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04 – Conditional Execution

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Admin• Technicians (Babbage 205) can provide you

with free copies of (bring your own blank CDs):

– MS Windows XP Professional (1 CD), includes• MS Internet Information Services (term 2)

– MS Visual Studio 6.0 (4 CDs), includes• Visual BASIC 6.0• Visual InterDev 6.0• Visual C++ 6.0

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Session Aims & Objectives• Aims

– to introduce the main concepts involved in getting the computer to act differently under different circumstances

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

– evaluate conditional expressions, and– implement decision trees in code

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Adaptive Behaviour• So far

– every statement always executed in sequence

• Often necessary for software to– change behaviour under different circumstances

• Example:A Pizza shop provides a delivery service. If the delivery is within five miles of the shop, then no delivery fee is charged. If the cost of the goods is less than £10 then a £3 delivery fee is charged, otherwise a £1.50 delivery fee is charged.

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Decision Trees• Natural language

– ambiguous & difficult to follow

• Decision trees– express same information clearly

Delivery Fee

<= 5 miles

> 5 miles>= £10

< £10

Free

£1.50

£3.00

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Conditions & Relational Operators

• Conditions – expression, evaluates to:– true (stored as –1)

– false (stored as 0)

• contain relational operators:= is equal to> is greater than< is less than>= is greater than or equal to<= is less than or equal to<> is not equal to

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Examples: Conditions• Using literals:

(34 = 34) (evaluates to true)(34 = 12) (evaluates to false)(34 > 4) (evaluates to true)(18 <=18) (evaluates to true)

• Using controls' properties:picMain.Left = 2300 (picMain.Left = 2300) (true) (picMain.Left = 2309 (false) (picMain.Left <> 189 (true) (picMain.Left > 1900 (true)

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Logical Operators

• And True when both items are True(picMain.Top > 5) AND (picMain.Top < 35) (true)(picMain.Top < 10) AND (picMain.Top > 55) (false)(picMain.Top > 6) AND (picMain.Top < 23) (false)(picMain.Top >=6) AND (picMain.Top <= 23) (true)

• Or True when either item is True(picMain.Top = 23) OR (picMain.Top = 11) (true)(picMain.Top < 10) OR (picMain.Top > 55) (false)

• Not True when item is FalseNot (picMain.Top = 23) (false)

Use to join conditions (assume picMain.Top is 23):

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Exercise: Conditions• What is the result of (picMain.Left is 5589):

(picMain.Left > 4400)

• What is the result of (txtAge.Text is 19, txtSalary.Text is 10787):

(txtAge.Text < 21) AND (txtSalary.Text < 10787)

• Write an expression to:

check if picMain.height is larger than 167.78

• Write an expression to:

check if picMain.Top is larger than picBall.Top

true

false

(picMain.Height > 167.78)

(picMain.Top > picBall.Top)

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If Then statements• Use the following syntax:

If condition Then [statementblock]End If

• For example:If txtAge.Text < 21 Then lblResult.BackColor = vbRedEnd If

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If Then Else statements• Use the following syntax:

If condition1 Then [statementblock-1][Else [statementblock-2]]End If

• For example:If txtAge.Text Then lblResult.BackColor = vbRedElse lblResult.BackColor = vbBlueEnd If

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Example: DeliveryOption Explicit

Private Sub btnCalc_Click() If txtDist.Text <= 5 Then lblDel.Caption = 0 Else If txtCost.Text >= 10 Then lblDel.Caption = 1.5 Else lblDel.Caption = 3 End If End IfEnd Sub Delivery

lblDelbtnCalc

txtDist txtCost

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Example: Tax Calculator v1

Inland Revenue

Private Sub btnCalc_Click()

lblTaxableIncome.Caption = "Taxable Income: " & txtGrossSalary.Text - txtAllowance.Text

End Sub

Salary v1

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Example: Tax Calculator v2

Private Sub btnCalc_Click() If txtGrossSalary.Text > txtAllowance.Text Then lblTaxableIncome.Caption = "Taxable Income: £" & txtGrossSalary.Text - txtAllowance.Text Else lblTaxableIncome.Caption = "Taxable Income: £0" End IfEnd Sub

Salary v2

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Example: Tax Calculator v3

Private Sub btnCalc_Click() If Val(txtGrossSalary.Text) > Val(txtAllowance.Text) Then lblTaxableIncome.Caption = "Taxable Income: £" & txtGrossSalary.Text - txtAllowance.Text Else lblTaxableIncome.Caption = "Taxable Income: £0" End IfEnd Sub

Salary v3

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Example: BallChar v3

Option Explicit

Private Sub tmrLeft_Timer() ' You need to work this out!End Sub

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

BallChar v3

tmrLeft

picBallChar

tmrRight

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• Check Box - give user on/off, yes/no choice– Value:

• 1 (vbChecked) if selected• 0 (vbUnchecked) if not selected

• Option Box - Used as a group to give usermultiple options (only 1 item selected at a time)– Value:

• -1 (True) if selected• 0 (False) if not selected

• Can place option boxes in Frame control:– groups option boxes

Selection/Decision controls

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Example: Face v2Private Sub btnDraw_Click() picFace.Cls picFace.Circle (2400, 2400), 2000

If chkNose.Value = vbChecked Then picFace.Line (2400, 2200)-Step(0, 600) End If

If optOpen.Value = True Then picFace.Circle (1600, 1600), 500 picFace.Circle (3200, 1600), 500 Else picFace.Line (1100, 1600)-Step(1000, 0) picFace.Line (2700, 1600)-Step(1000, 0) End If

If optHappy.Value = True Then picFace.Circle (2400, 2400), 1200, , 3.4, 6 Else picFace.Circle (2400, 4400), 1200, , 0.6, 2.5 End IfEnd Sub

Face