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 coming chapters.

Example 5-1 illustrates the use of variables. To test this program, open Visual Studio .NET and create a console application. Type in the code as shown.

Example 5-1. 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);
   }
}

Output:
Initialized, myInt: 7
After assignment, myInt: 5

Example 5-1 initializes the variable myInt to the value 7, displays that value, reassigns the variable with the value 5, and displays it again.