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Chapter 6 - Procedures

Chapter 6 - Procedures.

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Chapter 6 - Procedures

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  1. Chapter 6 - Procedures Outline6.1 Introduction6.2   Modules, Classes and Procedures6.3   Sub Procedures6.4   Function Procedures6.5   Methods6.6   Argument Promotion6.7   Option Strict and Data Type Conversions6.8   Value Types and Reference Types6.9   Passing Arguments: Pass-by-Value vs. Pass-by-Reference6.10   Duration of Identifiers6.11   Scope Rules6.12   Random-Number Generation6.13   Example: Game of Chance6.14   Recursion6.15   Example Using Recursion: The Fibonacci Series6.16   Recursion vs. Iteration

  2. Outline6.17   Procedure Overloading and Optional Arguments 6.17.1  Procedure Overloading 6.17.2  Optional Arguments6.18   Modules

  3. 6.2 Modules, Classes and Procedures • Framework Class Library • Provides a rich collection of “prepackaged” classes and methods for performing many operations • Mathematical calculations • String manipulations • Character manipulations • Input/output operations • Error checking

  4. 6.2 Modules, Classes and Procedures • Programmer-defined procedures • FCL cannot provide every conceivable feature that a programmer could want • Three types of procedures • Sub procedures • Function procedures • Event procedures • A procedure is invoked by a procedure call

  5. 6.2 Modules, Classes and Procedures • Division of code into procedures • Several motivations to divide code into procedures • Divide-and-conquer approach makes program development more manageable • Software reusability • Avoids the repetition of code in a program

  6. 6.2 Modules, Classes and Procedures • Earlier programs had only one procedure that called FCL methods • Next program contains two customized procedures

  7. PrintPayreceives the values of each argument and stores them in the parameters variables hours and wage PrintPay executes when it is invoked by Main Notice that PrintPay appears within modPayment. All procedures must be defined inside a module or a class 1 ' Fig. 6.2: Payment.vb 2 ' Sub procedure that prints payment information. 3 4 Module modPayment 5 6 Sub Main() 7 8 ' call Sub procedure PrintPay 4 times 9 PrintPay(40, 10.5) 10 PrintPay(38, 21.75) 11 PrintPay(20, 13) 12 PrintPay(50, 14) 13 14 Console.ReadLine() ' prevent window from closing 15 End Sub' Main 16 17 ' print amount of money earned in command window 18 Sub PrintPay(ByVal hours AsDouble, ByVal wage AsDecimal) 19 20 ' pay = hours * wage 21 Console.WriteLine("The payment is {0:C}", hours * wage) 22 EndSub' PrintPay 23 24 EndModule' modPayment Payment.vbProgram Output The payment is $420.00 The payment is $826.50 The payment is $260.00 The payment is $700.00

  8. 6.3 Sub Procedures • Format of a procedure definition Sub procedure-name(parameter-list) declarations and statements EndSub • Procedure header • The first line is known as the procedure header • Procedure-name • Directly follows the Sub keyword • Can be any valid identifier • It is used to call this Sub procedure within the program • Procedure body • The declarations and statements in the procedure definition form the procedure body

  9. 6.4 Function Procedures • Similar to Sub procedures • One important difference • Function procedures return a value to the caller

  10. The For structure displays the results of squaring the Integers from 1-10 Square is invoked with the expression Square(i) The Return statement terminates execution of the procedure and returns the result of y^2 1 ' Fig. 6.3: SquareInteger.vb 2 ' Function procedure to square a number. 3 4 ModulemodSquareInteger 5 6 Sub Main() 7 Dim i As Integer ' counter 8 9 Console.WriteLine("Number" & vbTab & "Square" & vbCrLf) 10 11 ' square numbers from 1 to 10 12 For i = 1To10 13 Console.WriteLine(i & vbTab & Square(i)) 14 Next 15 16 End Sub ' Main 17 18 ' Function Square is executed 19 ' only when the function is explicitly called. 20 Function Square(ByVal y As Integer) As Integer 21 Return y ^ 2 22 End Function' Square 23 24 End Module' modSquareInteger SquareInteger.vb

  11. Number Square 1 1 2 4 3 9 4 16 5 25 6 36 7 49 8 64 9 81 10 100 SquareInteger.vbProgram Output

  12. 6.4 Function Procedures • Format of a Function procedure definition Function procedure-name(parameter-list) As return-type declarations and statements EndFunction • Return-type • Indicates the data type of the result returned from the Function to its caller • Return expression • Can occur anywhere in a Function • It returns exactly one value • Control returns immediately to the point at which that procedure was invoked

  13. 6.5 Methods • Definition of method • A method is any procedure that is contained within a class • FCL methods • Custom methods in programmer-defined classes

  14. Remember that all forms inherit from class System.Windows.Forms.Form Event handler cmdMaximum_ClickHandles the event in which ButtoncmdMaximum is clicked These are declarations of all the controls used in the GUI. Create these components visually, using the Toolbox 1 ' Fig. 6.4: Maximum.vb 2 ' Program finds the maximum of three numbers input. 3 4 PublicClass FrmMaximum 5 Inherits System.Windows.Forms.Form 6 7 ' prompts for three inputs 8 FriendWithEvents lblOne As System.Windows.Forms.Label 9 FriendWithEvents lblTwo As System.Windows.Forms.Label 10 FriendWithEvents lblThree As System.Windows.Forms.Label 11 12 ' displays result 13 FriendWithEvents lblMaximum As System.Windows.Forms.Label 14 15 ' read three numbers 16 FriendWithEvents txtFirst As System.Windows.Forms.TextBox 17 FriendWithEvents txtSecond As System.Windows.Forms.TextBox 18 FriendWithEvents txtThird As System.Windows.Forms.TextBox 19 20 ' reads inputs and calculate results 21 Friend WithEvents cmdMaximum As System.Windows.Forms.Button 22 23 ' Visual Studio .NET generated code 24 25 ' obtain values in each text box, call procedure Maximum 26 PrivateSub cmdMaximum_Click(ByVal sender As System.Object, _ 27 ByVal e As System.EventArgs) Handles cmdMaximum.Click 28 Maximum.vb

  15. The values in the three TextBoxes are retrieved using the Text property Call to methods that are defined in a class in the FCL must include the class name and the dot (.) operator Call to methods defined in the class that contains the method call need only specify the method name 29 Dim value1, value2, value3 AsDouble 30 31 value1 = txtFirst.Text 32 value2 = txtSecond.Text 33 value3 = txtThird.Text 34 35 lblMaximum.Text = Maximum(value1, value2, value3) 36 EndSub ' cmdMaximum_Click 37 38 ' find maximum of three parameter values 39 Function Maximum(ByVal valueOne AsDouble, _ 40 ByVal valueTwo AsDouble, ByVal valueThree AsDouble) 41 42 Return Math.Max(Math.Max(valueOne, valueTwo), valueThree) 43 EndFunction ' Maximum 44 45 EndClass ' FrmMaximum Maximum.vbProgram Output

  16. 6.5 Methods Parameter Info window Fig. 6.5 Parameter Info feature of the Visual Studio .NET IDE.

  17. 6.5 Methods Fig. 6.6 IntelliSense feature of the Visual Studio .NET IDE.

  18. 6.5 Methods Fig. 6.7 Math class methods.

  19. 6.5 Methods Fig. 6.7 Math class methods.

  20. 6.6 Argument Promotion • Coercion of arguments • The forcing of arguments to be appropriate data type so that they can be passed to a procedure • Widening conversion • Occurs when a type is converted to another type without losing data • Narrowing conversion • Occurs when there is potential for data loss during the conversion

  21. 6.6 Argument Promotion Fig. 6.8 Widening conversions.

  22. 6.7 Option Strict and Data-Type Conversions • Option Explicit • Set to On by default • Forces the programmer to declare explicitly all variables before they are used • Option strict • Set to Off by default • When set to On, it forces the programmer to perform an explicit conversion for all narrowing conversions • Class Convert • The methods in class Convert change data types explicitly

  23. 6.7 Option Strict and Data-Type Conversions Fig. 6.9 Property Pages dialog with Option Strict set to On.

  24. 6.8 Value Types and Reference Types • Variable of a value type • Contains the actual data • Used for a single piece of data • Integer • Double • Variable of a reference type • Contains a location in memory where • Known as objects • Literals • Values typed directly in program code • Corresponds to one of the primitive data types

  25. 6.8 Value Types and Reference Types Fig. 6.10 Visual Basic primitive data types.

  26. 6.8 Value Types and Reference Types Fig. 6.11 Literals with type characters.

  27. 6.9 Passing Arguments: Pass-by-Value vs. Pass-by-Reference • Pass-by-value • The program makes a copy of the argument’s value and passes that copy to the called procedure • Pass-by-reference • The caller gives the called procedure the ability to access and modify the caller’s original data directly.

  28. When number1 is passed, a copy of the value is passed to the procedure A reference to the value stored in number2 is being passed 1 ' Fig. 6.12: ByRefTest.vb 2 ' Demonstrates passing by reference. 3 4 Module modByRefTest 5 6 ' squares three values ByVal and ByRef, displays results 7 Sub Main() 8 Dim number1 As Integer = 2 9 10 Console.WriteLine("Passing a value-type argument by value:") 11 Console.WriteLine("Before calling SquareByValue, " & _ 12 "number1 is {0}", number1) 13 SquareByValue(number1) ' passes number1 by value 14 Console.WriteLine("After returning from SquareByValue, " & _ 15 "number1 is {0}" & vbCrLf, number1) 16 17 Dim number2 AsInteger = 2 18 19 Console.WriteLine("Passing a value-type argument"& _ 20 " by reference:") 21 Console.WriteLine("Before calling SquareByReference, " & _ 22 "number2 is {0}", number2) 23 SquareByReference(number2) ' passes number2 by reference 24 Console.WriteLine("After returning from " & _ 25 "SquareByReference, number2 is {0}" & vbCrLf, number2) 26 27 Dim number3 As Integer = 2 28 ByRefTest.vb

  29. ByVal indicates that value-type arguments should be passed by value Enclosing arguments in parenthesis forces pass-by-value ByRef gives direct access to the value stored in the original variable 29 Console.WriteLine("Passing a value-type argument" & _ 30 " by reference, but in parentheses:") 31 Console.WriteLine("Before calling SquareByReference " & _ 32 "using parentheses, number3 is {0}", number3) 33 SquareByReference((number3)) ' passes number3 by value 34 Console.WriteLine("After returning from " & _ 35 "SquareByReference, number3 is {0}", number3) 36 37 End Sub ' Main 38 39 ' squares number by value (note ByVal keyword) 40 Sub SquareByValue(ByVal number As Integer) 41 Console.WriteLine("After entering SquareByValue, " & _ 42 "number is {0}", number) 43 number *= number 44 Console.WriteLine("Before exiting SquareByValue, " & _ 45 "number is {0}", number) 46 End Sub' SquareByValue 47 48 ' squares number by reference (note ByRef keyword) 49 Sub SquareByReference(ByRef number As Integer) 50 Console.WriteLine("After entering SquareByReference" & _ 51 ", number is {0}", number) 52 number *= number 53 Console.WriteLine("Before exiting SquareByReference" & _ 54 ", number is {0}", number) 55 End Sub' SquareByReference 56 57 End Module' modByRefTest ByRefTest.vb

  30. Passing a value-type argument by value: Before calling SquareByValue, number1 is 2 After entering SquareByValue, number is 2 Before exiting SquareByValue, number is 4 After returning from SquareByValue, number1 is 2 Passing a value-type argument by reference: Before calling SquareByReference, number2 is 2 After entering SquareByReference, number is 2 Before exiting SquareByReference, number is 4 After returning from SquareByReference, number2 is 4 Passing a value-type argument by reference, but in parentheses: Before calling SquareByReference using parentheses, number3 is 2 After entering SquareByReference, number is 2 Before exiting SquareByReference, number is 4 After returning from SquareByReference, number3 is 2 Program Output

  31. 6.10 Duration of Identifiers • Identifier’s duration • Period during which the identifier exists in memory • Identifier’s scope • Portion of a program in which the variable’s identifier can be referenced • Automatic duration • Identifiers that represent local variables in a procedure have automatic duration • Instance variable • A variable declared in a class • They exist as long as their containing class is loaded in memory

  32. 6.11 Scope Rules • Possible scopes • Class scope • Begins at the class identifier after keyword Class and terminates at the EndClass statement • Module scope • Variable declared in a module have module scope, which is similar to class scope • Namespace scope • Procedures defined in a module have namespace scope, which generally means that they may be accessed throughout a project • Block scope • Identifiers declared inside a block, such as the body of a procedure definition or the body of an If/Then selection structure, have block scope

  33. This variable is hidden in any procedure that declares a variable named value Automatic variable value is destroyed when MethodA terminates None of the method calls modifies this variable – both methods refer to variables in other scopes 1 ' Fig. 6.13: Scoping.vb 2 ' Demonstrates scope rules and instance variables. 3 4 Public Class FrmScoping 5 Inherits System.Windows.Forms.Form 6 7 Friend WithEvents lblOutput As System.Windows.Forms.Label 8 9 ' Windows Form Designer generated code 10 11 ' instance variable can be used anywhere in class 12 Dim value AsInteger = 1 13 14 ' demonstrates class scope and block scope 15 PrivateSub FrmScoping_Load(ByVal sender As System.Object, _ 16 ByVal e As System.EventArgs) HandlesMyBase.Load 17 18 ' variable local to FrmScoping_Load hides instance variable 19 Dim value AsInteger = 5 20 21 lblOutput.Text = "local variable value in" & _ 22 " FrmScoping_Load is " & value 23 24 MethodA() ' MethodA has automatic local value 25 MethodB() ' MethodB uses instance variable value 26 MethodA() ' MethodA creates new automatic local value 27 MethodB() ' instance variable value retains its value 28 29 lblOutput.Text &= vbCrLf & vbCrLf & "local variable " & _ 30 "value in CScoping_Load is " & value 31 EndSub' FrmScoping_Load 32 33 ' automatic local variable value hides instance variable 34 Sub MethodA() 35 Dim value As Integer = 25' initialized after each call Scoping.vb

  34. When MethodB procedure refers to variable value, the instance variable value (line 12) is used. 36 37 lblOutput.Text &= vbCrLf & vbCrLf & "local variable " & _ 38 "value in MethodA is " & value & " after entering MethodA" 39 value += 1 40 lblOutput.Text &= vbCrLf & "local variable " & _ 41 "value in MethodA is " & value & " before exiting MethodA" 42 EndSub' MethodA 43 44 ' uses instance variable value 45 Sub MethodB() 46 lblOutput.Text &= vbCrLf & vbCrLf & "instance variable" & _ 47 " value is " & value & " after entering MethodB" 48 value *= 10 49 lblOutput.Text &= vbCrLf & "instance variable " & _ 50 "value is " & value & " before exiting MethodB" 51 EndSub' MethodB 52 53 End Class ' FrmScoping Scoping.vb

  35. 6.12 Random-Number Generation • Random class • Produces values at random • Keyword New creates an object of a specified type and returns the object’s location in memory • Next Method • Generates a positive Integer value between zero and the constant Int32.MaxValue (2,147,483,647) • When a single argument is passed to Next, the values returned will be in the range from 0 to the value of that argument • Scaling • By passing two arguments, the programmer is allowed to specify the bottom range too

  36. Note that we must use 7 as the second argument to produce integers in the range from 1-6 Go to the next line every time five numbers are generated 1 ' Fig. 6.14: RandomInteger.vb 2 ' Generating random integers. 3 4 Imports System.Windows.Forms 5 6 Module modRandomInteger 7 8 Sub Main() 9 Dim randomObject As Random = New Random() 10 Dim randomNumber AsInteger 11 Dim output AsString = "" 12 Dim i As Integer 13 14 For i = 1To20 15 randomNumber = randomObject.Next(1, 7) 16 output &= randomNumber & " " 17 18 If i Mod5 = 0Then ' is i a multiple of 5? 19 output &= vbCrLf 20 EndIf 21 22 Next 23 24 MessageBox.Show(output, "20 Random Numbers from 1 to 6", _ 25 MessageBoxButtons.OK, MessageBoxIcon.Information) 26 End Sub ' Main 27 28 End Module ' modRandomInteger RandomInteger.vb

  37. 6.14 Recursion • Recursive procedure • It is a procedure that calls itself either directly or indirectly • It is called to solve a problem • The procedure knows how to solve only the simples case (base case) • For complex problems, the procedure divides the problem into a piece that it can perform and a piece that it does not know how to perform • Recursive call • The procedure invokes a fresh copy of itself to work on the smaller problem

  38. 6.14 Recursion 5! 5! Final value = 120 5 * 4! 5 * 4! 5! = 5 * 24 = 120 is returned 4 * 3! 4 * 3! 4! = 4 * 6 = 24 is returned 3 * 2! 3 * 2! 3! = 3 * 2 = 6 is returned 2 * 1! 2 * 1! 2! = 2 * 1 = 2 is returned 1 1 1 returned (a) Procession of recursive calls (b) Values returned from each recursive call Fig. 6.18 Recursive evaluation of 5!.

  39. Conversion from String to Integer 1 ' Fig. 6.19: Factorial.vb 2 ' Calculating factorials using recursion. 3 4 PublicClass FrmFactorial 5 Inherits System.Windows.Forms.Form 6 7 FriendWithEvents lblEnter As Label ' prompts for Integer 8 FriendWithEvents lblFactorial As Label ' indicates output 9 10 FriendWithEvents txtInput As TextBox ' reads an Integer 11 FriendWithEvents txtDisplay As TextBox ' displays output 12 13 FriendWithEvents cmdCalculate As Button ' generates output 14 15 ' Visual Studio .NET generated code 16 17 PrivateSub cmdCalculate_Click(ByVal sender As System.Object, _ 18 ByVal e As System.EventArgs) Handles cmdCalculate.Click 19 20 Dim value AsInteger = Convert.ToInt32(txtInput.Text) 21 Dim i AsInteger 22 Dim output AsString 23 24 txtDisplay.Text = "" 25 26 For i = 0To value 27 txtDisplay.Text &= i & "! = " & Factorial(i) & vbCrLf 28 Next 29 30 EndSub ' cmdCalculate_Click Factorial.vb

  40. If number is greater than 1, a recursive call to Factorial is made with a slightly simpler problem Forgetting to return a value from a recursive procedure can result in logic errors 31 32 ' recursively generates factorial of number 33 Function Factorial(ByVal number AsLong) AsLong 34 35 If number <= 1Then' base case 36 Return1 37 Else 38 Return number * Factorial(number - 1) 39 EndIf 40 41 EndFunction ' Factorial 42 43 EndClass ' FrmFactorial Factorial.vb

  41. 6.16 Recursion vs. Iteration • Iteration • Involves an explicit repetition structure • Uses a repetition structure • For, While or Do/LoopUntil • Recursion • Achieves repetition through repeated procedure calls • Uses a selection structure • If/Then, If/Then/Else or Select • Recursive calls take time and consume additional memory

  42. 6.17 Procedure Overloading and Optional Arguments • Overloading • Allows multiple procedures with the same name, but differing numbers and types of arguments • The overloading of procedures that perform closely related tasks can make programs more readable and understandable • Optional arguments • Defining an argument as optional allows the calling procedure to decide what arguments to pass

  43. The compiler might use the logical name “Square of Integer” “Square of Double” for the Square method that specifies a Double parameter The compiler uses a logical name to differ between the two Square methods 1 ' Fig. 6.22: Overload.vb 2 ' Using overloaded methods. 3 4 PublicClass FrmOverload 5 Inherits System.Windows.Forms.Form 6 7 Friend WithEvents outputLabel As Label 8 9 ' Visual Studio .NET generated code 10 11 PrivateSub FrmOverload_Load(ByVal sender As System.Object, _ 12 ByVal e As System.EventArgs) Handles MyBase.Load 13 14 outputLabel.Text = "The square of Integer 7 is " & _ 15 square(7) & vbCrLf & "The square of Double " & _ 16 "7.5 is " & square(7.5) 17 EndSub ' FrmOverload_Load 18 19 Function Square(ByVal value AsInteger) AsInteger 20 Return Convert.ToInt32(value ^ 2) 21 End Function ' Square 22 23 Function Square(ByVal value AsDouble) AsDouble 24 Return value ^ 2 25 End Function ' Square 26 27 End Class ' FrmOverload Overload.vb

  44. Procedure calls cannot be distinguished by return type 1 ' Fig. 6.23: Overload2.vb 2 ' Using overloaded procedures with identical signatures and 3 ' different return types. 4 5 PublicClass FrmOverload2 6 Inherits System.Windows.Forms.Form 7 8 Friend WithEvents outputLabel As Label 9 10 ' Visual Studio .NET generated code 11 12 PrivateSub FrmOverload2_Load(ByVal sender As System.Object, _ 13 ByVal e As System.EventArgs) Handles MyBase.Load 14 15 outputLabel.Text = "The square of Integer 7 is " & _ 16 square(7) & vbCrLf & "The square of Double " & _ 17 "7.5 is " & square(7.5) 18 EndSub ' FrmOverload2_Load 19 20 Function Square(ByVal value AsDouble) AsInteger 21 Return Convert.ToInt32(value ^ 2) 22 End Function ' Square 23 24 Function Square(ByVal value AsDouble) AsDouble 25 Return value ^ 2 26 End Function ' Square 27 28 End Class ' FrmOverload2 Overload2.vb

  45. The creating of overloaded procedures with identical parameter lists and different return types produces a syntax error Overload2.vbProgram Output

  46. 6.17.2 Optional Arguments • Optional • Optional arguments are specified in the procedure header with keyword Optional • Syntax errors • Not specifying a default value for an Optional parameter is a syntax error • Declaring a non-Optional parameter to the right of an Optional parameter is a syntax error

  47. Determines whether txtPower contains a value 1 ' Fig 6.24 Power.vb 2 ' Calculates the power of a value, defaults to square. 3 4 PublicClass FrmPower 5 Inherits System.Windows.Forms.Form 6 7 FriendWithEvents txtBase As TextBox ' reads base 8 FriendWithEvents txtPower As TextBox ' reads power 9 10 FriendWithEvents inputGroup As GroupBox 11 12 FriendWithEvents lblBase As Label ' prompts for base 13 FriendWithEvents lblPower As Label ' prompts for power 14 FriendWithEvents lblOutput As Label ' displays output 15 16 FriendWithEvents cmdCalculate As Button ' generates output 17 18 ' Visual Studio .NET generated code 19 20 ' reads input and displays result 21 PrivateSub cmdCalculate_Click(ByVal sender As System.Object, _ 22 ByVal e As System.EventArgs) Handles cmdCalculate.Click 23 24 Dim value AsInteger 25 26 ' call version of Power depending on power input 27 IfNot txtPower.Text = ""Then 28 value = Power(Convert.ToInt32(txtBase.Text), _ 29 Convert.ToInt32(txtPower.Text)) 30 Else 31 value = Power(Convert.ToInt32(txtBase.Text)) 32 EndIf 33 34 lblOutput.Text = Convert.ToString(value) 35 EndSub ' cmdCalculate_Click Power.vb

  48. When omitted, the Optional argument defaults to the value 2 36 37 ' use iteration to calculate power 38 Function Power(ByVal base AsInteger, _ 39 OptionalByVal exponent AsInteger = 2) AsInteger 40 41 Dim total AsInteger = 1 42 Dim i AsInteger 43 44 For i = 1To exponent 45 total *= base 46 Next 47 48 Return total 49 EndFunction ' Power 50 51 EndClass ' FrmPower Power.vb

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