mark dixon, socce soft 136page 1 06 – information processing: data-types, constants, variables

Mark Dixon, SoCCE SOFT 136

06 – Information Processing:Data-types, Constants, Variables


Session Aims & Objectives• Aims

– Introduce you to data storage concepts, i.e. constants, data types and variables

• Objectives,by end of this week’s sessions, you should be able to:

– declare and use constants– declare a variable, selecting appropriate data type– assign a value to a variable,

• using combination of literal values, operators, functions, and identifiers


Types of Information• Numbers (numeric) 29 (integer/whole)

56.23 (decimal/real)

• Text “Hello there!” “BOO”

• Pictures

• Sound


Constants• similar to variable• value given in declaration• value can’t be changed• useful for removing 'magic numbers'• declaration syntax:

• name used to represent literal value

[Global] Const constantname = expression


Example: CirCalc v1Private Sub Form_Load() picCircle.ScaleMode = vbMillimetersEnd Sub

Private Sub btnOK_Click()Dim radius As SingleDim circum As SingleDim area As Single radius = Val(txtRadius.Text)

circum = 2 * 3.14159265359 * radius area = 3.14159265359 * (radius ^ 2)

picCircle.Cls picCircle.Circle (30, 30), radius picInfo.Cls picInfo.Print "Circumference:" picInfo.Print " " & circum & "mm" picInfo.Print "Area:" picInfo.Print " " & area & "mm²"End Sub

btnOK

txtRadius

picCircle

picInfo


Example: CirCalc v2Private Sub Form_Load() …

Private Sub btnOK_Click()Const Pi = 3.14159265359Dim radius As SingleDim circum As SingleDim area As Single radius = Val(txtRadius.Text)

circum = 2 * Pi * radius area = Pi * (radius ^ 2)

picCircle.Cls picCircle.Circle (30, 30), radius picInfo.Cls picInfo.Print "Circumference:" picInfo.Print " " & circum & "mm" picInfo.Print "Area:" picInfo.Print " " & area & "mm²"End Sub

CirCalc

btnOK

txtRadius

picCircle

picInfo


Data Types• Integer – whole numbers

• Long – whole numbers (large)

• Single – decimal numbers

• Double – decimal numbers (more precise)

• Currency – money

• String – text

• Boolean – True or False


Data Type Selection

Will the numberever have afraction?

Will the numberever be large?

number

What informationwill it hold?

String

Long

Integer

text

yes

no yes

no

Currencymoney Will the number

ever need to bevery precise?

Double

Single

yes

no


Data Storage• Data can be stored in

– Controls• visible to user (although can use visible property to hide)

• take lots of memory

• slow to access

– Variables

• Not visible to user

• take up very little memory

• fast to access


Example: GuessNum - AnalysisSPECIFICATION

• User Requirements – need to keep children occupied/entertained, while

learning about maths

• Software Requirements– Functional:

–computer picks a number between 0 and 100–user enters a number–compare numbers and display appropriate

message– Non-functional

should be easy and fun to use


Variables (why?)• Variables useful for:

– reducing memory use

– speed up execution

– storing information you don't want user to see

– storing intermediate results of calculations temporarily (makes code easier to understand)

– making code easier to read (short variable name instead of long object.property names)


Variables (what)• Variables have

– Identifier (name) – you choose this, used to refer to (reference) variable

– Type – you choose this (to suit purpose)– Value – you set/change this

23x IntegerName/Identifier

Value TypeMemory


Numeric Variables


String (Text) Variables


Variable declaration (how 1)• Variables must be declared,

using the following syntax (grammar):

Dim <identifier> As <type>

e.g. Dim weight As doubleDim x As longDim s As stringDim year As long


Questions: Variable declaration

• Write a line of code that:

– Declares a variable called x of type double

– Declares a variable called y of type integer

– Declares a variable called surname of type string

– Declares a variable called age of type integer

Dim x As double

Dim y As integer

Dim surname As string

Dim age As integer


Variable assignment (how 2)• Variables are assigned values,

using the following syntax:

<identifier> = <expression>

e.g. x = 5 weight = 109.45name = "Bob" s = "Hello "

Note: the data flows backwards (from right to left)


Questions: Variable assignment

• Write a line of code that:

– Assigns the value of 23 to the variable y

– Assigns the value of 14.6 to the variable x

– Assigns the value of ‘John’ to the variable surname

– Assigns the value of 21 to the variable age

y = 23

x = 14.6

surname = "John"

age = 21


Example: AddNum v3Private Sub btnAdd_Click()Dim num1 As DoubleDim num2 As DoubleDim res As Double num1 = Val(txtNum1.Text) num2 = Val(txtNum2.Text) res = num1 + num2 lblResult.Caption = resEnd Sub

AddNum

• Variables used to:– spread code over several lines– makes code easier to understand


Example: GuessNum - CodeOption ExplicitDim GuessNum As Long

Private Sub Form_Load() Randomize GuessNum = Rnd() * 100End Sub

Private Sub btnGuess_Click() If txtGuessNum.Text = GuessNum Then lblResult.Caption = "Correct" Else lblResult.Caption = "Wrong, please try again" End IfEnd Sub

txtGuessNum

btnGuess

lblResult


Variables: ErrorsOption ExplicitDim z as integer

Sub Form_Click ()Dim s As StringDim x As IntegerDim x As Integer Print y Print z x = 40000 x = "21" s = 21 x = 3.2End Sub

OK, forces explicit variable declarationOK

OKOKOK Duplicate definition error. Variable not defined error.OK, as z was declared at the form level. Overflow error. Type mismatch error. Type mismatch error.OK (however x will be 3).


Questions: Variable assignment 2• Write a line of code that:

– Increases the value of x by 2.89

– Decreases the value of z by y

– Divides Km by 1.6 and puts the result in Miles

– Joins two strings Surname and Forenames together, putting the result in LongName

x = x + 2.89

z = z - y

Miles = Km / 1.6

LongName = Surname & Forenames


Scope (what)

• Scope – accessibility/visibility

– Local (declared within procedure)

– Form/module/unit (general declarations)


Variable Scope (How)

• Module variables– general

declarations (top)

• Local variables:– in procedures

Option ExplicitDim mv as long

Private Sub btnCalc_Click()Dim lv1 as long ...End Sub

Private Sub btnAdd_Click()Dim lv2 As Long ...End Sub


Variable Scope


Scope (why)• In short – Robustness of code/software

– Protection from accidental outside interference

• One of many responses to code that is– Difficult to maintain, and– Unreliable– House of cards phenomenon

• Prevent:– Uncontrolled and ad hoc interactions between code

• Always define things at lowest level needed


Variable Scope: Errors

• In the following: Option Explicit

Private Sub btnCalc_Click()

Dim x As Integer

x = 0

lblTotal.Caption = "£" & x

End Sub

Private Sub btnQuit_Click()

x = 0

lblTotal.Caption = "£" & x

End Sub

Variable not defined error


Questions: Variable Scope• Will this compile?

Option Explicit …Dim x As integer … Private Sub thing() Dim z As Integer x = 23 y = "there" z = 12 end

Private Sub btnTest_Click() Dim y As String y = "hello" x = 67 z = 53 End Sub

Is x in scope?Is y in scope?Is z in scope?

Is y in scope?Is x in scope?Is z in scope?

YesNoYes

YesYesNo


Example: Ball Char (v3)Option Explicit

Private Sub tmrLeft_Timer() Me.picBallChar.Left = Me.picBallChar.Left - 100 If Me.picBallChar.Left < 0 Then tmrLeft.Enabled = False tmrRight.Enabled = True End IfEnd Sub

Private Sub tmrRight_Timer() Me.picBallChar.Left = Me.picBallChar.Left + 100 If Me.picBallChar.Left >= Me.ScaleWidth Then tmrRight.Enabled = False tmrLeft.Enabled = True End IfEnd Sub


Example: Ball Char (v4)

Option ExplicitDim xInc As Single

Private Sub Form_Load() xInc = 100End Sub

Private Sub tmrMain_Timer() picBallChar.Left = picBallChar.Left + xInc If picBallChar.Left <= 0 Or picBallChar.Left >= frmBounce.ScaleWidth Then xInc = -xInc End IfEnd Sub

• Variable (xInc)– reduces amount of code


Tutorial Exercise: GuessNum

• Task 1: Get GuessNum example working.

• Task 2: Modify GuessNum to tell the user whether their incorrect guess was higher or lower than the correct number.

• Task 3: Modify GuessNum to only allow 5 attempts before picking a new number.


Tutorial Exercise: Ball Char

• Task 1: Get Ball Char (v3) example working.

• Task 2: Add sound to Ball Char (v3) example

• Task 3: Get Ball Char moving diagonally, bouncing off all four sides of the window.

• Task 4: Modify your program so that it allows the user to control how fast the ball character moves.

mark dixon, socce soft 136page 1 06 – information processing: data-types, constants, variables

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