CSCI1413 Programming

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CSCI1413 Programming

• Week 5
• Topic Selection

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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.

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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.
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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

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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

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• 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

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• 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

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• 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

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• 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.

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• 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.

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• 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.

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• 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.

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• 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.

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• 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.

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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.

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• 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.

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• 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.

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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.

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• 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.

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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

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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

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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.

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• 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

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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

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• 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

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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.

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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

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• 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.

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• 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

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• 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.

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• 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.

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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

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• 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

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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.

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• 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

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• 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.

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• 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.

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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.

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And now

• Its over to you