Visual Basic Syntax

Visual Basic master in one flashcard!

If…Then Statement

Definition: Tests a condition. If it's true, a set of statements is executed; otherwise, nothing happens.
Syntax:

IF (condition) THEN

statements

ENDIF

Example:

IF age >= 18 THEN

Message “You are eligible to vote”

ENDIF

If…Then…Else Statement

Definition: Provides an alternative path. If the condition is true, one action happens; else, another does.
Syntax:

IF (condition) THEN

statements

ELSE

statements

ENDIF

Example:

IF age >= 18 THEN

Message “You are eligible to vote”

ELSE

Message “Sorry, you are not eligible to vote”

ENDIF

If…Then…ElseIf…Else Statement

Definition: Checks multiple exclusive conditions. The first true condition's statements are executed.
Syntax:

IF condition1 THEN

statements

ELSEIF condition2 THEN

statements

ELSEIF condition3 THEN

statements

ELSE

statements

ENDIF

Example:

IF age <= 4 THEN

Message “Your rate is free”

ELSEIF age <= 12 THEN

Message “Children’s rate”

ELSEIF age < 65 THEN

Message “Full rate”

ELSE

Message “Seniors”

ENDIF

Nested If Statements

Definition: Allows multiple condition checks inside one another. Enables deeper logical branching.
Syntax:

IF (expression1) THEN

IF (expression2) THEN

statement1

ELSE

statement2

ENDIF

ELSE

body-of-else

ENDIF

Syntax 2:

IF (expression1) THEN

body-of-if

ELSE

IF (expression2) THEN

statement1

ELSE

statement2

ENDIF

ENDIF

Syntax 3:

IF (expression1) THEN

IF (expression2) THEN

statement1

ELSE

statement2

ENDIF

ELSE

IF (expression3) THEN

statement3

ELSE

statement4

ENDIF

ENDIF

Select Case – Exact Match

Definition: Tests a single expression against multiple possible values. Cleaner than long If…ElseIf chains.
Syntax:

Select Case expression

Case value1

statements

Case value2

statements

Case Else

statements

End Select

Example:

Select Case byMonth

Case 1,3,5,7,8,10,12

number_of_days = 31

Case 2

number_of_days = 28

Case Else

number_of_days = 30

End Select

Select Case – Relational Test

Definition: Uses relational comparisons (e.g., <, >) to decide which block to execute.
Syntax:

Select Case expression

Case Is < value

statements

Case Is < value

statements

Case Else

statements

End Select

Example:

Select Case marks

Case Is < 50

Result = “Fail”

Case Is < 60

Result = “Grade B”

Case Is < 75

Result = “Grade A”

Case Else

Result = “Grade A+”

End Select

Select Case – Range Check

Definition: Evaluates if an expression falls within specific value ranges. Great for grouped conditions.
Syntax:

Select Case expression

Case value1 To value2

statements

Case value3 To value4

statements

Case Else

statements

End Select

Example:

Select Case Age

Case 2 To 4

Message “Prenursery”

Case 4 To 6

Message “Kindergarten”

Case 6 To 10

Message “Primary”

Case Else

Message “Others”

End Select

IF summary

Variables / Declaring Variables

Variables:

• Store data in memory, like a textbox but not a control.

• Must be given a name.

Declaring Variables:

• Syntax: Dim v_Number1, v_Number2 As Integer

• This must be placed in the declarations section at the start of the program.

Variable Types

Data Type	Description	Storage Requirements	Range of values
Byte	Whole numbers	1 byte	0 to 255
Integer	Whole numbers	2 bytes	-32,768 to 32,767
Long	Whole numbers	4 bytes	Around –2 billion to 2 billion
Single	Decimal numbers “floating point” number	8 bytes	Around -1.79e308 to 1.79e308 with 15 digit precision
Currency	To 4 decimal places “floating point” number	8 bytes	+ - 922,337,203,685,477.5808
String	Text	1 byte per character
Boolean	True or False	2 bytes	True (i.e. the value –1) or False (i.e. the value 0)
Date	Valid date	8 bytes

Validation Techniques

Validation Techniques:

1. Input must be numeric (IsNumeric):

If IsNumeric(txtOne.Text) And IsNumeric(txtTwo.Text) Then

txtTotal.Text = Val(txtOne.Text) + Val(txtTwo.Text)

Else

MsgBox("Please enter a valid number")

End If

2. Input must be in a range:

If Val(txtPostcode.Text) <= 2999 Or Val(txtPostcode.Text) > 3999 Then

MsgBox("Illegal postcode. Please try again")

End If

3. Character length restriction (Len):

If Len(txtSurname.Text) > 20 Then

MsgBox("Surname too long, maximum 20 characters")

End If

4. Validating text input (<>):

If txtAnswer.Text <> "Yes" And txtAnswer.Text <> "No" Then

MsgBox("Your answer must be Yes or No")

End If

5. Validate for Upper/Lowercase:

If UCase(txtAnswer.Text) = "YES" Then

' Proceed with validation

End If

Do...Loop Until / Do...Loop While:

Do...Loop Until:

• Runs until the condition is true (checked after each iteration).

Do

'Loop code here

Loop Until condition

Do...Loop While:

• Runs while the condition is true (checked after each iteration).

Do

'Loop code here

Loop While condition

Do Until...Loop:

• Same as Do...Loop While, checks condition at the beginning of each iteration.

Do Until condition

'Loop code here

Loop

Do While...Loop:

• Same as Do...Loop Until, checks condition at the beginning.

Do While condition

'Loop code here

Loop

Array

• Arrays are used to store multiple values of the same data type in a single variable.

• Declared using the Dim statement:

Dim myData() As Integer

Dim myData(9) As String

Declaring and Initializing Arrays:

• You can declare and initialize arrays at the same time:

Dim myData() As Integer = {11, 12, 22, 7, 47, 32}

Dim students() As String = {"John", "Alice", "Antony"}

Fixed - Size Arrays

Fixed-size arrays have a predetermined number of elements:

Dim students(2) As String

students(0) = "John"

students(1) = "Alice"

students(2) = "Antony"

Dynamic Arrays

Can grow in size as needed:

Dim nums() As Integer

ReDim nums(1)

nums(0) = 12

nums(1) = 23

ReDim Preserve nums(2)

nums(2) = 35

Retrieving Array Elements

Access array elements using their index:

txtDisplay.Text = students(1) ' Displays "Alice"

txtDisplay.Text = myData(0) ' Displays "11"

Adding New Elements (Dynamic Arrays)

Only possible with dynamic arrays using ReDim:

ReDim Preserve nums(3) ' Resize and preserve old values

Join Function

Combine array elements into a single string:

Dim classmates As String = Join(students, ", ")

' Result: "John, Alice, Antony"

Loops & Arrays

Example of looping through an array:

For Each element As String In array

Console.Write(element)

Console.Write("... ")

Next

• For a ComboBox:

For Each element As String In array

comboBox.Items.Add(element)

Next

Summary: Arrays

• Arrays store multiple data elements of the same type.

• Dynamic arrays can be resized with ReDim; fixed-size arrays cannot.

• Indexes: First element is at index 0, last at index n-1.

• Use Erase to delete arrays.

• Use Split and Join for splitting and combining arrays of strings.

What is a DataGridView?

• A control in VB.NET for displaying and manipulating data in a grid format.

• Key Features:

  • Displays data in rows and columns.

  • Supports adding, editing, deleting, and sorting data.

  • Can be bound to databases, arrays, or lists, or populated manually.

  • Customizable styling.

Setting Up a DataGridView

• Add DataGridView (DGV) to the form.

• Uncheck properties: Enable Adding, Editing, Deleting.

• Add columns:

  1. Click ‘Add Column…’.

  2. Enter Name and Header Text for each column (e.g., fName, lName, score).

  3. Click ‘Add’ for each column.

  4. Adjust column width: Select DGV → Triangle (top right) → Edit Columns… → Change column width.

Using Loops & Arrays with DataGridView

Example arrays to load data into DGV:

aFirst() As String = {"Mateo", "Aisha", "Hiroshi", "Elena", "Liam", "Mei", "Ravi"}

aLast() As String = {"Gonzalez", "Khan", "Takahashi", "Rodriguez", "O'Connor", "Zhang", "Patel"}

aScore() As Integer = {90, 45, 78, 87, 92, 55, 76}

Load data into DGV as the form loads (using loops).

Averaging a Column

Calculate the total and average of a column (e.g., scores):

For Each row As DataGridViewRow In DataGridView1.Rows

total += row.Cells(2).Value

count += 1

Next

average = total / count

Apply rounding to the average for formatting.

Deleting Rows

To delete a row:

DataGridView1.Rows.RemoveAt(DataGridView1.SelectedRows(0).Index)

Validation: Ensure a row is selected before deleting.

Sorting Rows

• Clicking on column headers sorts columns.

• Programmatically sort:

DataGridView1.Sort(DataGridView1.Columns(2), ComponentModel.ListSortDirection.Ascending)

DataGridView1.Sort(DataGridView1.Columns(2), ComponentModel.ListSortDirection.Descending)

Why Write to a Text File?

• Data is stored temporarily in RAM. When the program closes, data is lost.

• To permanently store data (e.g., in ROM), data needs to be saved to an external file.

• You can export data to various file types: .txt, .docx, .xlsx, .xml, etc.

Demo: Writing Data to a Text File

1. Import Libraries:

• Add System.IO to the top of your code:

• Imports System.IO

2. Setting Up the Button:

   • Create a button in the form that will trigger the file-saving process.

   • Example: Button1_Click event handler.

3. Getting Data in Organised Format:

   • Use a variable to hold the data as a String.

4. Example File Content: If you want to get data from a DataGridView:

   • Use vbCrLf for new lines.

   • Example:

Dim fileData As String = ""

For Each row As DataGridViewRow In DataGridView1.Rows

fileData &= row.Cells(0).Value.ToString() & vbCrLf

Next

Writing Data to a File

Writing Data to a File:

• Use StreamWriter to write data:

Using writer As New StreamWriter("C:\path\to\file.txt")

writer.Write(fileData)

End Using

• The Using block ensures proper disposal of resources after the writing process is complete.

Getting an Average and Writing & Locating File

Getting an Average and Writing:

• If you're writing averages (e.g., class average):

Dim avg As Double = total / count

fileData &= "Class Average: " & avg.ToString() & vbCrLf

Locating the File:

• After saving, your file will be found in the following directory for VS2022:

  • Project Folder/Bin/Debug/net8.0-windows