Chapter 3: Variables & Calculations

Starting Out with Visual Basic 8e — Tony Gaddis | Quiz 3 due Jun 12

3.1 — Gathering Text Input

Use a TextBox control to let users type text into your application.

Dim userName As String userName = TextBox1.Text

Text property: gets/sets the text displayed in the box
Text is always a String — must convert for numbers
MultiLine property: allows multiple lines
ReadOnly property: prevents user editing
Clear() method: erases all text
Focus() method: moves cursor to this box

💡 After processing input, use TextBox1.Clear() and TextBox1.Focus() to reset for next entry.

3.2 — Variables and Data Types

A variable is a named storage location in memory. Declare with Dim.

Dim age As Integer Dim price As Decimal = 29.99D Dim name As String = "Alice" Dim isActive As Boolean = True

Type	Stores	Example
`Integer`	Whole numbers (-2B to 2B)	`Dim x As Integer = 5`
`Double`	Floating-point numbers	`Dim y As Double = 3.14`
`Decimal`	High-precision (money)	`Dim z As Decimal = 19.99D`
`String`	Text	`Dim s As String = "Hello"`
`Boolean`	True/False	`Dim b As Boolean = True`
`Char`	Single character	`Dim c As Char = "A"c`

💡 Naming rules: Must start with letter, no spaces, no keywords, use camelCase (e.g., totalSales).

3.3 — Performing Calculations

VB supports standard arithmetic operators:

+ ' Addition - ' Subtraction * ' Multiplication / ' Floating-point division (3 / 2 = 1.5) \ ' Integer division (3 \ 2 = 1) ^ ' Exponentiation (2 ^ 3 = 8) Mod ' Modulus / remainder (7 Mod 3 = 1)

Operator Precedence (highest to lowest):

Parentheses ( )
Exponentiation ^
Multiplication/division * / \
Modulus Mod
Addition/subtraction + -

' Calculate total with tax Dim subtotal As Decimal = 45.00D Dim taxRate As Decimal = 0.0825D Dim total As Decimal = subtotal + (subtotal * taxRate)

3.4 — Mixing Different Data Types

VB converts types automatically (implicit conversion) unless Option Strict is On.

Option Strict On ' Forces explicit conversions Option Strict Off ' Allows automatic widening

Type conversion functions:

CInt(value) ' → Integer CDbl(value) ' → Double CDec(value) ' → Decimal CStr(value) ' → String CBool(value) ' → Boolean

💡 Always use CDec() for currency/money calculations to avoid floating-point rounding errors.

3.5 — Formatting Numbers and Strings

' Format function Format(1234.5, "$#,##0.00") ' → "$1,234.50" ' ToString with format specifiers price.ToString("C") ' Currency: "$1,234.50" price.ToString("N2") ' Number: "1,234.50" price.ToString("P1") ' Percent: "12.3 %" price.ToString("F0") ' Fixed: "1235"

"C" or "c" — Currency format with $ sign and 2 decimals
"N" or "n" — Number with commas and decimals
"P" or "p" — Percentage (multiplies by 100)
"F" or "f" — Fixed-point with specified decimals

3.6 — Class-Level Variables

Declared outside any procedure, inside the form class. Accessible from all procedures in the form.

Public Class Form1 ' Class-level variable — available everywhere in this form Private totalSales As Decimal = 0D Private Sub Button1_Click(...) Handles Button1.Click totalSales += CDec(TextBox1.Text) End Sub End Class

💡 Class-level variables retain their value between procedure calls. Local variables are recreated each time.

3.7 — Exception Handling

Use TryParse to safely convert input without crashing:

Dim number As Integer If Integer.TryParse(TextBox1.Text, number) Then MessageBox.Show("Valid number: " & number.ToString()) Else MessageBox.Show("Invalid input!") End If

Or use Try-Catch blocks:

Try result = CDec(TextBox1.Text) / CDec(TextBox2.Text) lblResult.Text = result.ToString("C") Catch ex As Exception MessageBox.Show("Error: " & ex.Message) End Try

💡 TryParse is preferred for input validation — doesn't throw exceptions, just returns True/False.

Practice Quiz — Chapter 3

Test your knowledge. 25 questions covering variables, calculations, types, and more.

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questions answered correctly

Flashcards — Chapter 3

Click a card to flip it. Review key terms and concepts.

What is a variable?

A named location in memory that stores a value. Declared with the Dim keyword.

Card 1 / 30

Cheat Sheet — Chapter 3

Quick reference for Quiz 3. All the essentials in one place.

🔑 Key SyntaxDim variableName As DataType [= initialValue]
Const PI As Double = 3.14159
Dim result As Decimal = CDec(TextBox1.Text) * 0.0825D

📊 Data Types Quick Reference

Type	Size	Range / Values	Suffix
`Integer`	32-bit	-2,147,483,648 to 2,147,483,647	—
`Double`	64-bit	±4.94×10⁻³²⁴ to ±1.8×10³⁰⁸	R or #
`Decimal`	128-bit	±7.9×10⁻²⁸ to ±7.9×10²⁸ (28-29 digits)	D or @
`String`	varies	0 to ~2 billion chars	" "
`Boolean`	16-bit	True or False	—
`Char`	16-bit	Single Unicode char	"A"c

🔄 Conversion Functions

Function	Result Type
`CInt(value)`	Integer (rounds)
`CDbl(value)`	Double
`CDec(value)`	Decimal ← use for money!
`CStr(value)`	String
`CBool(value)`	Boolean
`CChar(value)`	Char

📋 Format Specifiers

Specifier	Name	Example (1234.5)
`"C"`	Currency	$1,234.50
`"N2"`	Number (2 decimals)	1,234.50
`"F0"`	Fixed (0 decimals)	1235
`"P1"`	Percent	12.3%
`"E2"`	Scientific	1.23E+003

⚠️ Common Pitfalls

Integer division: 7 / 3 = 2.333 but 7 \ 3 = 2 (backslash = integer division)
Mod: 17 Mod 4 = 1 (remainder)
Decimal suffix: Always use 19.99D not 19.99 for Decimal literals
Option Strict: When On, all conversions must be explicit (use CInt, CDec, etc.)
TryParse: Integer.TryParse(text, variable) — returns True/False, puts value in variable
CurrentCulture: ToString("C") respects system locale for currency symbol