Transcript Chapter 4

Chapter 4
Variables and constants
4.1 Variables
-Use of variables is good programming style
-easier to modify
-easier for a programmer to understand
-Declaration Statement (variables must be declared before you can use them)
-Format:
Dim variable_name As type
-Examples: Dim sngLength As Single
Dim sngWidth As Single
-Single (Data type)
-any numeric value that can contain a decimal
-variable name begins with sng
-Examples: sngLength, sngWidth, sngHeight
-Initializing a value (not necessary but helpful)
- Example:
Dim sngLength As Single = 12.3
-Multiple Declarations in a single statement (not necessary but helpful)
- Example:
Dim sngRadius, sngDiameter As Single
4.2 Variable Assignments
-A value stored in a variable can be changed at run-time through assignment
-Format:
variable_name = value
-Example:
Dim sngRadius As Single = 10
‘declaration and initialization
sngRadius = 12.3
‘change value
-Using variables to write formulas
-Example:
sngCircleArea = 3.14 * sngRadius ^ 2
-Common Error
-Example
12.3 = sngRadius
-Think About it
-Example:
Dim sngX As Single
sngX = 5.5
sngX = 8
Me.lblResult.text = sngX
‘sngCircleArea = 475.06
(What’s wrong???)
(What will be displayed???)
Ch 3 – Review 1
-New Project
-New Form
-Add 2 labels (lblQuestion, lblAnswer)
-Add 1 button (btnAnswer)
-Add appropriate comments to your code
-Add btnanswer click event
-Add following statements to event procedure
Dim sngRadius As Single = 6
Dim sngCircleArea As Single
sngCircleArea = 3.14 * sngRadius ^ 2
Me.lblAnswer.text = sngCircleArea
-Run program (Answer should be 113.04)
‘area of circle formula
‘Displays answer in label
4.3 Obtaining a Value from the User
-TextBox Object (Allows users to enter text at run-time)
-Properties:
-Name:
txt + description
-Text:
What is displayed (usually left blank)
-TextAlign:
Sets alignment of textbox
-Prompt (A label that is placed near a text box to describe its contents/purpose)
-TextChanged Event
-Used to make changes when a user changes the text in a textbox
-Anything typed in the textbox is stored in the text property for the textbox
-Example:
sngRadius = Me.txtRadius.text
-If the textbox contains a number (Code as Follows!!!)
-sngRadius = Val(me.txtRadius.text)
‘converts text to number
-What will sngHeight be assigned in the following examples???
1. sngHeight = Val(“62.5 inches”)
2. sngHeight = Val(“Twenty Inches”)
3. sngHeight = Val(“six feet 2 inches”)
Ch 3 – Review 2
-New Project
-New Form
-Add 2 labels (lblPrompt, lblAnswer)
-Add 1 text box (txtRadius)
-Add 1 button (btnAnswer)
-Add btnAnswer click event
-Add following statements to event procedure
Dim sngRadius As Single
Dim sngCircleArea As Single
sngRadius = Val(Me.txtRadius.text)
sngCircleArea = 3.14 * sngRadius ^ 2
Me.lblAnswer.text = sngCircleArea
‘gets value from textbox
‘area of circle formula
‘Displays answer in label
-Run program (Enter 3 into text box. Answer should be 12.56)
4.4 Using Named Constants
-A constant is a named memory location which stores a value that can’t be
changed.
-Constant Declarations:
-Format:
Const variable_name As Single = Value
-Example:
Const sngPI As Single = 3.14159
-Good Programming Style
-Description part of name is all capitals
-Place before variable declarations
-Examples: sngPI, sngSPEEDOFLIGHT
-Common Error (trying to change value of constant)
Const sngPI As Single = 3.14159
sngPI = 22/7
4.5 Choosing Identifiers
-Legal Names for variables and constants
-must begin with a letter
-can only contain letters, digits, and the underscore character
-periods, spaces, and other special characters not allowed
-can’t be a keyword (any word used by VB)
-Show up in a different font than normal code
-Examples: Single, End, Sub, Private, etc.
4.6 Built – In Data Types
-Refer to (Visual Aide p. 17)
-Special Considerations:
-Char % String assignments must be in quotation marks
-Initialization Value
-Short, Integer, Long, Single, Double, Decimal are set to 0
-Date set to 12:00:00 AM
-Boolean set to FALSE
-Char, String set NOTHING
-NOTHING is the same as “ “
-Example: me.lblMember5.text = NOTHING
-Most common types to be used in class
-Single = and decimal value
-Integer = any whole number
-String = and thing containing non-numerical characters
4.7 Automatic Type Conversion
-VB will convert decimals to Integers following normal rounding rules
-Example:
Dim intX, intY As Integer
intX = 6.7
intY = 12.3
What value is stored in intX and intY???
4.8 Scope
-Scope explains which procedures can use a variable
-Local Scope = variable declared inside of event procedure
-only that procedure can use variable
-Global Scope = variable declared outside of event procedure
-all procedures can use variable
-Visual Aide (p. 18)
-Good Programming Style = only use global variables if completely necessary
Ch. 4 – Review 5
-New Project
-New Form
-Add 2 labels (lblPrompt, lblAnswer)
-Add 2 buttons (btnProc1, btnProc2)
-Add Comment Code
-Add Global Variable
Dim intX As Integer = 10
-Add btnProc1 Click Event
Dim intX As Integer = 10
Me.lblAnswer.Text = intX
-Add btnProc2 Click Event
Me.lblAnswer.Text = intX
-Run Program (Why does the Answer change???)
4.9 Special Division Operators
-Integer Division ( \ )
-divides 2 numbers, but gives answer as a whole number
-Example:
How many basketball teams can we make with 23 people?
23 \ 5 = 4 teams
-Modulus Division ( mod )
-divides 2 numbers, but gives answer as the remainder
-Example:
How many leftover people after we make full teams with 23 people?
23 mod 5 = 3 people
-Order of Operations
-Parentheses, Exponents, Multiply, Divide, Integer division, Modulus, Add, Subtract
4.10 Programming Errors
-Syntax Error (violates rules of VB.NET)
-not syntactically correct
-usually underlined with wavy blue line
-Logic / Semantic Error (syntactically correct but produces undesired results)
-Hard to detect
-Examples: wrong formulas, wrong data types, etc.
-Computer will not detect, programmer must find through good testing
-Run-time Error (halts program during testing)
-sometimes called an exception
-will display a message box with what caused the error
4.11 Debugging an Application
-Be familiar with vocabulary from this section
-Debugging
-Breakpoint
-Watch Window
Ch. 4 – Case Study
-Sketch Form
-Use Arrows to name all objects
-Fill out Initial Properties table
-Brief description of whole project
-Write PseudoCode for each event procedure
-Should include needed variables, input, operation, and output