Post on 02-Jan-2016
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Principals of Software Design and Development
VariablesConstantsData Types
Error HandlingStarter: Blockbusters
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Learning Objectives
Describe Variables and Constants Further describe the Error handling Identify the need for good
documentation of the above Improve the Business Card Program!
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Variables
A variable is a named piece of memory that holds information
Think of it like a glass that can hold a measured amount of something
The contents of the glass is called a data type
The amount of contents would be its value
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Declaring Variables
We declare variables to tell the program: Their Name What they will hold
Dim Glass as Beer Glass = “Full”
Declaration Keyword
Name Data type
Initialise the variable
Initial value: Full
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Declaring and Initialising Variables
Dim PupilName as String Dim PupilGrade as Integer
Name = “Hermione” Grade = 10
Variable names cannot contain
spaces. Begin each word with a capital
instead
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Variables
Its value can be changed within the code
Variables can be used to do mathematics
It is better to use a variable than just type in a number Makes code easier to read Changes can be made more easily
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Data Types
A variable can only be of one data type Some data types take up more memory
than others Always use the least amount of memory
possible to keep programs running efficiently
Some data types have specialist functions
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The Money Trap
The Currency data type cannot be formatted to display only two decimal places – this is a problem with VB6
To display money properly you must use Single
Single variables may be formatted to display a set number of decimal places Dim Cost as Single Cost = 5.99 The pound sign
will be added later when the value is
displayed.
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Quiz
Declare and Initialise variables for the following scenarios A telephone number – 511 6000 The name of a course – A Level English A student’s age – 17 Student’s date of birth – 01/02/1989 A bus fare - £3.50 Dinner tickets issued – Yes Student Name – Tom Brown
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Quiz
Dim TelNo as String
Dim Course as String
Dim Age as Integer
Dim DateOfBirth as Date
Dim BusFare as Single
Dim DinnerTickets as Boolean
Dim StudentName as String
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Quiz
TelNo = “511 6000”
Course = “A Level English”
Age = “Integer”
DateOfBirth = “01/02/1989”
BusFare = 3.50
DinnerTickets = True
Student Name = “Tom Brown”
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Constants
Constants are like variables that cannot be changed once initialised
Use them for values that will not change while the program is running, e.g. VAT
Make things easier to remember e.g. the colour constant vbGreen replaces the value &H0080FF80& !!
Their names usually begin with “c” e.g. cMin
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Calculations
Use variables just like numbers!
Dim FirstNo as IntegerDim SecondNo as IntegerDim Result as Integer
FirstNo = 2SecondNo = 5
FirstNo + SecondNo = 7SecondNo – FirstNo = 3
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Comparisons Operators are used to compare two values
x = y x is exactly the same as y
OR
make x the same as y
x > y x is bigger than y
x < y x is smaller than y
x >= y x is bigger than or equal to y
x <= y x is smaller than or equal to y
x <> y x is not the same as y
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Quick Quiz
Use comparison operators to write expressions for these scenarios Phone bill is higher than gas bill Pink is not equal to green Electric bill is the same or less than gas bill Starburst same as Opal Fruits
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Quick Quiz
PhoneBill > GasBill Pink <> Green ElectricBill <= GasBill Starburst = OpalFruits
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Option ExplicitYour New Best Friend
Who has had problems with spelling mistakes in code?
Put Option Explicit in the General Declarations of a form’s code Keywords, variable and constant names
are capitalised and corrected Errors are highlighted when you run
code
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Error Handling – Why Bother?
Errors can make programs Crash, freeze or end unexpectedly Difficult to use Give inaccurate information
Try to think of errors likely to occur Test for errors Trap errors – tell the computer what to do if
they occur, e.g. display a message It is never possible to trap all errors!
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Error Messages
Good error messages should Explain what the problem was
Use Plain English Give examples if necessary
Why it occurred How to fix it What the user should do next Give options as appropriate
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More about MsgBox
Title
Style (icon)
Style (buttons)
Message
Put line breaks into your messages by using the constant vbctrlf
e.g. MsgBox “This will be the top line” & vbcrlf & “and this will be the bottom line”
Default choice
Message boxes can be customised to be more user friendly