Lecture 13 - Totalling, Averaging & Validation

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Objectives

• Introduce students to totalling, averaging, counting numbers
• To introduce validation of inputs

Contents

• Totalling, Counting and Averaging
• Validation
• Key Points
• Further Reading

1. Totalling, Counting and Averaging

Problem Description

The user is to input prices of items for sale. At the end of the day the total of the sales made, the number of items sold and the average price of items sold is to be displayed.

Discussion

It isn't possible to hold all of the prices in separate variables - we don't know how many prices are to be added up. It is far simpler to keep a running total (the total so far), so at the end this will be the total of all of the numbers.

As prices must be more than zero, a sentinel value of 0 can be used (-1 or some other suitable value could also be used in this problem).

Example Runs

Run
Price (0 to exit) ? 1.5
Price (0 to exit) ? 2
Price (0 to exit) ? 0

Count = 2, Total = 3.50, Average = 1.75

Defining Diagram

Outline the Solution

Pseudo Code

sumPrices()
    totalPrice = 0
    count = 0
    Input price
    WHILE price <> 0 DO
        totalPrice = totalPrice + price
        count = count + 1
        Input price
    ENDWHILE
    Display  count, totalPrice
    IF count > 0 THEN
        average = totalPrice / count
        Display average
    ELSE
        Display "No values input, unable to calculate average"
    ENDIF
STOP

Data Dictionary

Desk Check

1  sumPrices()
2      totalPrice = 0
3      count = 0
4      Input price
5      WHILE price <> 0 DO
6          totalPrice = totalPrice + price
7          count = count + 1
8          Input price
9      ENDWHILE
10     Display count, totalPrice
11     IF count > 0 THEN
12         average = totalPrice / count
13         Display average
14     ELSE
15         Display "No values input, unable to calculate average"
16     ENDIF
17 STOP

Inputs: prices = 1.50, 2, 0. Correct results: average = 1.75, count = 2, totalPrice = 3.50.

Basic Code

' Name: sumPrices
' Purpose: Produce statistics on a series of prices displaying the total, average and count
' Author: Tim Whitfort

Option Compare Database
Option Explicit

Sub Main()

    Dim average As Double ' The average price of the items sold
    Dim count As Long     ' The number of items sold
    Dim price As Double   ' The price of the item for sale in $
    Dim totalPrice As Double  ' The total price of all the items in $

    totalPrice = 0
    count = 0
    price = InputBox("Price (0 to exit) ?")
    While price <> 0
        totalPrice = totalPrice + price
        count = count + 1
        price = InputBox("Price (0 to exit) ?")
    Wend

    Debug.Print "Total price: $"; Format(totalPrice, "0.00")
    Debug.Print "Number of items sold: "; count
    If count > 0 Then
        average =  totalPrice / count
        Debug.Print "Average price: $"; Format(average, "0.00")
    Else
        Debug.Print "No values input, unable to calculate average"
    End If

End Sub

Inputting using a Sentinel

Problems involving repeated input terminated by inputting a sentinel value have a similar format, as shown below:

inputWithSentinel()
    ...
    Input value
    WHILE value <> sentinel DO
        ...
        Input value
    ENDWHILE
    ...
STOP

Totalling/Summing

The above problem introduced totaling or summing. A variable is used to store a running total of the total of the numbers input so far. At the end the running total represents the total to all the numbers. The total variable can be called anything (e.g. total, sum, x). The data type of the total is usually the same as the data type of the value it is summing up.  The logic to totaling problems tends to be very similar:

inputAndTotaling()
    total = 0
    Input value
    WHILE condition DO
        Optionally do some processing on the value
        total = total + value
        Input value
    ENDWHILE
    Do something with the total
STOP

Counting

In earlier problems involving loops, a counter was used in controlling how many times the loop was executed. e.g. For counter = 1 To 10. In the following logic, the counter is simply used to count how many times the loop repeated, it is not used in deciding when the loop is to end.

inputAndCounting()
    count = 0
    Input value
    WHILE condition DO
        count = count + 1
        Optionally do some processing on the value
        Input value
    ENDWHILE
    Do something with count
STOP

Note: in the above example the count is not used to control the loop, it is simply counting how many values were input.

Averaging

Averaging requires totaling and counting. Care needs to be taken to ensure that there is something to average (if count = 0 then division by zero occurs, crashing the program)

inputAveraging()
    total = 0
    count = 0
    Input value
    WHILE condition DO
        total = total + value
        count = count + 1
        Optionally do some processing on the value
        Input value
    ENDWHILE
    Do something with total, count
    IF count > 0 THEN
        average = total / count
        DO something with average
    ENDIF
STOP

2. Validation

Introduction

Checking whether a value is correct is known as Validation. We will look at validating values input by a user. When inputting a value, a user may get a value right the first time, or may take a number of attempts to get it right. To ensure that the value input is valid a loop is used which repeats until the user gets it right.

Example Problems:

• Ensuring a mark is between 0 and 100 inclusive.
• Ensuring that a month number is a whole number between 1 and 12 inclusive.
• Ensure a loan amount is $5000 or more
• Ensure an ATM withdrawal amount is $20, $40, $50, $100, $200 or $400
• Ensuring a menu item is "A", "D", "M" or "X"

If validation isn't performed, the program needs to check each time the value is used for invalid values, this may require checking in multiple places, or alternatively incorrect results may occur because invalid input values are not detected and used in processing. (e.g. allowing mark of -50). Input validation requires the user to input a value while it is wrong (until they it is correct). This requires a loop as it is unknown how many times it will take the user to get the input correct. Therefore the only way out of the loop is to input a correct value.

Performing validation checks on input allows the checking to be performed in one place, rather than throughout the code, and simplifies the logic as the programmer can assume that the value is correct.

Example 1: Validating a range of values

Problem Description

Write a program that allows a user to input a mark and checks that the mark is between 1 and 100.

Note: this is really just part of a program. Once the mark is obtained something would normally be done with it.

Defining Diagram

Outline the Solution

A While loop is required to repeat asking for a mark until it is correct (the loop is repeated while the value is wrong).

Pseudo Code

validateMark()
    Input mark
    WHILE mark < 0 OR mark > 100 DO
        Display "Error: mark must be between 0 and 100"
        Input mark
    ENDWHILE
    Display mark, " is a valid mark"
STOP

Data Dictionary

Desk Check

1 validateMark()
2     Input mark
3     WHILE mark < 0 OR mark > 100 DO
4         Display "Error: mark must be between 0 and 100"
5         Input mark
6     ENDWHILE
7     Display mark, " is a valid mark"
8 STOP

Inputs: mark = 70; Correct result: "70 is a valid mark"

Inputs: mark = 101. Correct Results: "Error: mark must be between 0 and 100"; Input mark
Inputs: mark = 100. Correct Results: "100 is a valid mark"

Inputs: mark = -1. Correct Results: "Error: mark must be between 0 and 100"; Input mark
Inputs: mark = 0. Correct Results: "0 is a valid mark"

Access Basic Code

' Name: validateMark
' Purpose: Validate a mark input by a user
' Author: Tim Whitfort

Option Compare Database
Option Explicit

Sub Main()

    Dim mark As Long ' A subject mark. Range: 0 to 100 inclusive.

    mark = InputBox("Mark (0 to 100) ?")

    ' While the mark is wrong
    While mark < 0 Or mark > 100
        ' MsgBox displays a message on the screen in a dialog box
        MsgBox ("Error: mark must be between 0 and 100")
        mark = InputBox("Mark (0 to 100) ?")
    Wend

    Debug.Print mark; " is a valid mark"

End Sub

Example 2: Validating discrete values

Problem Description

Write a program that allows a user to input a character representing a club membership type (F=Full, J=Junior, L=Life) and checks that the membership type is valid.

Defining Diagram

Outline the Solution

A While loop is required to repeat asking for a membershipType until it is correct.

Pseudo Code

validateMembershipType()
 
 
 
 
 
 
STOP

Data Dictionary

Desk Check

Inputs: membershipType = "F". Correct Results: "The membership type input is F"

Inputs: membershipType = "Z". Correct Results: "Error: membership type must be F, J or L"; Input membershipType
Inputs: membershipType = "J". Correct Results: "The membership type input is J"

Access Basic Code

' Name: validateMembershipType
' Purpose: Validate a membership type input by the user
' Author: Tim Whitfort

Option Compare Database
Option Explicit

Sub Main()
 
 
 
 
 
 
 
 
 
 
 
 
End Sub

General Validation Logic

Typically validation has the following logic:

Input value
WHILE value is wrong DO
    Display "Error: value is wrong"
    Input value
ENDWHILE

Range of Values

Where the value is valid over a range (e.g. 1 to 12) the condition has the format:

Input value
WHILE value < minimumValidValue OR value > maximumValidValue DO
    Display error
    Input value
ENDWHILE

Discrete Values

Where the value is valid for a number of discrete values (e.g. one of the numbers 1, 7, and 23) the condition has the format:

Input value
WHILE value <> validValue1 AND value <> validValue2 AND ... DO
    Display error
    Input value
ENDWHILE

Key Points

• Loops are often used for totalling, counting and averaging of numbers
• Validation is used to ensure that a value is correct
• Validation simplifies processing by (a) centralising the code to check a value and (b) allowing the programmer ton assume that the value is correct elsewhere in the program

Further Reading

• Pseudo Code Guide
• Desk Checking Guide
• Basic Language Guide
• Robertson, L. A. (2003) Simple Program Design: A Step-by-Step Approach. 4th ed. Melbourne: Thomson. chps 5-6

Written by Tim Whitfort.