Microsoft Visual Basic 2005: Reloaded Second Edition

1
Microsoft Visual Basic 2005 Reloaded Second
Edition

• Chapter 3
• Variables, Constants, Methods, and Calculations

2
Objectives

• After studying this chapter, you should be able
  to
• Declare variables and named constants
• Assign data to an existing variable
• Convert data to the appropriate type using the
  TryParse method and the Convert class methods
• Write arithmetic expressions
• Understand the scope and lifetime of variables
  and named constants

3
Objectives (continued)

• Understand the purpose of the Option Explicit,
  Option Strict, and Imports statements
• Use a TOE chart, pseudocode, and a flowchart to
  code an application
• Clear the contents of a controls Text property
  while an application is running

4
Objectives (continued)

• Send the focus to a control while the application
  is running
• Explain the difference between syntax errors and
  logic errors
• Format an applications numeric output

5
Variables

• Variables computer memory locations used to
  store data while an application is running
• Every variable has a
• Name
• Data type
• Scope
• Lifetime

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Selecting a Data Type for a Variable

• Each variable must be assigned a data type
• Data type the type of data the variable can
  store
• Each data type is a class
• Unicode
• Universal coding scheme for characters
• Assigns a unique numeric value to each character

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Selecting a Data Type for a Variable (continued)
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Selecting a Name for a Variable

• Identifier descriptive name given to a variable
• Use a meaningful name that reflects the purpose
  of the variable
• Use camel casing for variable identifiers
• Variable names must conform to naming rules

9
Selecting a Name for a Variable (continued)
10
Declaring a Variable

• Declaration statement used to declare, or
  create, a variable
• Declaration statement includes
• Scope keyword Dim or Private or Static
• Name of the variable
• Data type
• Initial value (optional)

11
Declaring a Variable (continued)
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Assigning Data to an Existing Variable

• Assignment statement
• Used to assign values to properties of controls
• Used to assign values to variables
• Assignment operator ()
• Value on the right of the operator is assigned
  to the variable on the left of the operator

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Assigning Data to an Existing Variable (continued)
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Assigning Data to an Existing Variable (continued)

• String group of characters enclosed in quotation
  marks
• Literal constant
• An item of data whose value does not change while
  the application is running
• Can be a numeric or a string literal constant
• A numeric literal with a decimal place is treated
  as a Double type
• Literal type character forces a literal constant
  to assume a specific data type

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Assigning Data to an Existing Variable (continued)
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Using the TryParse Method

• Method a specific portion of a classs
  instructions that performs a task for the class
• TryParse method
• Part of every numeric data types class
• Used to convert a string to that numeric data
  type
• TryParse method has 4 arguments
• String string value to be converted
• Variable location to store the result
• IFormatProvider (optional) specifies formatting
• NumberStyles (optional) allows formatting
  characters to be in the data to be converted

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Using the TryParse Method (continued)

• IFormatProvider argument formats numbers, dates,
  and times
• NumberFormatInfo.CurrentInfo value
• Uses the formatting characters specified in the
  Windows Customize Regional Options dialog box

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Using the TryParse Method (continued)
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Using the TryParse Method (continued)
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Using the TryParse Method (continued)

• Assign the TryParse methods return value to a
  Boolean variable
• If True, the conversion was successful
• If False, the value could not be converted
• Line continuation character the underscore (_)
• Breaks up a long instruction into two or more
  lines
• Must appear at end of line, preceded by a space
• Must have an Imports statement in the General
  Declarations section of code to use NumberStyles
  and NumberformatInfo.CurrentInfo
• Imports System.Globalization

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Using the TryParse Method (continued)
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Using the Convert Class

• Convert class
• Contains methods for converting numeric values to
  specific data types
• Use the dot member access operator to separate
  the class name from the method name

23
Using the Convert Class (continued)
24
Writing Arithmetic Expressions

• Precedence number indicates the order in which
  an operation in an expression is performed
• If an expression has two operators with the same
  precedence, they are evaluated from left to right
• Use parentheses to change the order of evaluation
• Integer division operator (\) divides two
  integers and returns an integer value
• Modulus arithmetic operator (Mod) divides two
  numbers and returns the remainder

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Writing Arithmetic Expressions (continued)
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Writing Arithmetic Expressions (continued)
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The Scope and Lifetime of a Variable

• Scope indicates where the variable can be used
• Lifetime indicates how long the variable remains
  in memory
• Variables are usually declared in two places
• Within a procedure
• In the forms Declarations section
• Procedure-level variable declared within a
  procedure
• Procedure scope only the procedure can use the
  variable

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The Scope and Lifetime of a Variable (continued)
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The Scope and Lifetime of a Variable (continued)

• With procedure-level scope, two procedures can
  each use the same variable names
• Comments
• Used to internally document the procedure
• Are ignored by the compiler
• Appear in green in the code editor

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The Scope and Lifetime of a Variable (continued)
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The Scope and Lifetime of a Variable (continued)

• Module scope variable can be used by all
  procedures in the form
• Module-level variable
• Declared in the forms Declarations section
• Use Private keyword in declaration
• Module-level variables retain their values until
  the application ends

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The Scope and Lifetime of a Variable (continued)
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The Scope and Lifetime of a Variable (continued)

• Block scope variable can be used within a
  specific block of code
• Block-level variable declared within a specific
  block of code

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Static Variables

• Static variable
• Procedure-level variable that retains its value
  even after the procedure ends
• Retains its value until the application ends
• Can be used instead of a module-level variable
• A static variable has
• Same lifetime as a module-level variable
• Narrower scope than a module-level variable
• Declared using the Static keyword

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Static Variables (continued)
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Named Constants

• Named constant memory location whose value
  cannot be changed while the application is
  running
• Declared using the Const keyword
• Good programming practice to specify the data
  type as well

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Named Constants (continued)
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Named Constants (continued)
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Named Constants (continued)
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Option Explicit and Option Strict

• Option Explicit
• When on, all variables used must first be
  declared
• Protects against misspelled variable names in
  code
• Placed in the General Declarations section of
  code editor
• Implicit type conversion can occur if the value
  on the right side of an assignment statement is
  not the same data type as the variable on the
  left side

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Option Explicit and Option Strict (continued)

• Promoting when a value is converted to another
  data type that stores larger numbers
• Demoting when a value is converted to another
  data type that stores only smaller numbers
• Data loss can occur with demoting
• Option Strict
• Can be used to enforce correct data typing
• Placed in the General Declarations section of the
  code editor

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Option Explicit and Option Strict (continued)

• Option Strict On follows these conversion rules
• Strings are not implicitly converted to numbers
  or vice versa
• Narrower data types are implicitly promoted to
  wider data types
• Wider data types are not implicitly demoted to
  narrower data types

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Option Explicit and Option Strict (continued)
44
Coding the Skate-Away Sales Application
45
Coding the Skate-Away Sales Application
(continued)
46
Using Pseudocode to Plan a Procedure

• Pseudocode short phrases to describe the steps a
  procedure needs to take to accomplish its goal

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Using Pseudocode to Plan a Procedure (continued)
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Using a Flowchart to Plan a Procedure

• Flowchart uses standardized symbols to show the
  steps a procedure must take to accomplish its
  goal
• Can be used in place of pseudocode for planning
• Three symbols
• Start/stop symbol (oval) indicates start and
  stop points
• Process symbol (rectangle) represents tasks
• Input/output symbol (parallelogram) represents
  input or output tasks

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Using a Flowchart to Plan a Procedure (continued)
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Using a Flowchart to Plan a Procedure (continued)
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Coding the clearButtons Click Event Procedure
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Clearing the Contents of a Controls Text Property
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Clearing the Contents of a Controls Text
Property (continued)

• Zero-length string (or empty string)
• Removes the contents in the Text property of a
  control
• Use empty set of quotation marks
• String.Empty used to assign an empty string to a
  controls Text property
• For TextBox control, use the Clear method

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Setting the Focus

• Focus method moves the focus to a specified
  control at runtime

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Setting the Focus (continued)
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Setting the Focus (continued)
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Coding the calcButtons Click Event Procedure
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Coding the calcButtons Click Event Procedure
(continued)
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Coding the calcButtons Click Event Procedure
(continued)
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Testing and Debugging the Application

• Valid data data that the application is
  expecting
• Invalid data data that is unexpected
• Debugging locating errors in a program
• Syntax errors usually caused by mistyping
• Logic errors occur when you enter an instruction
  that does not give the expected results
• Test a program with both valid and invalid data

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Testing and Debugging the Application (continued)
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Testing and Debugging the Application (continued)
63
Formatting Numeric Output

• Formatting specifying the number of decimal
  places and any special characters to display
• Format specifier specifies the type of
  formatting to use
• Precision specifier controls the number of
  significant digits or zeros to the right of the
  decimal point

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Formatting Numeric Output (continued)
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Formatting Numeric Output (continued)
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Formatting Numeric Output (continued)
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Formatting Numeric Output (continued)
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Formatting Numeric Output (continued)
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Programming Tutorial
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Programming Example
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Summary

• Variables and named constants are memory
  locations that store data
• Variables can change value, but constants cannot
• Variables and constants have a name, data type,
  scope, and lifetime
• Use Dim to declare a variable at block or
  procedure level
• Use Private to declare a variable at module level

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Summary (continued)

• Assignment statement is used to assign values to
  an existing variable
• Literals are constant items of data
• Use the TryParse method to convert a string to a
  number
• Use the Imports statement to import a namespace
• The Convert class contains methods to convert
  values to a specified data type

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Summary (continued)

• A procedure-level variable is usable only by the
  procedure in which it is declared
• A module-level variable is usable by all
  procedures in the form
• A block-level variable is usable only within the
  block in which it is declared
• A static variable is a procedure-level variable
  that retains its value when the procedure ends
• Option Explicit On forces declaration of all
  variables before use

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Summary (continued)

• Option Strict On disallows any implicit type
  conversions that may cause a loss of data
• Pseudocode or a flowchart is used to plan a
  procedures code
• Use the Clear method or empty string to clear a
  textbox
• The Focus method moves the focus to a control
• Test a program with both valid and invalid data