Lecture 16 - Subroutines & Functions

Objectives

• To introduce subroutines, when to use them and how this is done.

Contents

• Subroutine Example 3 - Compare Numbers
• Subroutine Example 4 - Swap Two Values
• Functions
• What's Next?
• Key Points
• Further Reading

1. Subroutine Example 3 - Compare Numbers

Problem Description

Write a program that allows a user to input three numbers and then displays the smallest number. This technique is similar to that used in latter programming subjects to find the smallest in a list of values.

Example Run

Run
Number 1 ? 5
Number 2 ? 3
Number 3 ? 8
Smallest number = 3

Defining Diagram

Outline the Solution

Pseudo Code

Data Dictionary

Desk Check

Input data: number1 = 5, number2 = 3, number3 = 8; Correct result smallest = 3

Code

' Name: CompareNumbers
' Purpose: Compare 3 numbers and display the smallest
' Author: Tim Whitfort

Option Compare Database
Option Explicit

' Declare global variables.
' Global variables are known throughout the program
Dim number1 As Double  ' First number to be compared
Dim number2 As Double  ' Second number to be compared
Dim number3 As Double  ' Third number to be compared
Dim smallest As Double ' The smallest of he number input


Sub Main()

    ' Call the inputNumbers subroutine
    inputNumbers

    ' call the getSmallestNumber subroutine
    getSmallestNumber

    ' Call the displaySmallest subroutine
    displaySmallest

End Sub


' Input the 3 numbers
Sub inputNumbers()

    number1 = InputBox("Number 1 ?")
    number2 = InputBox("Number 2 ?")
    number3 = InputBox("Number 3 ?")

End Sub


' Determine the smallest number
Sub getSmallestNumber()

 

End Sub


' Display the smallest of the 3 numbers
Sub displaySmallest()
    Debug.Print "Smallest is "; smallest
End Sub

2. Subroutine Example 4 - Swap Two Values

Problem Description

Write a program that uses a subroutine to swap two values. This technique is used in latter programming subjects to sort a list into order.

Example Run

Run
Number 1 ? 5
Number 2 ? 3
After swap
Number 1 = 3
Number 2 = 5

Defining Diagram

Outline the Solution

We can't just assign number2 to number1 and vice-versa as this would loose the original value of number1. A temporary variable is needed to hold the value during the swap.

Pseudo Code

swapTwoNumbers()
    inputNumbers()
    swap()
    displayNumbers()
STOP

inputNumbers()
    Input number1, number2
EXIT

swap()
    temp = number1
    number1 = number2
    number2 = temp
EXIT

displayNumbers()
    Display number1, number2
EXIT

Data Dictionary

Desk Check

1  swapTwoNumbers()
2     inputNumbers()
3     swap()
4     displayNumbers()
5  STOP

6  inputNumbers()
7      Input number1, number2
8  EXIT

9  swap()
10     temp = number1
11     number1 = number2
12     number2 = temp
13 EXIT

14 displayNumbers()
15     Display number1, number2
16 EXIT

Input data: number1 = 5, number2 = 3; Correct results: number1 = 3, number2 = 5

Code

' Name: SwapTwoNumbers
' Purpose: Input 2 numbers, swap them then display the new values
' Author: Tim Whitfort

Option Compare Database
Option Explicit

' Declare global variables.
Dim number1 As Double 
Dim number2 As Double 

Sub Main()
    inputNumbers
    swap
    displayNumbers
End Sub

' Input the 2 numbers
Sub inputNumbers()
    number1 = InputBox("Number 1 ?")
    number2 = InputBox("Number 2 ?")
End Sub

' Swap the two numbers
Sub swap()
    ' Local variable. Temporary storage for number1
    Dim temp As Double 
    temp = number1
    number1 = number2
    number2 = temp
End Sub

' Display the numbers
Sub displayNumbers()
    Debug.Print "After swap"
    Debug.Print "Number 1 = "; number1
    Debug.Print "Number 2 = "; number2
End Sub

3. Functions

We have been using functions for some time!

Functions are a special type of subroutine that return a single value. We won't be writing functions in this subject, however we will look at a few functions that are provided by Basic.

InputBox function and the Format function are examples of functions that we have been using.

InputBox Function

• The InputBox function displays a message and returns the value input by the user. It is used to input values by the user.
• InputBox has 1 parameter:
  • a prompt string that is to be displayed by the InputBox, so the user knows what type of value to input
• The function returns the value input by the user.

A parameter is supplied (passed to) the function to provide data required for the function to work, in this case the input prompt to be displayed.

A return value is the result that we are interested in finding out. In this case it is the value input by the user.

Example

Dim cost As Double
cost = InputBox("Cost of an item ? ")

In the above example:

•  "Cost of an item ? " is the input prompt parameter
• the user inputs a value which is returned by the function, and substituted in place of the function call
• the value is assigned (stored) in the variable cost

Format Function

• The Format function provides a formatted representation of a value. It is usually used in Debug.Print statements to format numbers for display.
• Format has 2 parameters:
  • the value (usually a number) to be formatted and
  • a format string describing how the value is to be formatted.
• The function returns a string representation of the value which is formatted

Example

costPer100Grams = 12.3 
Debug.Print Format("Cost per 100 grams = "; Format(costPer100Grams, "0.00")

In the above example:

•  costPer100Grams is the value parameter (12.3 in the example)
• "0.00" if the format string parameter (this indicates the formatted number is to have 2 decimal places)
• the result is calculated (e.g. "12.30") and returned by the function, and substituted in place of the function call
• the string "Cost per 100 grams = 12.3" is then displayed

Mathematical Functions

Here are some examples of mathematical functions provided in Basic:

Round Function

• Rounds a floating point number (Double) number to the specified number of decimal places. Typically this is assigned to (stored in) a variable or used in a calculation
• Round has 2 parameters:
  • the value to be rounded
  • the number of decimal places the value is to be rounded to
• The function returns a Double that is the value round as specified

Example

Dim cost As Double
cost = 12.347
cost = Round(cost, 2) ' cost is now 12.35

Sqr Function

• The Sqr function calculates the square root of a number. Typically this is assigned to (stored in) a variable or used in a calculation
• Sqr has 1 parameter:
  • the value that you want to find the square root of
• The function returns a Double that is the square root of the number passed to the function

Example

Dim y As Double
y = Sqr(x)

Example

' Name: MathsFunctionDemo
' Author: Tim Whitfort
' Purpose: Demonstrate using built-in functions

Option Compare Database
Option Explicit

Sub Main()

    ' Local Declarations
    Dim price As Double ' The price of an item before tax
    Dim tax As Double ' The GST on an item
    Dim x As Double ' A number
    Dim y As Double ' The square root of x

End Sub

What's Next?

That's the end of the lectures on Software Development. For the next few weeks we will be looking at the UNIX operating system.

Key Points

• Functions are a special type of subroutine that return a single value
• Basic has many built-in functions e.g. InputBox, Format, Sqr, Round

Further Reading

• Pseudo Code Guide
• Desk Checking Guide
• Basic Language Guide