microsoft visual basic 2005 for windows and mobile ... · visual basic will, by default,...
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Chapter 4: Variables and Arithmetic Operations 2
Objectives
►Create, modify, and program a TextBox
object►Use code to place data in the Text property of a
Label object►Use the AcceptButton
and CancelButton
properties►Understand and declare String and Numeric
variables►Use assignments statements to place data in
variables
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Chapter 4: Variables and Arithmetic Operations 3
Objectives
►Use literals and constants in coding statements►Understand scope rules for variables►Convert string and numeric data►Understand and use arithmetic operators and
arithmetic operations
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Chapter 4: Variables and Arithmetic Operations 4
Objectives
►Format and display numeric data as a string►Create a form load event ►Create a concatenated string►Debug a program
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Chapter 4: Variables and Arithmetic Operations 6
TextBox Objects
►With Visual Studio open and the Form1.vb [Design] tabbed page visible, point to the TextBox
.NET component in the Toolbox►Drag the TextBox
.NET component onto the
Windows Form object at the desired location►When the upper-left corner of the pointer is
located where you want the TextBox
object’s upper-left corner, release the left mouse button
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Chapter 4: Variables and Arithmetic Operations 8
Sizing and Positioning a TextBox Object
►Select the TextBox
object. Scroll in the Properties window until the Text property is visible and then click the right column for the Text property
►Type the maximum number of characters the user normally will enter into the text box and then press the ENTER key. When entering numbers, the digit 8 often is entered because it is wider than other digits. In this example, the value 888 is entered because three digits is the maximum number of digits the user normally will enter
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Chapter 4: Variables and Arithmetic Operations 9
Sizing and Positioning a TextBox Object
►Using the Font property in the Properties window, change the Font property to the correct font and font size. For this application, change the font to Times New Roman and change the font size to 12. Then, drag the right edge of the TextBox
object to resize the TextBox
object so it is slightly wider than the 888 entry
►To horizontally align the text in the label and the text in the text box, drag the text box up until a red snap line indicates the bottoms of the text are aligned. Then, release the left mouse button
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Chapter 4: Variables and Arithmetic Operations 11
Aligning Text in a TextBox Object
►Select the TextBox
object. In the Properties window, scroll until the TextAlign
property is
visible, click the TextAlign
property in the left column, and then click the list arrow in the right column of the TextAlign
property
►Click Center in the TextAlign
property list►Because the TextBox
object is sized properly,
remove the digits in the TextBox
object. Select the characters 888 in the Text property, press the DELETE key on your keyboard, and then press the ENTER key
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Chapter 4: Variables and Arithmetic Operations 13
Creating a MultiLine Text Box
►Select the TextBox
object, click the Action tag, and point to the MultiLine
check box
►Click the MultiLine
check box
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Chapter 4: Variables and Arithmetic Operations 15
Creating a MaskedTextBox Object
►Drag a MaskedTextBox
.NET component from the Toolbox to the Windows Form object. Then, click the Action tag on the TextBox
object and
point to the Set Mask command►Click Set Mask on the MaskedTextBox
Tasks list
and then click the Short date mask description in the Input Mask dialog box
►Click the OK button in the Input Mask dialog box and then click anywhere in the Windows Form object
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Chapter 4: Variables and Arithmetic Operations 17
Label Objects
►Drag a Label object onto the Windows Form object to the correct location. Name the label lblTotalCostOfDownloads. Change the label to the appropriate font size (Times New Roman, 12 point). In the Text property for the Label object, enter the maximum number of characters ($888.88) that will appear in the label during execution of the program
►Drag the Label object up until the red snap line appears. Then release the left mouse button
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Chapter 4: Variables and Arithmetic Operations 19
Accept Button in Form Properties
►The program will carry out the event handler processing associated with the Accept Button if the user clicks the button or if the user presses the ENTER key
►Click a blank area in the Windows Form object to select it. Scroll in the Properties window until the AcceptButton
property is visible. Click the
AcceptButton
property name in the left column and then click the AcceptButton
property list
arrow in the right column►Click btnCalculateCost
in the AcceptButton
property list
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Chapter 4: Variables and Arithmetic Operations 20
Cancel Button in Form Properties
►When the user presses the ESC key, the event handler processing for the button identified as the Cancel button will be executed
►Click a blank area in the Windows Form object to select it
►Click the CancelButton
property name in the left column in the Properties window for the Windows Form object, and then click the CancelButton
list
arrow►Click the button name (btnClear) in the
CancelButton
property list
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Chapter 4: Variables and Arithmetic Operations 21
Visual Studio Preparation for Code Entry
►With the Toolbox visible, click the Toolbox Close button. The Toolbox closes and the work area expands in size. To reshow the Toolbox after it has been closed, click the Toolbox button on the Standard toolbar
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Chapter 4: Variables and Arithmetic Operations 22
String Data Type
►A String data type allows every character available on the computer to be stored in it
►The data type for the value the user enters in a TextBox
object and that is stored in the Text
property of the TextBox
object is string►A variable is a named location in RAM where
data is stored►A String variable is a named location in RAM
that can store a string value
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Chapter 4: Variables and Arithmetic Operations 24
Assignment Statements
►One method to place data in the variable is to use an assignment statement
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Chapter 4: Variables and Arithmetic Operations 25
Assignment Statements
►With Visual Studio displaying the code editing window and the insertion point located in the desired column, type Dim followed by a space. Then, type the name of the String variable you want to define, strNumberOfSongs on your keyboard
►Press the SPACEBAR, type the word As and then press the SPACEBAR again
►Because the entry should be String, type str on your keyboard
►Press the ENTER key
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Chapter 4: Variables and Arithmetic Operations 26
Assignment Statements
►To begin the assignment statement, press CTRL+SPACEBAR in order to display the IntelliSense list of allowable entries. Then, type strn to highlight the strNumberOfSongs
variable name in the IntelliSense list
►Press the SPACEBAR, press the EQUAL SIGN key, and then press the SPACEBAR
►Type me. to display the IntelliSense list, and then type txt to identify the txtNumberOfSongs
TextBox
object in
the IntelliSense list►Press the PERIOD key and then, if necessary, type te to
highlight the Text entry in the IntelliSense list►Press the ENTER key
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Chapter 4: Variables and Arithmetic Operations 28
Numeric Data Types
►A numeric data type must be used in arithmetic operations
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Chapter 4: Variables and Arithmetic Operations 34
Literals
►A value is called a literal when the value being used in the assignment statement is literally the value that is required
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Chapter 4: Variables and Arithmetic Operations 35
Forced Literal Types
►Sometimes you might want a literal to be a different data type than the Visual Basic default
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Chapter 4: Variables and Arithmetic Operations 36
Constants
►A constant variable will contain one permanent value throughout the execution of the program
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Chapter 4: Variables and Arithmetic Operations 37
Constants
►The declaration of a constant variable begins with the letters Const, not the letters Dim
►You must assign the value to be contained in the constant on the same line as the definition of the constant.
►You cannot attempt to change the value in the constant variable anywhere in the program. If you attempt this, you will produce a compiler error
►The letter c often is placed before the prefix of the constant variable name to identify throughout the program that it is a constant variable and cannot be changed
►Other than the letter c constant variable names are formed using the same rules and techniques as nonconstant
names
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Chapter 4: Variables and Arithmetic Operations 38
Referencing a Variable
►When a variable is declared, it will be underlined with a green squiggly line until it is referenced in a statement
►It is mandatory when using a variable in a program that the variable is defined prior to using the variable name in a statement
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Chapter 4: Variables and Arithmetic Operations 39
Scope of Variables
►The scope of a variable specifies where within the program the variable can be referenced in a Visual Basic statement
►A variable can be referenced only within the region of the program where it is defined
►The code between the Sub statement and the End Sub statement is a procedure
►A variable that can only be referenced within the region of the program where it is defined is called a local variable• Local variables have a certain lifetime in the program
►Global variables can be used in multiple regions of a program
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Chapter 4: Variables and Arithmetic Operations 40
Converting Variable Data
►Visual Basic includes several procedures that allow you to convert one data type to another data type
►A procedure to convert a String data type to an Integer data type is named ToInt32
►The procedure is found in the Convert class, which is available in a Visual Studio 2005 class library
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Chapter 4: Variables and Arithmetic Operations 41
Using a Procedure
►A procedure that performs its task but does not return a value is called a Sub procedure
►A procedure that returns a value is called a Function procedure, or a function
►An argument identifies a value required by a procedure
►Every procedure is part of a class
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Chapter 4: Variables and Arithmetic Operations 43
Option Strict On
►Visual Basic will, by default, automatically convert data types if the data type on the right side of the equal sign in an assignment statement is different from the data type on the left side of the equal sign
►To prevent automatic conversion of values, the developer must insert the Option Strict On statement in the program prior to any event handler code in the program
►The Option Strict On statement explicitly disallows any default data type conversions in which data loss would occur and any conversion between numeric types and strings
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Chapter 4: Variables and Arithmetic Operations 47
Multiple Operations
►A single assignment statement can contain multiple arithmetic operations
►Hierarchy of Operations• Exponentiation (^) is performed first• Multiplication (*) and division (/) are performed next• Integer division (\) is next• MOD then occurs• Addition (1) and subtraction (2) are performed last• Within these five steps, calculations are performed left
to right
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Chapter 4: Variables and Arithmetic Operations 49
Format Specifications for the ToString Function
►Use the format specifier to identify the format for the numeric data to be returned by the ToString
function
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Chapter 4: Variables and Arithmetic Operations 50
Precision Specifier
►The precision specifier is a number that is included within the quotation marks in the function call to identify the number of positions to the right of the decimal point that should be returned
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Chapter 4: Variables and Arithmetic Operations 51
Clear Procedure
►The Clear procedure clears any data currently placed in the Text property of a TextBox
object
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Chapter 4: Variables and Arithmetic Operations 52
Clearing the Text Property of a Label
►The Clear procedure cannot be used with a Label object
►You must write an assignment statement that assigns a null length string to the Text property of a Label object
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Chapter 4: Variables and Arithmetic Operations 53
Setting the Focus
►When the focus is on a TextBox
object, the insertion point is located in the text box
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Chapter 4: Variables and Arithmetic Operations 54
Form Load Event
►A form load event occurs when the program starts and the Digital Downloads form is loaded
►This event handler completes the following tasks:• Display the cost per download heading• Clear the placeholder from the
lblTotalCostOfDownloads
Text property• Set the focus on the txtNumberOfSongs
text
box
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Chapter 4: Variables and Arithmetic Operations 55
Concatenation
►The process of joining two different values into a single string is called concatenation
►The values being concatenated must be String data types
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Chapter 4: Variables and Arithmetic Operations 57
Class Scope
►When a variable is referenced in two different event handling procedures, it must be defined at the class level instead of the procedure (event handler) level
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Chapter 4: Variables and Arithmetic Operations 58
Debugging Your Program
►A Format Exception occurs when the user enters data that a statement within the program cannot process properly
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Chapter 4: Variables and Arithmetic Operations 60
Debugging Your Program
►An Overflow Exception occurs when the user enters a value greater than the maximum value that can be processed by the statement
►It is not possible to divide by zero, so if your program contains a division operation and the divisor is equal to zero, the Divide By Zero Exception will occur
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Chapter 4: Variables and Arithmetic Operations 64
Summary
►Create, modify, and program a TextBox
object►Use code to place data in the Text property of a
Label object►Use the AcceptButton
and CancelButton
properties►Understand and declare String and Numeric
variables►Use assignments statements to place data in
variables
4
Chapter 4: Variables and Arithmetic Operations 65
Summary
►Use literals and constants in coding statements►Understand scope rules for variables►Convert string and numeric data►Understand and use arithmetic operators and
arithmetic operations
4
Chapter 4: Variables and Arithmetic Operations 66
Summary
►Format and display numeric data as a string►Create a form load event ►Create a concatenated string►Debug a program