Introduction to Numeric Data Types and Variables

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Chapter 3

• Introduction to Numeric Data Types and Variables

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Objectives (1)

• Create a text box for input and output and define
  an accept and cancel button for a form
• Define the order in which the user navigates from
  one control instance to another by pressing the
  Tab key
• Explicitly set input focus and create ToolTips
  for control instances
• Create variables to store information while a
  solution runs
• Understand the organization of namespaces

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Objectives (2)

• Learn how to create comments
• Understand the events that occur as a user
  navigates between text boxes
• Perform computations with variables using
  assignment statements
• Understand the role of precedence, perform type
  conversions, and format data
• Write a statement on multiple lines and calculate
  solution output

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The Text Box Control

• Used to display output to the user and get text
  input from the user
• Similar to a Label control but a Label control
  only displays output
• Standard prefix for a text box is "txt"
• User enters text into one text box at a time
• This text box is said to have input focus or focus

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The TextBox Control Properties (1)

• BorderStyle - defines the border surrounding the
  text box
• ForeColor, BackColor - defines the text color and
  background color respectively
• Enabled - controls whether the text box can get
  focus and will respond to events
• Locked - controls whether user can change the
  text appearing in the text box
• MultiLine - controls whether text will appear on
  multiple lines
• MaxLength - defines the maximum number of
  characters that the user can enter into the text
  box

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The TextBox Control Properties (2)

• Name - defines name used to reference control
  instance with code
• Again, use a standard prefix of "txt"
• ScrollBars - defines whether vertical or
  horizontal scroll bars appear in text box
• Text - contains the text appearing in text box
• TextAlign - justifies text within the visible
  region of the control instance
• X, Y, Height, Width properties define the
  position and size of the control instance

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The TextBox Control Methods and Events

• Focus method sets input focus to a text box
• Enter event fires when text box gets focus
• Leave event fires when text box loses focus
• TextChanged event fires when the changes the
  textual contents at run time
• The event fires for each character typed

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The AcceptButton and CancelButton Properties

• One button on a form can be designated as the
  accept button and another the cancel button
• Set the forms AcceptButton property to a Button
  control instance to define the accept button
• Click event handler executes when user presses
  the Enter key
• Set the forms CancelButton property to Button
  control instance to define the cancel button
• Click event handler executes when user presses
  the Esc key

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Setting the Tab Order (1)

• Order in which control instances get focus is
  called the tab order
• The TabIndex property defines tab order
• Set the TabIndex property to a unique Integer
  value for each control instance that can get focus

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Setting the Tab Order (2)

• To set the tab order visually
• Activate the Win Form Designer
• On the menu bar, click View, and then click Tab
  Order
• Click the control instances in the order that you
  want them to get focus
• Click View, and then click Tab Order again to
  record your changes

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Setting the Tab Order (3)
Tab order of control instances appear in
upper-left corner of control instance region
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Explicitly Setting Input Focus

• Call the Focus method of a text box or other
  object that can get input focus
• Set the input focus to the text box named
  txtInitialValue
• txtInitialValue.Focus()

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ToolTips

• ToolTips appear as a pop-up when the mouse hovers
  over a control instance for a short period
• Standard prefix is "tip"
• InitialDelay property defines the time interval
  for mouse to hover before the ToolTip appears
• AutomaticDelay property defines the amount of
  time that the ToolTip appears
• After creating the ToolTip, set the ToolTip on
  control property for each control instance
• Note that the control instance appears in
  resizable tray below the form because it is not
  visible at run time

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ToolTips (Example)
ToolTip on TipGeneral Property
ToolTip control instance
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Storing Data in Variables

• Variable stores data while a solution runs
• A variable is simply a unique name that
  identifies a space in memory (RAM)
• Data may be a string of characters or a number
• Every variable has a data type
• A variable's data type determines the kind of
  information that the variable can store
• Numbers, characters, etc.
• Every variable has a name
• Use standard prefixes for variable names to
  denote data type

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Selected VB .NET Data Types (1)
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Selected VB .NET Data Types (2)
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Standards for Variable Names (1)

• Use a one-character prefix to denote scope
• The scope of a variable indicates which event
  handlers can use that variable
• Use a three-character prefix to denote data type
• Add an additional prefix character to denote a
  variable's scope
• Module-level variables carry a prefix of "m"

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Standards for Variable Names (1)
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Declaring a Variable (1)

• Private statement declares variable
• Private varname As type initexpr
• varname is variable name
• As type contains data type of variable
• Optional initexpr contains the initial value of a
  variable
• Declare variables having the Double and Integer
  data types
• Private mdblInterestRate As Double
• Private mintCounter As Integer

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Declaring a Variable (2)

• Initialize a variable when declaring it
• New to VB .NET
• Declare an Integer variable and store the value
  30 in the variable
• Private mintYearTerm As Integer 30

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Declaring a Variable (3)

• Do not include punctuation characters in a
  variable declaration
• The following declarations are illegal because of
  the sign and commas
• Private mdblInitialValue As Double 100,000.52
• Private mdblInitialValue As Double 100,000.52
• Possible to declare multiple variables on the
  same line
• Private mintYearTerm, mintMonthTerm As Integer

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Declaring Module-Level Variables
Declare module-level variables after the
statements generated by the Win Form Designer but
outside an event handler
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The Organization of Namespaces (1)

• VB .NET accesses predefined components via
  namespaces
• Namespaces developed by .NET team begin with
  System
• Namespaces developed by others at Microsoft being
  with Microsoft
• Namespaces are organized hierarchically

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The Organization of Namespaces (2)
System namespace System
Drawing namespace System.Drawing
Windows.Forms namespace System.Windows.Forms
Data namespace System.Data
Printing namespace System.Drawing.Printing
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Referencing Namespaces

• Use dot syntax to connect namespaces together
• A period separates each namespace name
• System.Drawing
• System.Windows.Forms
• System.Data

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Parts of a Namespace (1)

• A namespace contains
• Enumerations which define a symbolic name for a
  value
• Classes which are reference types
• Structures which are value types

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Parts of a Namespace (2)
Windows.Forms namespace System.Windows.Forms
Classes
Structures
Enumerations
Button
BorderStyle
Message
Label
FormBorderStyle
TextBox
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Namespace Enumerations

• Enumerations define a symbolic name for a value
• Enumerations improve code readability
• The BorderStyle enumeration applies to the
  System.Windows.Forms namespace
• lblFileName.BorderStyle _ System.Windows.Form
  s.BorderStyle.FixedSingle

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Namespace Classes

• Classes are reference types
• Variable stores the memory address of an object
  rather than the object itself
• Controls appearing in the Toolbox are all classes

Variable btnCalculate 023448
Variable contains memory address of Button
control instance
btnCalculate (Memory address 023448) Data
Caption "Calculate"
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Namespace Structures (1)

• Structures are value types
• Store data directly in the variable
• Primary data types such as Boolean Integer,
  Single, and Double are all structures
• Note that the String data type is a class though

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Namespace Structures (2)
Private mcolCurrent As System.Drawing.Color
  mcolCurrent System.Drawing.Color.Navy
 
Allocate memory for the variable
Store the value Navy directly in the variable
variable mcolCurrent Navy
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Object Browser (1)

• Use the Object Browser to identify the properties
  and methods supported by classes
• Object Browser does not create code for you
• Think of it as a roadmap to the namespaces,
  classes, structures, and enumerations defined by
  VB .NET

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Object Browser (2)
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Creating Well-Written Comments (1)

• A Comment is a full or partial line of text
  ignored by VB .NET
• Create comments to describe what your code is
  doing or how code performs a particular task
• Multiple comments are called a comment block
• Blank lines between statements are ignored and
  are called whitespace
• A comment can appear on its own line and begins
  with a single quote ( ' )
• A comment can appear at the end of a line

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Creating Well-Written Comments (2)

• The following is a comment appearing on its own
  line
• ' This line is a comment.
• The following comment appears at the end of a
  line. Note a space must precede the comment
  character
• txtDemo.text "Dog" ' Store "Dog" in the text
  box

Comment
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Creating Well-Written Comments (3)
VB .NET displays comments in in green
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Writing Assignment Statements Using Variables

• Store a property in a variable
• mintResult txtExample.Height
• Store a literal value in a variable
• mintResult 3
• Store another variable in a variable
• mintResult mintExample

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Arithmetic Operators (1)

• Expressions consists of literal values,
  constants, variables and arithmetic operators
• An arithmetic operator performs a mathematical
  calculation
• Multiply the literal values 3 and 2 together and
  store the result (6) in the variable mintResult
• mintResult 3 2

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Arithmetic Operators (2)
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Arithmetic Operators Example

• Divide two numbers and multiply the intermediate
  result by 100
• Private mdblNumerator As Double 10
• Private mdblDenominator As Double 5
• Private mdblResult As Double
• mdblResult mdblNumerator / mdblDenominator 100

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The Role of Precedence in Expressions

• Programming languages evaluate arithmetic
  expressions from left to right in a predefined
  order known as precedence
• Perform exponentiation first
• Next perform multiplication and division
• Next perform Integer division and then apply the
  Mod operator
• Next perform addition and subtraction
• Parenthesis override default precedence
• Parenthesis may be nested

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Precedence (Example)
(Var1 Var2) / (Var3 Var4) (Var5 Var6)
1 (addition)
2 (addition)
3 (exponent)
4 (division)
5 (multiplication)
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Type Conversion

• VB .NET must convert data from one data type to
  another when evaluating mathematical expressions
• The Option Strict On statement enables strict
  type checking
• Strict type checking requires explicit type
  conversion from less restrictive types to more
  restrictive types
• Use strict type checking to minimize hard to find
  errors resulting from implicit type conversion

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Strict Type Checking (Example)

• The following statement will cause an error if
  strict type checking is enabled. If disabled,
  mintPI will store the value 3
• Note that the result if the implicit type
  conversion is truncation
• Private mintPI As Integer 3.14159

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Option Explicit

• Option Explicit On statement requires that you
  declare variables before using them the first
  time
• Also reduces hard to find errors resulting from
  typos
• Note that Option Explicit and Option Strict
  statements appear at the beginning of a module

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Type Conversion Methods

• Methods belong to the System.Convert class
• ToInt16 converts value to a Short
• ToInt32 converts value to an Integer
• ToInt64 converts value to a Long
• ToDouble converts value to a Double
• ToSingle converts value to a Single
• ToString converts value to a String
• Each method takes one argument the value to
  convert
• Note that these methods supercede older intrinsic
  functions like CInt and CLng

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Type Conversion Examples

• Private msngInput As Single 3.44
• Private mstrInput As String "3.95"
• Private mintOutput As Integer
• Private msngOutput As Single
• Convert a Single to an Integer
• mintOutput System.Convert.ToInt32(msngInput)
• Convert a String to an Integer
• mintOutput System.Convert.ToInt32(mstrInput)
• Convert a String to a Single
• msngOutput System.Convert.ToSingle(mstrInput)

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Numeric Overflow (1)

• Numeric overflow occurs when trying to store too
  large a number in a variable
• The following statements will cause a numeric
  overflow error because the largest number that
  can be stored in a short is 32767
• Private mshoExample As Short
• mshoExample 46000
• mshoExample 2500 2500

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Numeric Overflow (2)

• VB .NET performs arithmetic using less
  restrictive type on right side of expression
• The last statement will cause numeric overflow
• Private mshoArg1 As Short, mshoArg2 As Short
• Private mintArg3 As Integer
• mshoArg1 32000
• mshoArg2 32000
• mintArg3 mshoArg1 mshoArg2

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Formatting a Numeric Value with Methods

• The ToString type conversion method accepts a
  string argument to format data
• Special characters are used to define the format
• lblGain.Text mdblGain.ToString(",.")

variable
ToString method
argument defines how to format the value
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Formatting Placeholders
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Formatting Examples
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Continuation Lines

• Use when statement will not fit on a line
• Underscore (_) is the continuation character
• Rules for use
• Space must precede continuation the character
• Comment following continuation character is not
  allowed
• Do not break up words
• Multiple continuation lines allowed

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Continuation Lines (Example)

• The following statement appears on two lines
• mdblInterestRate _
• System.Convert.ToDouble(txtInterestRate.Text) /
  100

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The Intrinsic FV Function

• Calculate the future value of a fixed amount of
  money based on a constant interest rate
• Syntax
• result FV( rate, periods, payment ,
  presentvalue , type )
• Rate is interest rate
• Periods is number of time periods
• Regular payments, if made stored in payment
• Value at start of term stored in present value
• Type indicates when payments are made

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FV Function - Example

• Calculate the future value of 1000.00 for 12
  periods at a rate of 1 per period
• mdblFutureValue FV(.01, 12, 0, 1000)

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The System.Math Class

• Performs common mathematical operations
• Static methods Do not create an instance of the
  class
• Abs method calculates absolute value
• Sqrt method calculates square root of a number
• Sin, Cos, and Tan compute sine, cosine and
  tangent of a number
• Many others exist