04 – Conditional Execution

1. 04 – Conditional Execution

2. 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

3. 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

4. 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.

5. Decision Trees • Natural language • ambiguous & difficult to follow • Decision trees • express same information clearly <= 5 miles Free Delivery Fee >= £10 £1.50 > 5 miles < £10 £3.00

6. Conditions & Relational Operators • Conditions – expression, evaluates to: • true (stored as –1) • false (stored as 0) • contain relationaloperators:= is equal to> is greater than< is less than>= is greater thanor equal to<= is less thanor equal to<> is not equal to

7. 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)

8. Logical Operators Use to join conditions (assume picMain.Top is 23): • And True when both itemsare 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 itemis True(picMain.Top = 23) OR (picMain.Top = 11) (true)(picMain.Top < 10) OR (picMain.Top > 55) (false) • Not True whenitemis FalseNot (picMain.Top = 23) (false)

9. 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)

10. If Then statements • Use the following syntax:IfconditionThen [statementblock]EndIf • For example:If txtAge.Text < 21 Then lblResult.BackColor = vbRed End If

11. If Then Else statements • Use the following syntax:Ifcondition1Then [statementblock-1] [Else [statementblock-2]]EndIf • For example:If txtAge.Text Then lblResult.BackColor = vbRed Else lblResult.BackColor = vbBlue End If

12. Example: Delivery Option 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 If End Sub txtDist txtCost btnCalc lblDel Delivery

13. Example: Tax Calculator v1 Private Sub btnCalc_Click() lblTaxableIncome.Caption = "Taxable Income: " & txtGrossSalary.Text - txtAllowance.Text End Sub Salary v1 Inland Revenue

14. 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 If End Sub Salary v2

15. 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 If End Sub Salary v3

16. Example: BallChar v3 tmrRight tmrLeft 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 If End Sub picBallChar BallChar v3

17. Selection/Decision controls • 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

18. Example: Face v2 Private 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 If End Sub Face