1. 10.3 Procedures
Function Procedures
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2. Learning Objectives
Describe the difference between functions and sub procedures.
Explain how to write and call functions.
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3. Function Procedures
Known as just functions for short (sub procedures are known as just procedures for short).
Always returns one item of data to the calling procedure.
Remember that:
Sub procedures:
May return one or more items of data or no data at all to the calling procedure.
If only one value is to be returned then a Function is more suitable, if two or more values (or no values) need to be returned then a procedure is more suitable.

4. Built-in & User-defined Functions
Built-in Functions
Supplied by VB,
some of which you have used previously e.g. Format, Mid, Today
User-defined Functions
Written by the programmer.
You will learn to write these here.
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5. AreaTriangle = ½ * TriangleHeight * TriangleBase
Functions:
CalcTriangleArea
Main Program
TriangleHeight
AreaTriangle = ½ * TriangleHeight * TriangleBase
Input TriangleHeight
Input TriangleBase
Input RectangleWidth
Input RectangleLength
CalcTriangleArea
CalcRectangleArea
CalcHouseArea
Output HouseArea
TriangleBase
TriangleArea
CalcAreaRectangle
RectangleLength
AreaRectangle = RectangleWidth * RectangleBreadth
RectangleWidth
RectangleArea
AreaRectangle
CalcHouseArea
AreaTriangle
HouseArea = AreaTriangle + AreaRectangle
Parameters
HouseArea
Note that the variable to be returned no longer has to be sent to Function procedures (as they do to Sub Procedures – see 10.2 Procedures), so all formal parameters are value parameters. A function just returns one answer and it is left up to the main program to “know” what that answer means.

6. Writing a Function Similar to Sub procedures. Differences:
Its job is to return only one item of data.
So all formal parameters are value parameters (see the last slide).
i.e. Declared with ByVal
Starts with Private Function instead of Sub.
The first line of the function (after the Private Function…) has to declare the variable which will be returned:
Dim (Variable Name to be returned) As (Data Type)
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Continued on the next slide.

7. Writing a Function
The last line of the function (before the End Function) will return the one item of data to the calling procedure:
Return (Variable name to be returned)
When the function is called you treat it as a value (i.e. the value it returns).
Store it in a variable:
VariableName = FunctionName(Variables to be passed)
Display it directly:
Control.Text = FunctionName(Variables to be passed)
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8. Writing a Function i.e. Private Function …(ByVal … As …, By Val… As …)
Dim (Variable name to be returned) As (Data Type) …
Return (Variable name to be returned)
End Function
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9. Program 10.3 Calculating Interest
Specification:
Write a function to calculate the amount of interest earned on an investment.
The amount invested, the interest rate and the number of years the investment lasts, are to be entered by the user.
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10. Program 10.3 Calculating Interest
If you invested €1000 over 2 years at an interest rate of 10% per year then the interest paid after one year would be €100.
i.e. 10% of €1000
In year 2 you would get 10% of (€ €100 = €1100)
i.e. €110
So the total interest paid would be €210 after 2 years.
This program will calculate the interest paid for a given number of years.
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11. Program 10.3 Calculating Interest
Create a new project named ‘Calculating Interest’.
txtAmount
txtInterestRate
txtYears
lblInterest
butCalcInterest

12. Program 10.3 Calculating Interest
butCalcInterest code:
Dim Interest As Decimal
Dim Amount As Decimal
Dim InterestRate As Single
Dim Years As Integer
Amount = txtAmount.Text
InterestRate = txtInterestRate.Text
Years = txtYears.Text
‘Call the CalcInterest function by passing it the relevant
‘parameters and assigning it to the variable Interest.
Interest = CalcInterest(Amount, InterestRate, Years)
lblInterest.Text = “€” & Interest
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13. Program 10.3 Calculating Interest
Create a function:
Private Function CalcInterest(ByVal Amount _
As Decimal, ByVal InterestRate As Single, _
ByVal Years As Integer)
Dim Interest As Decimal ‘Variable to be returned.
‘For the loop to calculate year on year up to the number of
‘years the user entered.
Dim Year As Integer
For Year = 1 To Years
Interest = Interest + ((Amount + Interest) * _
InterestRate / 100)
Next Year
Return Interest ‘Return the variable Interest.
End Function

14. Program 10.3 Calculating Interest
Run the program and test it.
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15. Program 10.3 Calculating Interest
Write a “Boolean” function to test if the data entered in the text boxes is numeric.
Private Function NumericFN
Dim Numeric As Boolean = True
If IsNumeric (txtAmount.Text) = False Or IsNumeric (txtInterestRate.Text) = False Or IsNumeric (txtYears.Text) = False Then
Numeric = False
End If
Return Numeric
End Function
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16. Program 10.3 Calculating Interest
Test this function before the Dim lines in the butCalcInterest code:
If NumericFN = False Then
MsgBox (“Please only enter numbers!”)
Exit Sub
End If
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17. Commenting on Functions
In presentations 10.1 – 10.3 I will only ask for comments to procedures/functions.
Your comments MUST explain:
What is the procedure/function for?
Why and when (after and before what) are you calling it?
What parameters are being sent to it and why?
What parameter/s is/are being returned and why?

18. Extension “Average” Program 1
Open Program 3.4 Average you used last lesson.
Make the sub-procedure CalcMean into a function.
This is appropriate because CalcMean returns only one variable i.e. Mean.
Write a Boolean function to test for numeric data.
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19. Extension “Factorial Iterative Function” Program 1
Change the “Factorial” Program Loops so that it uses a function.
This is called an iterative function.
Make a copy of the previous program’s whole folder to keep the original program but rename this folder with the same name but add (Iterative Function Version).
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20. Extension Programs 2
Open any of the programs you have written previously and move appropriate lines into appropriate functions.
Particularly look at the original programs written in
3.1 Working with Data.
e.g.
“Discount” Program
“Selling Price” Program
“Interest” Program
etc....
Make a copy of the previous program’s whole folder to keep the original program but rename this folder with the same name but add (Function Version).
Do this for as many programs as you need to until you are confident in creating functions.
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21. Extension “Factorial Recursive Function” Program 3
Change the “Factorial” Program (Iterative Version) in presentation 5.1 Loops so that it uses a recursive function.
Hint:
FUNCTION calc(n)
IF n = 1 THEN
calc ← 1
ELSE
calc ← n * calc(n-1)
ENDIF
RETURN calc
Make a copy of the previous program’s whole folder to keep the original program but rename this folder with the same name but add (Recursive Function Version).
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22. Extension “Adding Numbers from 1 to a chosen number” Program 4
Change the “Adding Numbers from 1 to a chosen number” Program (Iterative Version) in presentation 5.1 Loops so that it uses a recursive function.
Make a copy of the previous program’s whole folder to keep the original program but rename this folder with the same name but add (Recursive Function Version).

23. Extension “Adding Numbers between two chosen numbers” Program 5
Change the
“Adding Numbers between two chosen numbers” Program (Iterative Version) in presentation 5.1 Loops so that it uses a recursive function.
Make a copy of the previous program’s whole folder to keep the original program but rename this folder with the same name but add (Recursive Function Version).

24. Extension “Adding Numbers from one chosen number to 6 then add 1” Program 6
Change the
“Adding Numbers from one chosen number to 6 then add 1” Program (Iterative Version) in presentation 5.1 Loops so that it uses a recursive function.
Make a copy of the previous program’s whole folder to keep the original program but rename this folder with the same name but add (Recursive Function Version).

25. When to Use Recursion?
Neat, cleaner and more elegant answer compared to the iterative version.
Writing recursive functions can give you greater confidence that you are coding correctly. Often, writing code that ought to be recursive without recursion (i.e., as loops) produces messy code.
Recursion often produces solutions that are very compact (requires few lines of typed code). So easier to debug than code with many lines.
Recursive solutions can be VERY inefficient though.
In general, recursion should be used when you know the number of recursive calls isn't excessive which depends on how much memory you have. Maximum: ~ 1000 recursive calls.
A large number of function calls could cause stack overflow.
Iteration can be simpler to understand and easier to write so less chance of error though.
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26. Plenary What is the difference between functions and sub procedures?
Function Procedures
Always returns one item of data to the calling procedure.
Sub procedures:
May return one or more items of data or no data at all to the calling procedure.
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27. Plenary How do we write functions?
Private Function …(ByVal … As …, ByVal… As …)
Dim (Variable name to be returned) As (Data Type) …
Return (Variable name to be returned)
End Function
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28. Plenary How do we call functions?
Assign a function’s return value to a variable.
(Variable name) = (Function Name)(Parameters)
Display a function’s return value e.g. to the text property of a control.
(Control Name).Text = (Function Name)(Parameters)
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