Lecture 16 - Subroutines & Functions

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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 later 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

1  compareNumbers()
2     inputNumbers()
3     getSmallestNumber()
4     displaySmallest()
5  STOP

6  inputNumbers()
7     Input number1, number2, number3
8  EXIT

9  getSmallestNumber()
10     smallest = number1
11     IF number2 < smallest THEN
12         smallest = number2
13     ENDIF
14     IF number3 < smallest THEN
15         smallest = number3
16     ENDIF
17 EXIT

18 displaySmallest()
19     Display smallest
20 EXIT     

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
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 


 
 
 

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 later 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

Dim costPer100Grams as Double
Dim speed as Double
costPer100Grams = 12.3 
Debug.Print "Cost per 100 grams = "; Format(costPer100Grams, "$0.00")
speed = 1234567.89
Debug.Print "Speed = "; Format(speed, "#,##0.0kph") 'Speed = 1,234,567.9kph

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 = $12.30" is then displayed
• Note: the value in costPer100Grams is not changed

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
cost = 12.347
cost = Round(cost, 1) ' cost is now 12.3

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

    ' Input x and calculate its square root
    x = InputBox("x ?")
    y = Sqr(x)
    Debug.Print "The square root of "; x; " is "; y
    Debug.Print

    ' Input price and calculate its GST
    price = InputBox("Price before GST ?")
    tax = price * 10 / 100
    Debug.Print "GST before rounding is = "; tax
    tax = Round(tax, 2)
    Debug.Print "GST rounded to the nearest cent is = "; tax
    Debug.Print

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

Written by Tim Whitfort.