Variables and Expressions

1
Variables and Expressions
7

• Programming Right from the Start with Visual
  Basic .NET 1/e

2
Objectives

• Distinguish between Numeric, String, Char,
  Boolean, Date, and Object data types
• Explain the similarities and differences between
  variables and constants
• Understand and use the Dim and Const statements
• Understand and use arithmetic, comparison, and
  logical operators

3
Objectives (cont.)

• Explain the advantages of intrinsic functions for
  performing common calculations
• Understand and effectively use the TextBox,
  ListBox, and ComboBox controls
• Successfully write Windows applications involving
  calculations that require complex expressions and
  intrinsic functions

4
7-1 Data Types

• A variable is a storage location that can be
  accessed and changed by developer code.
• A variable has a name and an associated data
  value.
• A variable has a data type that determines the
  kind of data values the variable can store.

5
Numeric Types

• Numeric types have two classes
• Integral data types are those that represent only
  whole numbers.
• Integer is the most common.
• Floating-point data types are those that
  represent numbers with both integer and
  fractional parts.
• Double is the most common.

6
String and Char Types

• The Char data type stores a single character in
  Unicode format.
• The String data type stores a sequence of Unicode
  characters.
• Unicode is a 16-bit international character
  encoding system that covers values for more than
  45,000 characters.
• The first 128 characters correspond to the ASCII
  (American Standard Code for Information
  Interchange) character set.

7
Boolean, Data, and Object Types

• The Boolean data type is an unsigned value that
  is interpreted as either true or false.
• The Date data type stores a date and time value
  that includes second, minute, hour, day, month,
  and year values.
• The Object data type is the universal data type
  in VB. NET.

8
7-2 The Dim Statement

• Variables are declared with the Dim statement
  (short for Dimension statement).
• One or more variables can be declared with a
  single Dim statement.
• The Dim statement can be used to specify the
  variables initial value by placing an equal sign
  after the data type.

9
Naming Rules for Variables

• A variable must have a unique name, called an
  identifier.
• Identifiers must follow three rules
• Begin with a letter or underscore
• Contain only letters, digits, and underscores
• Cannot be reserved words
• Dim identifier , identifier As datatype
  initialvalue

10
Working with Constants

• A constant is a storage location whose value
  cannot change during the execution of the
  program.
• Constants are declared with the Const
  declaration.
• Const identifier As datatype value

11
7-3 Operators and Expressions

• An operator is a symbol that indicates an action
  to be performed.
• Value-returning code elements such as variables,
  constants, and expressions can be combined with
  one or more operators to form an expression.

12
Arithmetic andString Operators

• Arithmetic operators require numeric operands and
  produce numeric results.
• The string operator concatenation () requires
  String operands and produces a String result such
  as
• S sun set
• S becomes sunset

13
Comparison Operators

• Comparison operators require numeric operands and
  produce a logical result.
• Greater than gt
• Less than lt
• Greater than or equal gt
• Less than or equal lt
• Equal
• Not equal ltgt

14
Logical Operators

• Logical Operators require logical operands and
  produce a logical result.
• NOT Logical opposite
• AND Both values are true
• OR At least one value is true
• XOR Exactly one value is true

15
Operator Precedence

• When several operations occur in an expression,
  each part is evaluated and resolved in a
  predetermined order called operator precedence.
• First Evaluate all arithmetic/concatenation
  operators
• Second Evaluate all comparison operators
• Third Evaluate all logical operators

16
7-4 The Assignment Statement

• An assignment statement is used to store a value
  into a variable or an object property.
• variable expression
• object.property expression
• The equal sign () is the assignment operator.

17
7-5 Intrinsic Functions

• VB .NET provides a large number of intrinsic
  functions to improve developer productivity.
• Intrinsic functions support a broad range of
  activities, including math calculations, business
  calculations, time and date functions, string
  formatting functions, and many more.

18
Simple Functions
19
Financial Annuity Functions

• Pmt is a function for determining monthly
  payments.
• Pmt(Rate, NPer, PV)
• FV is a function for determining the future value
  of an annuity based on periodic, fixed payments
  and a fixed interest rate.
• FV(Rate, NPer, Pmt)

20
7-6 InputBox and MsgBox

• InputBox and MsgBox are intrinsic functions that
  facilitate communication with the user by means
  of a dialog box.
• A dialog box is a pop-up window that informs the
  user of some information or accepts information
  from the user.

21
The InputBox Function

• The InputBox function is a simple means for
  prompting the user for keyboard input.
• Result InputBox(prompt, title)
• When executed, InputBox displays a dialog box
  containing the specified prompt and a text box.

22
The MsgBox Function

• The MsgBox function displays a message in a
  dialog box.
• MsgBox(prompt)
• The prompt argument is a string expression that
  gets displayed as the message in the dialog box.

23
The MsgBox Function (cont.)
24
7-7 TextBox Control

• The TextBox control provides an area for the user
  to enter data while the program is executing.
• TextBox controls can be used to display output on
  the form.

25
TextBox Properties
26
TextBox Methods
27
7-8 ListBox Control

• The ListBox control makes a visible list of
  items.
• The user can select items in the list using mouse
  clicks.

28
ListBox Properties
29
ListBox Methods
30
ListBox IllustratedSelect Ice Cream Toppings

• The scroll bar is automatically created by VB.
  NET if the number of items in the list is too big
  for the display size of the list box.
• Double-click on an item in the first list box to
  add the item to the second list box.
• Double-click on an item in the second list box to
  remove that item.

31
7-9 ComboBox Control

• The ComboBox control combines the functionality
  of the TextBox and ListBox controls.
• The user can select values from the drop-down
  list or enter values directly into the text box
  area.

32
ComboBox Properties
33
ComboBox IllustratedMovie Voting

• Select a movie from each combo box, then press
  the vote button.
• The default DropDownStyle property value is
  DropDown, which makes the text box editable.
• Change the first combo boxs DropDown-Style
  property to DropDownList and rerun the
  application.

34
Chapter Summary

• A variable is a storage location that can be
  accessed and changed by developer code.
• A constant is a storage location with a name and
  an associated data value.
• A data type specifies the kind of data that a
  storage location can hold.
• The unique name for a variable, constant, object,
  or subroutine is called an identifier.

35
Chapter Summary (cont.)

• It is good practice to make variable names
  descriptive with mixed case.
• The Dim statement is used to declare a variable
  by specifying its name and data type.
• Expressions are combinations of operators and
  value-returning code elements such as variables,
  constants, and literals.

36
Chapter Summary (cont.)

• One of the common uses of an expression is to
  perform a calculation for an assignment
  statement.
• The process of changing a value from one data
  type to another is called conversion.
• Pmt is an intrinsic function that returns the
  payment for a loan based on periodic, constant
  payments and a constant interest rate.

37
Chapter Summary (cont.)

• For a program to be useful, it must perform some
  type of input/output.
• A TextBox control provides GUI form input by
  providing an area on the form where the user can
  enter data while the program is executing.

38
Chapter Summary (cont.)

• A ListBox control provides GUI form input as a
  list of items that the user can select by
  clicking.
• A ComboBox control combines the functionality of
  the TextBox and ListBox controls.

39
Variables andExpressions
7

• Programming Right from the Start with Visual
  Basic .NET 1/e