IE 411511: Visual Programming for Industrial Applications

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IE 411/511Visual Programming for Industrial
Applications

• Lecture Notes 8
• Methods A Deeper Look

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OBJECTIVES

• In this chapter you will learn
• To construct programs modularly from methods
• That Shared methods are associated with a class
  rather than a specific instance of the class
• To use common Math methods from the Framework
  Class Library
• To create new methods

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OBJECTIVES

• The mechanisms used to pass information between
  methods
• How the visibility of identifiers is limited to
  specific regions of programs
• To write and use recursive methods(methods that
  call themselves)

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Modules, Classes and Methods

• Programs consist of many pieces, including
  modules and classes
• Modules and classes are composed of methods,
  fields and properties
• You combine new modules and classes with those
  available in class libraries
• Related classes are grouped into namespaces

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Modules, Classes and Methods (cont.)

• A method is invoked by a method call
• The method call specifies the method name and
  provides information
• When the method completes its task, it returns
  control to the caller
• In some cases, the method also returns a result
  to the caller

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Modules, Classes and Methods (cont.)

• A boss (the caller) asks a worker (the callee) to
  perform a task and return the results when the
  task is done
• The worker might call other workers
• However, the boss would be unaware of this

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Modules, Classes and Methods (cont.)

• To promote reusability, the capabilities of each
  method should be limited to the performance of a
  single, well-defined task
• Also, the name of the method should express that
  task effectively
• If you cannot choose a concise method name that
  expresses the task performed by a method, the
  method could be attempting to perform too many
  diverse tasks
• Consider dividing such a method into several
  smaller methods

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Modules, Classes and Methods (cont.)

• Subroutines (such as Console.WriteLine) do
  notreturn a value
• The console application below uses a
  subroutineto print a workers payment information

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Subroutines

• Subroutines are methods that do not return a
  value
• The format of a subroutine declaration is
• The parameter-list is a comma-separated list of
  each parameters type and name
• The type of each argument must be consistent with
  its corresponding parameters type
• If a method does not receive any values, the
  method name is followed by an empty set of
  parentheses

Sub method-name(parameter-list)
Declarations and statements End Sub
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Subroutines (cont.)

• Declaring a variable in the methods body with
  the same name as a parameter variable in the
  method header is a compilation error
• Although it is allowable, an argument passed to a
  method should not have the same name as the
  corresponding parameter name in the method
  declaration
• This prevents ambiguity that could lead to logic
  errors

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Subroutines (cont.)

• The method body (also referred to as a block)
  contains declarations and statements
• The body of a method declared with Sub must be
  terminated with End Sub
• Method names tend to be verbs because methods
  typically perform actions
• By convention, method names begin with an
  uppercase first letter
• e.g., a method that sends an e-mail message might
  be named SendMail
• Small methods are easier to test, debug and
  understand than large methods

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Functions

• Functions are methods that return a valueto the
  caller
• The console application on the next slide uses
  the function Square to calculate the squares of
  the integers from 1 to 10

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Functions (cont.)
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Functions (cont.)

• The Return statement terminates the method and
  returns the result
• The return statement can occur anywhere in a
  function body
• Return expression
• The return-type indicates the type of the result
  returned from the function

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Functions (cont.)

• If the expression in a Return statement cannot be
  converted to the functions return-type, a
  runtime error is generated
• Failure to return a value from a function (e.g.,
  by forgetting to provide a Return statement)
  causes the function to return the default value
  for the return-type, possibly producing incorrect
  output

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Shared Methods and Class Math

• A method which performs a task that does not
  depend on an object is known as a Shared method
• Place the Shared modifier before the keyword Sub
  or Function
• To call a Shared method, do the following
• ClassName.MethodName(arguments)
• Class Math provides a collection of Shared
  methods
• Math.Sqrt(900.0)

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Shared Methods and Class Math (cont.)

• It is not necessary to add an assembly reference
  to use the Math class methods in a program,
  because class Math is located in the assembly
  mscorlib.dll, which is referenced by every .NET
  application
• Also it is not necessary to import class Maths
  namespace (System), because it is implicitly
  imported in all .NET applications

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Shared Methods and Class Math (cont.)

• Summary of several Math class methods (1 of 3)
• In the figure, x and y are of type Double

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Shared Methods and Class Math (cont.)

• Summary of several Math class methods (2 of 3)
• In the figure, x and y are of type Double

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Shared Methods and Class Math (cont.)

• Summary of several Math class methods (3 of 3)
• In the figure, x and y are of type Double

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Shared Methods and Class Math (cont.)

• Math.PI and Math.E are declared in class Math
  with the modifiers Public and Const
• Const declares a constanta value that cannot be
  changed.
• Constants are implicitly Shared

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Notes on Declaring and Using Methods

• There are three ways to call a method
• Using a method name by itself within the same
  class or module
• Using a reference to an object, followed by a dot
  (.) and the method name to call a method of the
  object
• Using the class or module name and a dot (.) to
  call a Shared method
• A Shared method can call only other Shared methods

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Notes on Declaring and Using Methods

• Common Programming Errors with Methods
• Declaring a method outside the body of a class or
  module declaration or inside the body of another
  method is a syntax error
• Omitting the return-value-type in a method
  declaration, if that method is a function, is a
  syntax error
• Redeclaring a method parameter as a local
  variable in the methods body is a compilation
  error
• Returning a value from a method that is a
  subroutine is a compilation error

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Implicit Argument Conversion (cont.)

• An important feature of argument passing is
  implicit argument conversion
• A widening conversion occurs when an argument is
  converted to another type that can hold more data
• A narrowing conversion occurs when there is
  potential for data loss during the conversion

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Implicit Argument Conversion (cont.)

• Converting a primitive-type value to a value of
  another primitive type may change the value if
  the conversion is not a widening conversion
• For example, converting a floating-point value to
  an integral value truncates any fractional part
  of the floating-point value (e.g., 4.7 becomes 4)
• Conversions occur in expressions containing two
  or more types
• Temporary copies of the values are converted to
  the widest type in the expression
• The value of integerNumber is converted to type
  Single for this operation
• singleNumber integerNumber

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Implicit Argument Conversion (cont.)

• The figure below lists the widening conversions
  supported by Visual Basic (1 of 2)

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Implicit Argument Conversion (cont.)

• The figure below lists the widening conversions
  supported by Visual Basic (2 of 2)

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Option Strict and Data-Type Conversions

• Option Explicit forces an explicit declaration of
  variables before they are used
• To set Option Explicit to Off
• Double click My Project in Solution Explorer
• Click the Compile tab, then select the value Off
  from the Option Explicit drop-down list

Drop-down list to modify Option Infer
Drop-down list to modify Option Strict
Compile tab
Drop-down list to Modify Option Explicit
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Option Strict and Data-Type Conversions (cont.)

• Option Strict requires explicit conversion for
  all narrowing conversions
• An explicit conversion uses a cast operator or a
  method
• The methods of class Convert explicitly convert
  data from one type to another, e.g.,
• number Convert.ToDouble(Console.ReadLine())

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Value Types and Reference Types

• All Visual Basic types can be categorized as
  either value types or reference types
• A variable of a value type contains data of that
  type
• A variable of a reference type contains the
  address of the location in memory where the data
  is stored

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Value Types and Reference Types (cont.)

• Primitive Types (1 of 3)

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Value Types and Reference Types (cont.)

• Primitive Types (2 of 3)

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Value Types and Reference Types (cont.)

• Primitive Types (3 of 3)

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Framework Class Library Namespaces

• The .NET framework contains many classes grouped
  into over 100 namespaces
• A program includes the an Imports declaration to
  use classes from that namespace, e.g.,
• Imports System.Windows.Forms

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Framework Class Library Namespaces (cont.)

• Some key namespaces (1 of 2)

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Framework Class Library Namespaces (cont.)

• Some key namespaces (2 of 2)

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Passing Arguments

• Arguments are passed pass-by-value
  orpass-by-reference
• When an argument is passed by value, the program
  makes a copy of the arguments value and passes
  the copy
• Changes to the copy do not affect the original
  variable
• When an argument is passed by reference, the
  original data can be accessed and modified
  directly

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Passing Arguments (cont.)

• The following program demonstrates passing
  arguments by value and by reference (1 of 4)

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Passing Arguments (cont.)

• The following program demonstrates passing
  arguments by value and by reference (2 of 4)

Squaring number2 by reference.
Squaring number1 by value.
The inner set of parentheses evaluates number3 to
its value and passes this value.
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Passing Arguments (cont.)

• The following program demonstrates passing
  arguments by value and by reference (3 of 4)

ByVal indicates that its argument is passed by
value.
Squaring the number parameter value.
ByRef receives its parameter by reference.
Squaring the number parameters reference.
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Passing Arguments (cont.)

• The following program demonstrates passing
  arguments by value and by reference (4 of 4)

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Passing Arguments (cont.)

• When passing arguments by value, changes to the
  called methods copy do not affect the original
  variables value
• This prevents possible side effects that could
  hinder the development of correct and reliable
  software systems
• Always pass value-type arguments by value unless
  you explicitly intend for the called method to
  modify the callers data

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Scope of Declarations

• A declarations scope is the portion of the
  program that can refer to the declared entity by
  its name without qualification
• The basic scopes are
• Block scope ? From the point of the declaration
  to the end of the block
• Method scope ? From the point of the declaration
  to the end of that method
• Module scope ? The entire body of the class or
  module
• Namespace scope ? Accessible to all other
  elements in the same namespace

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Scope of Declarations (cont.)

• If a local variable has the same name as a field
  of the class, the field is hidden
• This is called shadowing
• A variables lifetime is the period during which
  the variable exists in memory
• Variables normally exist as long as their
  container exists
• Use different names for fields and local
  variables
• Helps prevent subtle logic errors that occur when
  a method is called and a local variable of the
  method shadows a field of the same name in the
  class

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Scope of Declarations (cont.)

• Example (1 of 3)

This instance variable is shadowed by any local
variable named x.
Method Begin declares a local variable x
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Scope of Declarations (cont.)

• Example (2 of 3)

Subroutine UseLocalVariable declares a local
variable x
Modifying the local variable x
UseInstanceVariable does not declare local
variables, so x refers to the instance variable.
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Scope of Declarations (cont.)

• Example (3 of 3)

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Method Overloading

• Method overloading allows you to create methods
  with the same name but different parameters
• Overloading methods that perform closely related
  tasks can make programs clearer

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Method Overloading (cont.)

• Overload resolution determines which method to
  call
• This process first finds methods that could be
  used
• Visual Basic converts variables as necessary when
  they are passed as arguments
• The compiler selects the closest match
• Creating overloaded methods with identical
  parameter lists and different return types is a
  compilation error

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Method Overloading (cont.)
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Optional Parameters

• Optional parameters specify a default value that
  is assigned to the parameter if that argument is
  not passed
• Sub ExampleMethod(ByVal value1 As Boolean, _
  Optional ByVal value2 As Integer 0)
• Both of these calls to ExampleMethod are valid
• ExampleMethod(True)
• ExampleMethod(False, 10)

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Optional Parameters (cont.)

• Declaring a non-Optional parameter to the right
  of an Optional parameter is a syntax error
• Not specifying a default value for an Optional
  parameter is a syntax error

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Optional Parameters (cont.)

• Example (1 of 3)

Using values from both TextBoxes.
Using the value of one TextBox and omitting the
optional parameter.
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Optional Parameters (cont.)

• Example (2 of 3)

Using values from both TextBoxes.
baseTextBox
powerTextBox
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Optional Parameters (cont.)

• Example (3 of 3)

Power not necessary, optional parameter
calculateButton