microsoft visual basic 2010: reloaded fourth edition chapter three memory locations and calculations
TRANSCRIPT
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Microsoft Visual Basic 2010: Reloaded Fourth Edition
Chapter ThreeMemory Locations and Calculations
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Objectives
After studying this chapter, you should be able to:
• Declare variables and named constants
• Assign data to an existing variable
• Convert data to the appropriate type using the TryParse method and the Convert class methods
• Write arithmetic expressions
• Understand the scope and lifetime of variables and named constants
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Objectives (cont'd.)
• Understand the purpose of the Option statements
• Use a TOE chart, pseudocode, and a flowchart to code an application
• Clear the contents of a control’s Text property during run time
• Send the focus to a control during run time
• Explain the difference between syntax errors and logic errors
• Format an application’s numeric output
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Internal Memory
• Internal memory: a component inside a computer comprised of memory locations
• Each memory location has a unique numeric address and can hold only one item at a time
• A programmer can reserve memory locations for a program by assigning each location a name, a data type, and an initial value
• Data type: indicates the type of data the memory location will store
• Two types of memory locations that a programmer can declare: variables and constants
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Internal Memory (cont'd.)
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Figure 3-1: Illustration of storage bins
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Variables
• Variables: computer memory locations used to temporarily store data while an application is running– Contents can change during run time
• Use a meaningful variable name that reflects the purpose of the variable
• Use camel casing for variable identifiers
• Variable names should conform to naming rules
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Figure 3-2: How to name a variable
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Variables (cont'd.)
• Each variable must be assigned a data type, which determines the memory location’s data type
• Each data type is a class– Integer, Long, or Short data types can store
integers (whole numbers)– Decimal, Double, and Single data types: store
real numbers (numbers with a decimal place)– Char data type: stores one Unicode character– String data type: stores multiple Unicode
characters
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Variables (cont'd.)
• Unicode: – Universal coding scheme for characters that assigns
a unique numeric value to each character
• Other data types– Boolean data type: stores a value of True or False
– Date data type: stores date and time information– Object data type: stores any type of data
• Computer must determine the data type at run time, making it more inefficient
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Figure 3-3: Basic data types in Visual Basic
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Declaring a Variable in Code
• Declaration statement: used to declare, or create, a variable– Declaration statement includes:
• Scope keyword: Dim, Private, or Static• Name of the variable and data type• Initial value (optional)
• Initialization– Numeric data types: automatically initialized to 0– String data type: automatically initialized to Nothing– Boolean data type: initialized to False
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Figure 3-4: How to declare a variable
Declaring a Variable in Code (cont’d.)
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Figure 3-4: How to declare a variable (cont’d.)
Declaring a Variable in Code (cont’d.)
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Assigning Data to an Existing Variable
• Assignment statement:– Used to assign values to properties of controls– Used to assign values to variables
• Assignment operator (=):– Expression on the right of the = operator is assigned
to the variable on the left of the = operator
• Expression: can contain literal constants, object properties, variables, keywords, or arithmetic operators
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Assigning Data to an Existing Variable (cont'd.)
• Literal constant: – An item of data whose value does not change while
the application is running– Can be a numeric or a string literal constant
• A numeric literal without a decimal place is treated as an integer
• A numeric literal with a decimal place is treated as a Double type
• String literals are enclosed in quotation marks
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Figure 3-5: How to assign a value to a variable
Assigning Data to an Existing Variable (cont'd.)
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Figure 3-5: How to assign a value to a variable (cont’d.)
Assigning Data to an Existing Variable (cont'd.)
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Using the TryParse Method
• Method: a specific portion of a class’s instructions that performs a task for the class
• TryParse method: – Part of every numeric data type’s class– Used to convert a string to that numeric data type
• Argument: a value that is provided to a method
• Basic syntax of TryParse method has two arguments:– String: string value to be converted– Variable: location to store the result
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Using the TryParse Method (cont'd.)
• If TryParse conversion is successful, the method stores the value in the variable
• If unsuccessful, a 0 is stored in the numeric variable
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Figure 3-6: How to use the basic syntax of the TryParse method
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Figure 3-7: Result of the TryParse method for the Double, Decimal, and Integer data types
Using the TryParse Method (cont'd.)
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Using the Convert Class Methods
• Convert class:– Contains methods for converting numeric values to
specific data types
• Commonly used methods of the Convert class include:– ToDouble– ToDecimal– ToInt32– ToString
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Figure 3-8: How to use the Convert class methods
Using the Convert Class Methods (cont’d.)
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Including Variables in Arithmetic Expressions
• Arithmetic operators: used to perform calculations
• Precedence number: indicates the order in which an operation in an expression is performed
• If an expression has two operators with the same precedence, they are evaluated from left to right
• Use parentheses to change the order of evaluation
• Integer division operator (\): divides two integers and returns an integer value
• Modulus arithmetic operator (Mod): divides two numbers and returns the remainder
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Including Variables in Arithmetic Expressions (cont'd.)
Figure 3-9: Most commonly used arithmetic operators
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Including Variables in Arithmetic Expressions (cont'd.)
Figure 3-10: How to use the integer division and Mod operators
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Including Variables in Arithmetic Expressions (cont'd.)
Figure 3-11: Expressions containing more than one operator having the same precedence
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Figure 3-12: How to use variables and arithmetic operators
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Figure 3-12: How to use variables and arithmetic operators (cont’d.)
Including Variables in Arithmetic Expressions (cont'd.)
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Arithmetic Assignment Operators
• Arithmetic assignment operators: abbreviate an assignment statement that contains an arithmetic operator for specific cases
• Statement must be of the form:variableName =
variableName arithmeticOperator value
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Arithmetic Assignment Operators (cont’d.)
Figure 3-13: How to use the arithmetic assignment operators
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Figure 3-13: How to use the arithmetic assignment operators (cont’d.)
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The Scope and Lifetime of a Variable
• Scope: indicates where the variable can be used
• Lifetime: indicates how long the variable remains in memory
• Variables can have module scope, procedure scope, or block scope
• A variable’s scope and lifetime are determined by where you declare the variable– Variables declared in the form’s Declarations section
have class scope– Variables declared within a procedure have either
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Variables with Procedure Scope
• Procedure-level variable: declared within a procedure– Use the Dim keyword in the declaration
• Procedure scope: only the procedure can use the variable– With procedure-level scope, two procedures can each
use the same variable names
• Comments: – Used to internally document the procedure– Are ignored by the compiler– Appear in green in the Code Editor
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Figure 3-14: The MainForm in the Sales Tax application
Figure 3-15: Examples of using procedure-level variables
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Variables with Procedure Scope (cont’d.)
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Figure 3-15: Examples of using procedure-level variables (cont’d.)
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Variables with Class Scope
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• Class scope: variable can be used by all procedures in the form
• Class-level variable: – Declared in the form’s Declarations section– Use Private keyword in declaration
• Class-level variables retain their values until the application ends
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Figure 3-17: Example of using a class-level variable
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Static Variables
• Static variable: – Procedure-level variable that remains in memory and
retains its value even after the procedure ends– Retains its value until the application ends (like a
class-level variable), but can only be used by the procedure in which it is declared
• A static variable has:– Same lifetime as a class-level variable– Narrower scope than a class-level variable
• Declared using the Static keyword
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Figure 3-18: Example of using a static variable
Static Variables (cont’d.)
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Named Constants
• Named constant: memory location whose value cannot be changed while the application is running– Declared using the Const keyword– Good programming practice to specify the data type
as well– Many programmers use Pascal case for named
constants
• Literal type character: forces a literal constant to assume a specific data type
• Named constants help to document the program code
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Figure 3-19: How to declare a named constant
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Figure 3-20: Area Calculator application’s interface
Figure 3-21: Example of using a named constant
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Option Explicit, Option Infer, and Option Strict
• Undeclared variable: a variable that does not appear in a declaration statement (such as Dim)– Is assigned a data type of Object
• Misspelling a variable name can result in an undeclared variable unless Option Explicit is on
• Option Explicit On statement– Appears in the General Declarations section of the
Code Editor window (above Public Class statement)
– Enforces that all variables must be declared before being used
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Option Explicit, Option Infer, and Option Strict (cont'd.)
• Option Infer Off statement: ensures that every variable is declared with a data type
• Implicit type conversion: occurs when you attempt to assign data of one type to a variable of another type without explicitly attempting to convert it– If converted to a data type that can store larger
numbers, the value is said to be promoted– If converted to a data type that can store only
smaller numbers, the value is said to be demoted• Can cause truncation and loss of precision
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Figure 3-22: Rules and examples of type conversions
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Option Explicit, Option Infer, and Option Strict (cont'd.)
• Option Strict On statement: ensures that values cannot be converted from one data type to a narrower data type, resulting in lost precision
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Option Explicit, Option Infer, and Option Strict (cont'd.)
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Figure 3-23: Option statements entered in the General Declarations section
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Coding the Sunshine Cellular Application
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Figure 3-24: Sunshine Cellular interface from Chapter 2
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Coding the Sunshine Cellular Application (cont'd.)
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Figure 3-25: Sunshine Cellular TOE chart from Chapter 2
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Using Pseudocode to Plan a Procedure
• Pseudocode: short phrases that describe the steps a procedure needs to take to accomplish its goal
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Using Pseudocode to Plan a Procedure (cont’d.)
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Figure 3-26: Pseudocode for the Sunshine Cellular application
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Using a Flowchart to Plan a Procedure
• Flowchart: uses standardized symbols to show the steps a procedure must take to accomplish its goal
• Can be used in place of pseudocode for planning
• Three symbols:– Start/stop symbol (oval): indicates start and stop
points– Process symbol (rectangle): represents tasks– Input/output symbol (parallelogram): represents
input or output tasks
• Flowlines: connect the symbols to show the direction
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Figure 3-27: Flowcharts for the Sunshine Cellular application
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Coding the Calculate Order Button’s Click Event Procedure
Figure 3-28: Pseudocode for the calcButton’s Click event procedure
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Coding the Calculate Order Button’s Click Event Procedure (cont'd.)
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Figure 3-29: Named constants and variables for the calcButton’s Click event procedure
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Coding the Calculate Order Button’s Click Event Procedure (cont'd.)
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Figure 3-25: Declaration statements entered in the procedure
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Coding the Calculate Order Button’s Click Event Procedure (cont'd.)
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Figure 3-31: User input stored in variables
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Completing the Sunshine Cellular Application
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• Empty string: set of quotation marks with nothing between them; also called zero-length string
• String.Empty: a value used to clear an object’s Text property
• Focus method: moves the focus to a specified control when the application is running
Figure 3-28: Pseudocode for the clearButton’s Click event procedure
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Figure 3-34: Sunshine Cellular application’s code
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Figure 3-34: Sunshine Cellular application’s code (cont'd.)
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Testing and Debugging the Application
• To test an application:– Select a set of sample data– Manually compute the expected output– Run the application and compare its output with the
expected output
• Bug: an error in the program code
• Syntax error: an error that violates the programming language’s syntax – Usually caused by mistyping
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Testing and Debugging the Application (cont'd.)
• Logic error: when the application does not perform as expected
• Valid data: data that the application is expecting
• Invalid data: data that is unexpected
• Test a program with both valid and invalid data
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Testing and Debugging the Application (cont'd.)
Figure 3-35: Sample test data for the Sunshine cellular application
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Testing and Debugging the Application (cont'd.)
Figure 3-36: Result of using the data from Set 1 in Figure 35
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Testing and Debugging the Application (cont'd.)
Figure 3-37: Result of using the data from Set 2 in Figure 3-35
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Formatting Numeric Output
• Formatting: specifying the number of decimal places and any special characters to display
• ToString method of a variable can be used to format a number
• FormatString argument: specifies the type of formatting to use
• Precision specifier: controls the number of significant digits or zeros to the right of the decimal point
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Figure 3-38: How to format a number
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Figure 3-39: The calcButton’s modified Click event procedure
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Formatting Numeric Output (cont'd.)
Figure 3-40: Formatted output shown in the interface
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Programming Tutorial 1
Figure 3-51: Result of entering 255, 255, and 0 as the RGB values
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Programming Tutorial 2
Figure 3-55: MainForm for the Vans & More Depot application
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Programming Example
Figure 3-63: Pseudocode
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Summary
• Variables and named constants are memory locations that store data
• Variables can change value, but constants cannot
• Variables and constants have a name, data type, initial value, scope, and lifetime
• Use Dim or Static to declare a variable at block or procedure level
• Use Private to declare a variable at class level
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Summary (cont'd.)
• Assignment statement is used to assign values to an existing variable during run time
• Literals are constant items of data that do not change during run time
• String literal constants are enclosed in quotation marks
• Use the TryParse method to convert a string to a number
• The Convert class contains methods to convert values to a specified data type
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Summary (cont'd.)
• A procedure-level variable is usable only by the procedure in which it is declared
• A class-level variable is usable by all procedures in the form
• A static variable is a procedure-level variable that retains its value even when the procedure ends
• Use comments to document your code
• Use Const to declare a named constant• Option Explicit On forces declaration of all
variables before use
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Summary (cont'd.)
• Option Infer Off warns if a variable declaration does not include a data type
• Option Strict On disallows any implicit type conversions that may cause a loss of data
• Pseudocode or a flowchart is used to plan a procedure’s code
• You can clear the contents of a text box or label control by assigning an empty string or String.Empty value
• The Focus method moves the focus to a control
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Summary (cont'd.)
• Test a program with both valid and invalid data
• Use the ToString method to format a program’s numeric output with special characters, such as for currency, percentages, and number of decimal places
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