NOTES: Procedures (module 15)

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Computer Science 110 NAME: NOTES: Procedures (module 15) Introduction to Procedures When you use a top-down, structured program design, you take a problem, analyze it to determine what the outcome

After following these steps from beginning to end, you should arrive at a satisfactory resolution to the problem. For example, you may be presented with the problem of writing a payroll application.

1 Computer Science 110 NAME: NOTES: Procedures (module 15) Introduction to Procedures When you use a top-down, structured program design, you take a problem, analyze it to determine what the outcome should be and then decide the best way to arrive at that output. Your analysis should result in the development of a series of steps that you must follow to arrive at the desired output. After following these steps from beginning to end, you should arrive at a satisfactory resolution to the problem. For example, you may be presented with the problem of writing a payroll application. You determine that you will need, among other things, a way to calculate the amount of income tax that must be deducted from each employee. In your payroll application, you will use a procedure to calculate the income tax. Visual Basic procedures are used to break problems into manageable sections of code. A procedure is a block of code that performs a specific task. In this module, you will learn how to write both general procedures and function procedures. Sub Procedures To review, a procedure is a block of code that performs a specific task. The now familiar cmddone_click( ) event is an example of a procedure. This procedure performs the task of ending a program. There are two types of Sub procedures: 1. Event procedures, which are referred to as Sub Procedures 2. General procedures, which are user-defined procedures that must be called from an event procedure General Procedures There are many tasks that must be carried out as a program runs. The programmer writes general procedures, or small blocks of code, to carry out each task. Even if a task must be performed many times, the code only needs to be written once. Each procedure that is written simplifies the program by breaking the program down into smaller blocks of code that are easier to work with. Let's look at an example of how a general procedure is used to clear a caption. In this example, a label has a caption. This caption must be cleared several times as the program runs. To do this, the programmer would write a general procedure that contains a block of code to clear the caption. This procedure is then called whenever the caption needs to be cleared. The syntax for a general procedure is shown: The following code shows an example of a general procedure: Sub ClearLabelInstructions( ) lblinstructions.caption = "" A general procedure must be called from another procedure in order to be executed. The Call Statement A general procedure such as the one shown from the previous lesson: Sub ClearLabelInstructions( ) lblinstructions.caption = "" must be called from another procedure in order to be executed. The Call statement is used to execute a general procedure. The following is an example: Private Sub optbeachinstructions_click ( ) Call ClearLabelInstructions Call ToBeach

In the event procedure shown above, there are two Call statements (shown in green type). The first one executes the ClearLabelInstructions() general procedure.

2 In the event procedure shown above, there are two Call statements (shown in green type). The first one executes the ClearLabelInstructions() general procedure. The second one executes the ToBeach()general procedure (not shown). Let's look at a section of code that contains both the ClearLabelInstructions and the ToBeach procedures. The user interface is as shown above. Notice, in the code that follows, the use of the line continuation character ( _ ) to join long statements from one line to the next. Private Sub Form_Load( ) optconveniencestoreinstructions.value = False optvideostoreinstructions.value = False optbeachinstructions.value = False Private Sub optbeachinstructions_click( ) Call ClearLabelInstructions Call ToBeach Sub ClearLabelInstructions() lblinstructions.caption = "" Sub ToBeach() Call ToOceanViewDrive lblinstructions.caption = lblinstructions.caption & _ "Head east on Ocean View Drive. Beach is to the_ left." & vbcrlf Sub ToOceanViewDrive() Call ToMaplesTrail lblinstructions.caption = lblinstructions.caption &_ "Head south on Maples Trail to Ocean View Drive." &_ vbcrlf Sub ToMaplesTrail() lblinstructions.caption = "Head east on Sunny Lane to_ Maples Trail." _ & vbcrlf When the user clicks the Beach button on the interface, the optbeachinstructions_click event procedure is executed: This procedure calls ClearLabelInstructions to clear the lblinstructions label. When ClearLabelInstructions is finished executing, control of the program passes back to optbeachinstructions_click. The next statement in the optbeachinstructions_click procedure calls the ToBeach procedure. This procedure calls the ToOceanViewDrive procedure. ToOceanViewDrive procedure calls the ToMaplesTrail procedure. ToMaplesTrail procedure assigns to lblinstructions a string of instructions to Maples Trail and a carriage return-line feed. ToOceanViewDrive procedure and ToBeach concatenate additional instructions and a carriage return-line feed to the lblinstructions label. Creating a General Procedure in the Code Window General procedures may be added to the Code window in two ways: 1. Type Sub, followed by the procedure name and then press Enter. Visual Basic then automatically adds an statement and places the cursor in the body of the procedure. 2. With the Code window open, select Tools Add Procedurefrom the menu bar. The Add Procedure command displays a dialogue box, as shown. From the dialogue box, the programmer selects the procedure name, type and scope. After selecting OK, the procedure is placed in alphabetical order in the Code window.

Parameters and Passing Arguments by Value A procedure often needs additional data to be able to complete its task. The extra data is enclosed in parentheses at the end of the procedure call.

3 Parameters and Passing Arguments by Value A procedure often needs additional data to be able to complete its task. The extra data is enclosed in parentheses at the end of the procedure call. When the procedure is called, the extra data is given, or passed, to it. For example, in the statement Call GiveClue (intsecretnumber, intguess) the GiveClue procedure is called and two values are passed to it intsecretnumber and intguess. A variable or a value that is passed to a procedure is called an argument. In our example above, intsecretnumber and intguess are arguments that will be used by the procedure. A procedure that requires arguments is declared with parameters. The following syntax is used for a procedure with parameters: Sub ProcedureName(ByVal parameter1 As Type, ) statements Where: ProcedureName is the name of the procedure. ByVal indicates that the parameter is a value parameter. parameter1 is the name of the parameter. As Type is the data type of the parameter....statements is the body of the procedure. It is possible to have multiple parameters. Each parameter must be separated by a comma. ByVal and ByRef: ByVal indicates that the parameter is a value parameter. This means that the parameter's value is the only thing passed. ByRef indicates that the parameter is a reference parameter. This means that the reference to a variable would be passed as the parameter. Therefore, changes could be made to the variable s value which is not the case with ByVal. Parameters and Passing Arguments by Value (Continued) Once again, let's look at the GiveClue procedure: Sub GiveClue(ByVal intfirstnum As Integer, ByVal intsecondnum As Integer) If intfirstnum > intsecondnum Then MsgBox "Too low" Else MsgBox "Too high" End If When the GiveClue procedure is called with this statement: Call GiveClue (intsecretnumber, intguess) The value of the first argument, intsecretnumber, is assigned to the first parameter, intfirstnum. The value of the second argument, intguess, is assigned to the second parameter, intsecondnum. The image shows the two arguments in the Call statement being passed to the parameters declared in the GiveClue procedure. When a procedure is called, the number of arguments passed to it must match the number of parameters that have been declared. It is important to remember the following points: The order in which arguments are passed is important, as: The order of the arguments must correspond to the order of the parameters. For example, the first argument in the procedure call (intsecretnumber) corresponds to the first parameter(intfirstnum) and so on. Arguments passed by value can be in the form of a constant, a variable, values or expressions.

For example, the GiveClue Call statement could look like any of these: Call GiveClue(intSecretNumber, intguess) Call GiveClue(2*5, 10*2) Call GiveClue(10, intguess) Call GiveClue(10,20) Variable

4 For example, the GiveClue Call statement could look like any of these: Call GiveClue(intSecretNumber, intguess) Call GiveClue(2*5, 10*2) Call GiveClue(10, intguess) Call GiveClue(10,20) Variable arguments that are passed by value are not changed by the procedure. For example, examine each line of code and its corresponding comment in green type: Sub Demo( ) Dim intcounter As Integer 'declares variable intcounter intcounter = 1 'assigns value of 1 to intcounter Call ShowCount(intCounter) 'call ShowCount procedure and passes_ copy of intcounter value(1) as argument to ShowCount procedure lblnumber.caption = intcounter 'Demo procedure displays value of_ intcounter(1) in the label. Value of intcounter(1) is_ unaffected by the ShowCount procedure Sub ShowCount(ByVal intcounter As Integer) 'parameter intcount_ receives value of 1 passed to it intcounter = intcounter + 1 'ShowCount procedure increments its_ intcounter variable by one to make value of intcounter 2 MsgBox intcounter 'ShowCount procedure displays value of_ intcounter(2) in the message box Static Variables Recall from Module 12 that variables may be local or global. Also, we saw that the scope of a variable refers to its accessibility to various procedures. A local variable is only available to the procedure in which it is declared, while a global variable is available to every procedure in the program module. In addition to scope, variables also have a lifetime in which they exist in memory. The lifetime of a local variable begins when the procedure is declared and ends when the procedure finishes execution. In other words, the local variable exists in memory while the procedure is executing. The end of the procedure signifies the end of the local variable's lifetime in memory. On the other hand, a global variable exists in memory for the duration of the program regardless of whether a procedure is executing or not. When the program is terminated, the global variable's lifetime is terminated. Variables may also be declared as static. When a variable is declared as static, the variable's: Scope is local to the procedure in which it is declared Lifetime is the duration of the program A static variable is declared using the Visual Basic keyword Static instead of the keyword Dim. It is good programming style to use static variables rather than global variables because the use of static variables does not interfere with other procedures. In the next exercise, we will write a program to add numbers. The program will use a static variable to maintain a running total. 1. Start the Visual Basic IDE. 2. Create a new project. 3. Design the interface as shown on the right: 4. Set the properties as shown in the table below: Object Name Caption Text Form1 frmaddingmachine Adding Machine! Label1 lbldirections Number to be added to the total is Text1 txtnewnum empty Label2 lblnumbersadded empty Label3 lbltotal empty Command1 cmdadd Click to Add Command2 cmddone Done

5 Once you have set the properties: 5. Save the project. Save the form as frmaddingmachine. Save the project as Adding Machine. 6. Double-click the Click to Add command button to display the Code window. Add the following code to the cmdadd_click procedure: Private Sub cmdadd_click() Dim dblnumberentered As Double Static dbltotal As Double 'declares static variable dblnumberentered = txtnewnum.text dbltotal = dbltotal + dblnumberentered 'keeps running total lblnumbersadded.caption = lblnumbersadded & " " _ & dblnumberentered & "+" lbltotal.caption = "Total = " & dbltotal Private Sub cmddone_click() Unload Me Resave your project. Run the application. Test the application by entering numbers and clicking the Click to Add button. Function Procedures A function, sometimes called a function procedure, performs a specific task and returns a value. Functions may be: Built-in User-defined We have already looked at several Visual Basic built-in functions in previous lessons, such as the built-in functions called Int, Lcase and Len. Functions may also be user-defined. User-defined functions use the following syntax: A function usually has at least one parameter. This parameter is for data that is required for the function to be able to perform its task. Parameters are ByVal because a function performs a task and returns a single value. The function should not change the arguments that it has been passed. Functions are useful for verifying user input. For example, an application that asks the user to enter a value between one and ten could use a user-defined Boolean function to determine if the user input falls within the correct range. The following is the code for a user-defined function called ValidEntry. This function will be used to check that the values entered by the user are within a predetermined range. The function parameters set the lower limit of the range to one and the upper limit of the range to ten.

6 Function ValidEntry(ByVal intusernum As Integer, ByVal intupperlimit As Integer, _ ByVal intlowerlimit As Integer)As Boolean If intusernum > intupperlimit Or intusernum < intlowerlimit Then ValidEntry = False Else ValidEntry = True End If End Function This code uses the ValidEntry function to check the user input: intguess = txtuserguess.text If Not ValidEntry(intGuess, 10,1) Then MsgBox "Invalid Guess. Please try again." End If The Boolean value that is returned by the ValidEntry function is then used as the condition for the If Then statement in the ValidEntry function. When working with functions, remember: The order of the arguments must match the order of the parameters. Only ByVal parameters should be declared in a function. A function should not change the arguments that are passed. A function returns a single value. This value will be used in another statement such as an assignment statement. For example, in the code shown in the preceding lesson, the Boolean value that is returned becomes the condition of the If Then statement.

NOTES: Variables & Constants (module 10)

Computer Science 110 NAME: NOTES: Variables & Constants (module 10) Introduction to Variables A variable is like a container. Like any other container, its purpose is to temporarily hold or store something.