21 – User Defined Functions
Mark Dixon, SoCCE
SOFT 131
Page 1
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
Mark Dixon, SoCCE
SOFT 131
Page 2
Meet George
• Common Boa Constrictor
– boa constrictor imperator
• Native to Central & South America
• No venom (no poison)
Mark Dixon, SoCCE
SOFT 131
Page 3
Meet George (cont.)
Mark Dixon, SoCCE
SOFT 131
Page 4
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
Mark Dixon, SoCCE
SOFT 131
Page 5
George (cont.)
• To convert from Fahrenheit to Celsius:
( f  32)  5
c
9
e.g. 50 Fahrenheit is:
(50  32)  5
c
9
c = 10
Mark Dixon, SoCCE
SOFT 131
Page 6
Example: Temp v1
Temp v1
Option Explicit
Calculation
repeated
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
Mark Dixon, SoCCE
SOFT 131
Page 7
Procedures and Functions
• Both Procedures and Functions
– Group of statements
– Identified by unique name
– mirror real life activities
• Procedures – just do something
• Functions – return a value
– used to perform calculations
Mark Dixon, SoCCE
SOFT 131
Page 8
Built in Functions: Sqr
Option Explicit
Private Sub txtNum_Change()
Dim tmpNum As Double
tmpNum = Val(Me.txtNum.Text)
Me.lblResult.Caption = Sqr(tmpNum)
End Sub
Square
Root
• Sqr – gives square root of a number:
X: Double
Sqr
Double
– Examples
Sqr(4)
Sqr(3)
Sqr(2)
Mark Dixon, SoCCE
returns
returns
returns
SOFT 131
16
9
4
Page 9
Built in Functions: Rnd
Option Explicit
Private Sub Form_Load()
Randomize
End Sub
Random
Numbers
Private Sub btnRandom_Click()
Me.lblNum.Caption = Rnd()
End Sub
• Rnd() function
– generates pseudo-random numbers
– >= 0 and <1
Random
Single
• Randomize – initialises random number
generator
Mark Dixon, SoCCE
SOFT 131
Page 10
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
Mark Dixon, SoCCE
SOFT 131
Page 11
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:
num: integer
Double
integer
Function Double(num As integer) As integer
Double = num * 2
End Function
Mark Dixon, SoCCE
SOFT 131
Page 12
FtoC Animation
Mark Dixon, SoCCE
SOFT 131
Page 13
FtoC Function
• The declaration:
Function FtoC(F As double) As double
FtoC = ((f-32) * 5) / 9
End Function
• The call:
lblResult.Caption = FtoC(50)
Mark Dixon, SoCCE
SOFT 131
Page 14
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
Mark Dixon, SoCCE
SOFT 131
Page 15
Question: Function Diagrams
• Draw function diagram for the following
code:
Function Thing() As Double
Thing
double
Function Miles(km As Double) As Double
km: double
Miles
double
Function Twice(num As Long) As Long
num: long
Mark Dixon, SoCCE
Twice
SOFT 131
long
Page 16
Question: Function Headers
• Generate the code for the following
diagrams:
Pounds: integer
Euros
integer
Function Euros(Pounds As Integer) As Integer
Mins: integer
Hours: integer
Minutes
integer
Function Minutes(Mins As Integer, _
Hours As Integer) As Integer
Mark Dixon, SoCCE
SOFT 131
Page 17
Function Animation: Total
Mark Dixon, SoCCE
SOFT 131
Page 18
Example: SnakeTemp v1
Option Explicit
Sub Draw()
Dim t As Long
picTemp.Cls
SnakeTemp v1
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
Mark Dixon, SoCCE
SOFT 131
Page 19
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).
SnakeTemp v2
Sub Draw()
Dim t As Long
picTemp.Cls
x1 st yO) * yS)-(x1, (en - yO) * yS)
picTemp.Line (
,( For t = st To en Step 1
picTemp.Line (
Next
x1, (t - yO) * yS)-(x2, (t - yO) * yS)
For t = st To en Step 5
x2 yO yS x3 yO) * yS)
picTemp.Line (
, (t )*
)-(
, (t picTemp.Print t & "F = " & (((t - 32) * 5) / 9) & "C"
Next
x1
yO yS x4
yO) * yS), vbRed
picTemp.Line (
, (txtCur.Text )*
)-(
, (txtCur.Text 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
Mark Dixon, SoCCE
SOFT 131
Page 20
Example: SnakeTemp v3
Option Explicit
Const st = 80
Const en = 95
SnakeTemp v3
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 = " &
Next
FtoC(t) & "C"
picTemp.Line (x1, (txtCur.Text - yO) * yS)-(x4, (txtCur.Text - yO) * yS), vbRed
picTemp.Print txtCur.Text & "F = " &
End Sub
FtoC(txtCur.Text) & "C"
Private Sub Form_Load()
Me.Show
Draw
End Sub
Private Sub txtCur_Change()
Draw
End Sub
Mark Dixon, SoCCE
SOFT 131
Page 21
Tutorial Exercises: Temp
• Task 1: Get the temperature examples v1
and v2 (from the lecture) working
Mark Dixon, SoCCE
SOFT 131
Page 22
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
Mark Dixon, SoCCE
SOFT 131
Page 23
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
Tuesday 25 June 2002
Years
6
5½
5
4½
4
3½
3
2½
2
1½
1
Monday 25 June 2001
Sunday 25 June 2000
Saturday 25 December 1999
Friday 25 June 1999
Friday 25 December 1998
Thursday 25 June 1998
Thursday 25 December 1997
Wednesday 25 June 1997
Inches
70.5
70.0
68.0
66.0
61.0
57.5
48.5
43.0
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).
Mark Dixon, SoCCE
SOFT 131
Page 24