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Objectives

• Declare both a variable and a named constant
• Write an assignment statement
• Use the Convert class methods to convert data to
  an appropriate type
• Write arithmetic expressions
• Understand the scope of both a variable and a
  named constant

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Objectives (continued)

• Include internal documentation in the code
• Use the Option Strict and Option Explicit
  statements
• Use a TOE chart to code an application
• Use pseudocode and a flowchart to plan an
  objects code
• Send focus to a control

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Objectives (continued)

• Explain the difference between syntax errors and
  logic errors
• Format an applications numeric output

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Variables

• Computer memory locations where users can
  temporarily store data
• Must be assigned a data type by programmer
• Data type determines type of data variable can
  store
• Contents can change as application runs

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

• Integers ? whole numbers
• Integer, Long, Short
• Floating-point numbers ? real numbers
• Single, Double use exponential notation
• Example 32000 3.2E4
• Decimal ? uses fixed decimal point
• Character Types
• Char one character
• String 0 to 2 billion characters

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Selecting a Name for a Variable

• Name should be descriptive
• Use three-letter prefix
• Use m for module-scope variables
• Punctuate using Pascal-case
• Type m and 3 letter prefix using lowercase, then
  capitalize the first letter of each word in the
  variable name

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

• accessibility variablename as datatype
  initialvalue
• accessibility - sets scope of variable
• Dim, Public, Private
• variablename programmer chosen name
• datatype data type of the variable
• Anything in brackets is optional
• initialvalue default value at start of program
• If no value specified numeric types 0,
  character types Nothing, boolean false

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Assigning Data to an Existing Variable

• assignment statement
• strName Mary
• intAge 35
• sign is called assignment operator
• Literal constant value does not change
• Number 35 is numeric literal constant
• Mary is a string literal constant
• String - a group of characters enclosed in quotes

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

• Literal type characters force literal constants
  to assume new data type

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Using the Convert Class

• Convert class creates object that is either a
  number or a string by converting value to a
  specified data type and returning result strAge
  Convert.ToString(intAge)

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Writing Arithmetic Expressions

• Precedence number indicates order in which VB
  performs operation in an expression

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Writing Arithmetic Expressions (continued)

• Integer division operator (\)
• Returns integer portion of division discarding
  remainder
• 8\3 results in answer of 2
• Modulus arithmetic operator (Mod)
• Returns remainder portion of division discarding
  integer portion
• 7 Mod 3 results in an answer of 1
• Often used to determine Leap years
• Valid only with division of integer values

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How To
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The Scope and Lifetime of a Variable

• Scope
• Indicates where in the applications code the
  variable can be used
• Lifetime
• Indicates how long the variable remains in the
  computers internal memory

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The Scope and Lifetime of a Variable (continued)

• Procedure Scope
• Procedure-level variable
• Variable declared within a procedure
• Variable can only be used in procedure in which
  declared
• Module scope
• Module-level variable
• Variable declared in General Declarations section
  of the form
• Variable can be used within all procedures in the
  form

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Procedure-level variables example
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Module-level variable example
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The Scope and Lifetime of a Variable (continued)

• Block scope
• Block-level variable
• Declared inside specific blocks of code
• IfThenElse or For.Next
• Can be used only inside the block in which
  declared

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Named Constants

• A memory location inside a computer whose value
  cannot be changed while the application is
  running

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Named Constants (continued)
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Internally Documenting the Program Code

• Internal documentation
• Term used by programmers for comments placed in
  code
• Place an apostrophe () before text you want
  treated as internal documentation (comment)
• VB ignores everything after apostrophe on line
• Place comments at beginning of each procedure

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Internally Documenting the Program Code
(continued)

• Include comments
• as necessary to explain various sections of code
• at beginning of application explaining
  applications
• Name
• Purpose
• Author
• Date of creation or modification

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Internally Documenting the Program Code
(continued)
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Option Explicit and Option Strict

• Option Explicit On
• Tells computer to warn you if your code contains
  name of an undeclared variable
• Prevents unintentional declaration of variables
  which automatically are of type Object
• Option Strict On
• Tells computer not to perform any implicit type
  conversions which may lead to loss of data
• Implicit type conversions are automatic
  conversions from one data type to another
  performed by the computer to fit data into an
  assigned memory location

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Option Explicit and Option Strict (continued)
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Coding the SKATE-AWAY SALES Application

• Application calculates and displays total number
  of skateboards ordered and total price of
  skateboards including 5 sales tax
• Planning steps
• Identify tasks the application needs to perform
• Identify objects to which you will assign those
  tasks
• Identify events required to trigger an object
  into performing its assigned tasks
• Design the user interface

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Coding the SKATE-AWAY SALES Application
(continued)
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Using Pseudocode to Plan a Procedure

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

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Using a Flowchart to Plan a Procedure

• Flowchart uses standardized symbols to show
  steps procedure must follow to reach its goal
• Standard symbols utilized
• Flowlines - connection lines between symbols
• Start/stop ovals indicating starting and ending
  points of procedure
• Process rectangles designating tasks such as
  calculations
• Input/Output parallelogram designating input
  and output tasks such as getting information from
  user

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

• Coding the one task of bntExit_Click event using
  either pseudocode or flowchart tools

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Coding the btnClear controls Click Event
Procedure
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Assigning a Value to the Property of a Control

• Zero-length string -
• Also called empty string
• Set of quotation marks with nothing between them
• Assigned to the text property of a control at
  run-time removes contents of control
• Me.txtname.Text
• Me refers to the current form
• txtName refers to a textbox control on the form
• Text refers to a property of the txtName control

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HOW TO
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Using the Focus Method

• Focus indicates the control that can accept input
  and/or is awaiting an action from user

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btnClear_Click Event Procedure code
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Coding the btnCalc Controls Click Event Procedure
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Coding the btnCalc Controls Click Event
Procedure (continued)
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Completed Code for Application
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Completed Code for Application (continued)
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Testing and Debugging the Application

• Invalid data - data the application is not
  expecting
• Debugging
• Process of locating syntax and logic errors in
  the program
• Syntax errors
• Typographical errors that violate rules governing
  valid syntax of the language
• Me.Colse() instead of Me.Close()
• Logic errors
• Instruction that does not give expected results
  
• decAverage decNum1 decNum2 / 2

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Testing and Debugging the Application (continued)

• First test with valid data

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Testing and Debugging the Application (continued)

• Then test with invalid data

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Formatting Numeric Output

• Formatting
• Specifying number of decimal places and special
  characters to display in a number
• Format string
• String used to specify format
• Example Axx must be enclosed in quotes
• Format specifier - A
• Alphabetic character that specifies format you
  wish to use
• Precision specifier xx
• xx is the sequence of digits desired

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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 Example Currency Calculator

• Application allows user to enter number of
  American dollars that he or she wants to convert
  to British pounds and Mexican pesos
• Make appropriate calculations and then display
  results on screen

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TOE Chart
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User Interface
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Objects, Properties, and Settings
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Tab Order
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Pseudocode

• btnExit Click event procedure
• close application
• btnCalc Click event procedure
• 1. assign input value to a variable
• 2. calculate number of British pounds by
  multiplying American dollars by .626881
• 3. calculate number of Mexican pesos by
  multiplying American dollars by 10.392
• 4.display number of British pound and Mexican
  pesos in lblBritish and lblMexican
• 5.send the focus to the txtAmerican text box

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Code
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Code (continued)
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Summary

• Variables and named constants are computer memory
  locations that store data
• Variables are declared with a name and data type
• Scope refers to where a program can access a
  variable or named constant
• Module scope variables can be accessed anywhere
  in the module
• Procedure scope variables can only be accessed in
  the procedure in which declared
• Block scope variables can only be accessed within
  the block in which declared

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Summary (continued)

• Lifetime refers to how long a variable remains in
  the computers memory
• Literal constants are items of data
• Convert class contains methods to convert values
  to a specified data type
• Integer division operator divides 2 integers and
  returns the result discarding any remainder
• Modulus division operator divides 2 integers and
  returns the remainder

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Summary (continued)

• Option Explicit warns if undeclared variables are
  used
• Option Strict prevents implicit type conversions
  that may result in loss of data
• Use pseudocode (short phrases) or flowcharts
  (standardized symbols) to plan the application
• Test application with both valid and invalid data
• Format numeric output to display special
  characters