Chapter 4Variables 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 ‘sngCircleArea = 475.06

-Common Error-Example 12.3 = sngRadius (What’s wrong???)

-Think About it-Example: Dim sngX As Single

sngX = 5.5sngX = 8Me.lblResult.text = sngX (What will be

displayed???)

Ch 4 – 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 = 6Dim sngCircleArea As Single

sngCircleArea = 3.14 * sngRadius ^ 2 ‘area of circle formulaMe.lblAnswer.text = sngCircleArea ‘Displays answer in label

-Run program (Answer should be 113.04)

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 4 – 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) ‘gets value from textboxsngCircleArea = 3.14 * sngRadius ^ 2 ‘area of circle formulaMe.lblAnswer.text = sngCircleArea ‘Displays answer in label

-Run program (Enter 2 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 = 3.14159sngPI = 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 = any decimal value-Integer = any whole number-String = anything containing non-numerical characters-Char = anything containing a single chararcter

4.7 Automatic Type Conversion-VB will convert decimals to Integers following normal rounding rules

-Example:Dim intX, intY As IntegerintX = 6.7intY = 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 = 3Me.lblAnswer.Text = intX

-Add btnProc2 Click EventMe.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

Ch 4 – Review 6-New Project

-New Form -Add 1 text box (txtSkyHooks)

-Add 1 button (btnShip)

-Add 4 labels

-lblPrompt, lblBoxAnswer, lblBoxes

-lblEnvelopeAnswer, lblEnvelopes

-Add following statements to btnShip click event

Dim intSkyHooks As Integer

Dim intBoxes, intEnvelopes As Integer

intSkyHooks = Val(Me.txtSkyHooks.text) ‘gets value from textbox

intBoxes = intSkyHooks \ 3 ‘Calculate number of boxesintEnvelopes = intSkyHooks mod 3 ‘Calculate number of Envelopes

Me.lblBoxAnswer.text = intBoxes ‘Displays # of boxes in label

Me.lblEnvelopeAnswer.text = intEnvelopes ‘Displays 3 of envelops in label

-Add textchanged event to clear out both labels

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

Pseudocode -> ACTUAL CODEradExponent_Click1. What variable do you need??? (Variable Declarations)

intNum1 ->Integer Dim intNum1 As IntegerintNum2 -> Integer Dim intNum2 As IntegersngAnswer -> Single Dim sngAnswer As Single

2. What variables come from the user???intNum1 = txtFirst intNum1 = Val(Me.txtFirst.text)intNum2 = txtSecond intNum2 = Val(Me.txtSecond.text)

3. What action/formula/task does the event complete???sngAnswer = intNum1 ^ intNum2 sngAnswer = intNum1 ^ intNum2

4. What is displayed and where???lblAnswer = sngAnswer Me.lblAnswer.text = sngAnswer