21 – User Defined Functions

1. 21 – User Defined Functions

2. Session Aims & Objectives • Aims • To introduce user defined functions • Objectives,by end of this week’s sessions, you should be able to: • Create your own function definitions • Call these functions

3. Meet George • Common Boa Constrictor • boa constrictor imperator • Native to Central & South America • No venom (no poison)

4. Meet George (cont.)

5. George (cont.) • Problem: • Difficult to keep • Require temperature and humidity controlled environment • Much of the literature is from the US • Temperature in Fahrenheit • Solution • Need a program to convert from Celsius to Fahrenheit

6. George (cont.) • To convert from Fahrenheit to Celsius: e.g. 50 Fahrenheit is: c= 10

7. Example: Temp v1 Calculationrepeated Temp v1 Option Explicit Private Sub Form_Load() lblResult.Caption = ((txtFah.Text - 32) * 5) / 9 End Sub Private Sub txtFah_Change() lblResult.Caption = ((txtFah.Text - 32) * 5) / 9 End Sub

8. Procedures and Functions • Both Procedures and Functions • Group of statements • Identified by uniquename • mirror real life activities • Procedures – just do something • Functions – return a value • used to perform calculations

9. Built in Functions: Sqr X: Double Sqr Double • Sqr – gives square root of a number: • Examples Sqr(4) returns 16 Sqr(3) returns 9 Sqr(2) returns 4 Option Explicit Private Sub txtNum_Change() Dim tmpNum As Double tmpNum = Val(Me.txtNum.Text) Me.lblResult.Caption = Sqr(tmpNum) End Sub SquareRoot

10. Built in Functions: Rnd Random Single • Rnd() function • generates pseudo-random numbers • >= 0 and <1 • Randomize – initialises random number generator RandomNumbers Option Explicit Private Sub Form_Load() Randomize End Sub Private Sub btnRandom_Click() Me.lblNum.Caption = Rnd() End Sub

11. User Defined Functions (how) • Syntax very similar to procedure definition: Function<name>(<parameters>) As <type> [<Statement-Block>] <name> = <value> End Function • Where • <name> represents function’s name you choose • <parameters> represent information needed • <type> represents the return type • <value> represent the return value

12. Function Implementation: Code • Function Header – gives: • name (e.g. Double), • parameter names and types (e.g. num: integer), and • return type (e.g. integer) • Function Body – gives code: Function Double(num As integer) As integer Double = num * 2 End Function num: integer Double integer

13. FtoC Animation

14. FtoC Function • The declaration: Function FtoC(F As double) As double FtoC = ((f-32) * 5) / 9 End Function • The call: lblResult.Caption = FtoC(50)

15. Example: Temp v2 Temp v2 Option Explicit Function FtoC(F As Double) As Double FtoC = ((F - 32) * 5) / 9 End Function Private Sub Form_Load() lblResult.Caption = FtoC(txtFah.Text) End Sub Private Sub txtFah_Change() lblResult.Caption = FtoC(txtFah.Text) End Sub

16. Question: Function Diagrams Thing double km: double Miles double num: long Twice long • Draw function diagram for the following code: Function Thing() As Double Function Miles(km As Double) As Double Function Twice(num As Long) As Long

17. Question: Function Headers Mins: integer Minutes integer Hours: integer Pounds: integer Euros integer • Generate the code for the following diagrams: Function Euros(Pounds As Integer) As Integer Function Minutes(Mins As Integer, _ Hours As Integer) As Integer

18. Function Animation: Total

19. Example: SnakeTemp v1 Option Explicit Sub Draw() Dim t As Long picTemp.Cls picTemp.Line (200, 1 * 200)-(200, 16 * 200) For t = 80 To 95 Step 1 picTemp.Line (200, (t - 79) * 200)-(300, (t - 79) * 200) Next For t = 80 To 95 Step 5 picTemp.Line (300, (t - 79) * 200)-(500, (t - 79) * 200) picTemp.Print t & "F = " & (((t - 32) * 5) / 9) & "C" Next picTemp.Line (200, (txtCur.Text - 79) * 200)-(600, (txtCur.Text - 79) * 200), vbRed picTemp.Print txtCur.Text & "F = " & (((txtCur.Text - 32) * 5) / 9) & "C" End Sub Private Sub Form_Load() Me.Show Draw End Sub Private Sub txtCur_Change() Draw End Sub SnakeTemp v1

20. Example: SnakeTemp v2 Option Explicit Const st = 80 Const en = 95 Const x1 = 200 Const x2 = 300 Const x3 = 500 Const x4 = 600 Const yS = 200 ' Vertical scaling factor. Const yO = st - 1 ' Vertical offset (gap at top). Sub Draw() Dim t As Long picTemp.Cls picTemp.Line (x1, (st - yO) * yS)-(x1, (en - yO) * yS) For t = st To en Step 1 picTemp.Line (x1, (t - yO) * yS)-(x2, (t - yO) * yS) Next For t = st To en Step 5 picTemp.Line (x2, (t - yO) * yS)-(x3, (t - yO) * yS) picTemp.Print t & "F = " & (((t - 32) * 5) / 9) & "C" Next picTemp.Line (x1, (txtCur.Text - yO) * yS)-(x4, (txtCur.Text - yO) * yS), vbRed picTemp.Print txtCur.Text & "F = " & (((txtCur.Text - 32) * 5) / 9) & "C" End Sub Private Sub Form_Load() Me.Show Draw End Sub Private Sub txtCur_Change() Draw End Sub SnakeTemp v2

21. Example: SnakeTemp v3 Option Explicit Const st = 80 Const en = 95 Const x1 = 200 Const x2 = 300 Const x3 = 500 Const x4 = 600 Const yS = 200 ' Vertical scaling factor. Const yO = st - 1 ' Vertical offset (gap at top). Function FtoC(F As Double) As Double FtoC = ((F - 32) * 5) / 9 End Function Sub Draw() Dim t As Long picTemp.Cls picTemp.Line (x1, (st - yO) * yS)-(x1, (en - yO) * yS) For t = st To en Step 1 picTemp.Line (x1, (t - yO) * yS)-(x2, (t - yO) * yS) Next For t = st To en Step 5 picTemp.Line (x2, (t - yO) * yS)-(x3, (t - yO) * yS) picTemp.Print t & "F = " & FtoC(t) & "C" Next picTemp.Line (x1, (txtCur.Text - yO) * yS)-(x4, (txtCur.Text - yO) * yS), vbRed picTemp.Print txtCur.Text & "F = " & FtoC(txtCur.Text) & "C" End Sub Private Sub Form_Load() Me.Show Draw End Sub Private Sub txtCur_Change() Draw End Sub SnakeTemp v3

22. Tutorial Exercises: Temp • Task 1: Get the temperature examples v1 and v2 (from the lecture) working

23. Tutorial Exercises: Snake Temp • Task 1: Get the snake temperature examples from the lecture working. • Task 2: There is another place that a function can be used to simplify the code. Hint: it’s to do with calculating the vertical plotting position

24. Tutorial Exercises: Snake Length • Task 1: Create a program that stores the following length (inch) data in an array, and plots a graph of it (using line method): Date Years Inches Tuesday 25 June 2002 6 70.5   5½   Monday 25 June 2001 5 70.0   4½   Sunday 25 June 2000 4 68.0 Saturday 25 December 1999 3½ 66.0 Friday 25 June 1999 3 61.0 Friday 25 December 1998 2½ 57.5 Thursday 25 June 1998 2 48.5 Thursday 25 December 1997 1½ 43.0 Wednesday 25 June 1997 1 37.5 • Task 2: Modify your code – add code that calculates an average length and plots it on the graph (use a function to calculate the total). • Task 3: Modify your code – to convert all values to cm (1 inch = 2.54 cm).