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CSCI1413 Programming

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What about those 'bigger and better idiots' ... only accepts numeric data what is to stop some idiot typing their name into it? ... – PowerPoint PPT presentation

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Title: CSCI1413 Programming

1
CSCI1413 Programming

Week 5
Topic Selection

2
Review of Last Week

Last week you were introduced to a range of data
types and will have seen how data needs to be
applied correctly to the correct variable types.
You will also have designed test data for your
programs.

3
Overview of This Week

Lab 1
You will be introduced to the process of
validating user input. In learning about
validation you will be introduced to an important
programming concept called selection.

Lab 2 You will be introduced to the debugging
environment and see how it may be used to locate
errors.
4
Preview of Next Week

Next week you will be given a chance to apply
everything you have learnt so far and create a
program from scratch. Both lab sessions will be
dedicated to this mini project.
Reading for this week Tutorial 8

5
Lab 1 Tutors Talk Validation

"Programming today is a race between software
engineers striving to build bigger and better
idiot-proof programs and the Universe trying to
produce bigger and better idiots. So far, the
Universe is winning."
Rich Cook

In last weeks work, you will have declared a set
of variables to accept data from a
pre-defined form.
You should have a section of code that looks
something like this

Dim FirstName As String
Dim SecondName As String
Dim Address As String
Dim PostCode As String
Dim TelephoneNo As String
Dim InvoiceNo As Integer
Dim InvoiceTotal As Single
Dim PaymentDate As Date
Dim Cash As Single
Dim Cheque As Single
Dim CreditCard As Single
Dim CreditCardNo As String
Dim Age As Integer

FirstName txtFirstName.Text
SecondName txtSecondName.Text
Age txtAge.Text
Address txtAddress.Text
PostCode txtPostCode.Text
TelephoneNo txtTelNo.Text
InvoiceNo txtInvoiceNo.Text
InvoiceTotal txtInvoiceTotal.Text
PaymentDate txtPaymentDate.Text
Cash txtCash.Text
Cheque txtCheque.Text
CreditCard txtCreditCard.Text
CreditCardNo txtCardNo.Text

You should have seen how selecting an incorrect
data type for a variable will result in your
program crashing.
For example
Dim FirstName as Integer
FirstNameFred
Would result in a crash because the Integer data
type only accepts whole numeric values within a
certain range.
Fred is textual data and inappropriate for the
Integer data type. For FirstName we should be
using the String data type.

So it is important that you select the right data
type for your variables, however
What about those bigger and better idiots? What
about the users that will be using the software
you create?
As you saw when you created your test data,
entering data of the wrong type will crash your
program.

If I have a field on my form called Age which
only accepts numeric data what is to stop some
idiot typing their name into it? What if instead
of entering 14 they type fourteen?
Sending the wrong data to the correct data type
will end in the same result as selecting the
wrong data type in the first place.
In order to stop users entering inappropriate
data, we need some system of checking the data
entered.

The process of checking data as it is entered is
called validation.
In looking at validation we shall introduce some
of the validation functions but more importantly
we shall introduce the idea of selection.
Selection is all about making decisions.

we make decisions all of the time
If its last orders then buy another round.
If its raining then take an umbrella.
If it is 930am then think about going to the 9am
lecture.
And so on.

Selection means we are making decisions based on
certain criteria that may or may not be the case.
If your wallet is empty then you go to the cash
point.
The decision is do we go to the cash point or
not.
The criteria is how much money we may or may not
have in our wallet.
In writing programs the ability to make decisions
is vital for any programming language.

15
The IF Statement

The main statement used in VB for making
decisions depending on different conditions is
called IF.
We have seen how we can allocate RAM as variables
to store data.
We have seen how we can perform processing on the
contents of these variables and place the results
into other variables.

The next step is to perform certain calculations,
only if a specific variable contains a certain
value.
For example if the time on the clock has reached
zero, we detonate the bomb.
If the washing machine control has reached a
certain point on the dial, we make the drum spin
to clear water out of the clothes.

The If statement takes the following form
If condition Then
execute the code here
End If
In the above example, if the condition is true,
then the code between If End If will run,
otherwise, the code will be ignored.

18
In the following code

Dim AreaCode as String
Dim City as String
AreaCode0116
If AreaCode 0115 Then
City Nottingham
End If
If AreaCode 0116 Then
City Leicester
End If
The value of the variable City would be set to
Leicester.

Since the value of AreaCode is 0116, only the
line of code
City Leicester
will run,
not the code
City Nottingham
The If statements may be used to control which
code runs under what set of circumstances.

20
However

Dim AreaCode as String
Dim City as String
AreaCode0117
City Unknown Area Code
If AreaCode 0115 Then
City Nottingham
End If
If AreaCode 0116 Then
City Leicester
End If
The value of the variable City would undefined

21
In the following better code

Dim AreaCode as String
Dim City as String
AreaCode0117
City Unknown Area Code
If AreaCode 0115 Then
City Nottingham
End If
If AreaCode 0116 Then
City Leicester
End If
The value of the variable City would be set to
Unknown Area Code

22
Using If for Validation.

Among many other things, If is helpful for
validation.
We can use If to look at our data and decide if
the data is useful/correct or not.
Assume we have a field to capture a persons age
on a form.

How can we check to see if the user has typed a
suitable age value or not?
We want the If statement to do something like
If the value entered by the user is a number Then
Assign the data to our variable
End If

24
The Boolean Data Type.

Before we do any more on If, we need to look at a
new data type called Boolean.
Like all other data types, the Boolean data type
is limited in its range and the data it is able
to store.
The Boolean data type may only contain one of two
values True or False

To declare a Boolean variable we might type
Dim IsItOK as Boolean
declares a new Boolean variable called IsItOk.
To assign the value True to it we might type
IsItOK True

26
Introduction to Validation Functions IsNumeric

VB provides a large range of functions that may
be used for validation.
What these functions do is that they examine data
and report back something about the state of the
data.
One such function is called IsNumeric.
IsNumeric (as the name suggests) looks at data
and reports back if it is numeric or not.
IsNumeric returns a Boolean value as a result of
its test.
We can assign this Boolean value to a variable
and use it in an If statement.

27
Bringing it all together.

Consider the following situation.
We have a form with an age field on it. we want
the user to enter a valid numeric value, not
text. If we try to process a non numeric value
our program is going to crash.
We want the user to enter 30 not thirty.
The text box on the form is called txtAge.
We could first declare a variable to store the
result of our test
Dim IsItOK as Boolean

We could then use IsNumeric to examine what the
user typed and see if the value is any use
IsItOk IsNumeric(txtAge.Text)
In the above line, IsNumeric will examine the
contents of the text box and assign a True or
False value to the IsItOk variable depending on
if the value in txtAge is a number or not.

Now the IsItOk variable may be used in an If
statement to see if it is ok to place the value
into a suitable variable
If IsItOk True then
Age txtAge.Text
End If

The whole section of code would read as follows
Dim IsItOK as Boolean
Dim Age as Integer
IsItOk IsNumeric(txtAge.Text)
If IsItOk True then
Age txtAge.Text
End If
Using the above example, should the user type
text into the txtAge text box, IsNumeric will
return False when it looks at the data and the
value will not be placed into the Integer
variable Age.

Validation is all well and good so long as the
program doesnt crash, but it would be nice if we
gave the user some idea of what they had done
wrong.
Take the age example, if the user types thirty
rather than 30. It would be nice if the user
received an error message telling them what they
had done wrong.
e.g.

32
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33
If Then Else

To do this we can extend the If statement to
include an Else option.
If the value entered is valid Then
assign the value to our variable
Else
display an error message
End If

The code for this would look like this
Dim IsItOK as Boolean
Dim Age as Integer
IsItOk IsNumeric(txtAge.Text)
If IsItOk True then
Age txtAge.Text
Else
MessageBox.Show("Age is not valid")
End If

35
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36
Exercise

In this exercise you will extend the application
used to illustrate various data types by adding
suitable validation.
Copy the source code for this exercise from
L\mdean\csci1413\samples\validation
Open the project and open the code for the click
event of the OK button.

The source code has been provided including
Dim FirstName As String
Dim SecondName As String
Dim InvoiceNo As Integer
Dim InvoiceTotal As Single
Dim PaymentDate As Date
FirstName txtFirstName.Text
SecondName txtSecondName.Text
InvoiceNo txtInvoiceNo.Text
InvoiceTotal txtInvoiceTotal.Text
PaymentDate txtPaymentDate.Text

to which you will add suitable validation using
the correct validation functions and suitably
placed If statements.
To validate numbers you need to use IsNumeric.
To validate dates you need to use IsDate.
Notice that the lines of code that do the
assignment have been commented out. (The lines in
green!)
This has been done to help you not get confused.

You will find it simpler if you write the code
processing each field one at a time. i.e. write
the code to process the First Name, get that
working and tested, then do Second Name, then do
Age.
So to activate the line of code that assigns the
First Name field to the FirstName variable remove
the comment symbol .
If you do too much at once you may have more
errors to fix and you might get confused.

40
Try to solve one error at a time!

When you think that a field has been suitably
validated, try entering your valid / invalid test
data from last weeks exercise to see if your
program crashes or not.

41
And now