Validation and Selection.

1
Validation and Selection.

• "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

2
Poorly Handled Data Causes Crashes!

• Dim FirstName as Integer
• FirstNameFred
• Or
• A field called Age ,
• What if instead of entering 14 you type fourteen?

3

• 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.

4
Selection

• Selection is all about making decisions.
• 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.

5
Decisions and Criteria

• Selection means making decisions based on certain
  conditions.
• E.g. If your wallet is empty then you go to the
  cash point.
• The decision - do we go to the cash point or
  not.
• The condition - how much money we may or may not
  have in our wallet.

6
The IF Statement

• To code decisions in VB we use the If statement.
• 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.

7
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
• Q. What would be the value of the variable City?

8
Using If for Validation.

• 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
• Use the data
• End If

9
Boolean Data Types and Validation Functions.

• Before looking any further at If Statements, we
  shall introduce two other programming topics
• 1. The Boolean Data Type
• 2. Validation Functions.

10
The Boolean Data Type.

• May only contain one of two values
• True or False
• To declare a Boolean variable
• Dim IsItOK as Boolean
• declares a new Boolean variable called IsItOk.

11
Validation Functions 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.

12
Bringing it all together.

• Dim IsItOK as Boolean
• Dim Age as Integer
• IsItOk IsNumeric(txtAge.Text)
• If IsItOk True then
• Age txtAge.Text
• End If

13
If Then Else.

• 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