microsoft visual basic 2010: reloaded fourth edition chapter three memory locations and calculations

Microsoft Visual Basic 2010: Reloaded Fourth Edition

Chapter ThreeMemory Locations and Calculations

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

Microsoft Visual Basic 2010: Reloaded, Fourth Edition 2

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


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


Internal Memory (cont'd.)


Figure 3-1: Illustration of storage bins

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



Figure 3-2: How to name a variable

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


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



Figure 3-3: Basic data types in Visual Basic


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


Figure 3-4: How to declare a variable

Declaring a Variable in Code (cont’d.)


Figure 3-4: How to declare a variable (cont’d.)

Declaring a Variable in Code (cont’d.)


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

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



Figure 3-5: How to assign a value to a variable



Figure 3-5: How to assign a value to a variable (cont’d.)


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


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



Figure 3-6: How to use the basic syntax of the TryParse method


Figure 3-7: Result of the TryParse method for the Double, Decimal, and Integer data types

Using the TryParse Method (cont'd.)

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



Figure 3-8: How to use the Convert class methods

Using the Convert Class Methods (cont’d.)


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


Including Variables in Arithmetic Expressions (cont'd.)

Figure 3-9: Most commonly used arithmetic operators



Figure 3-10: How to use the integer division and Mod operators



Figure 3-11: Expressions containing more than one operator having the same precedence


Figure 3-12: How to use variables and arithmetic operators


Figure 3-12: How to use variables and arithmetic operators (cont’d.)


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



Arithmetic Assignment Operators (cont’d.)

Figure 3-13: How to use the arithmetic assignment operators


Figure 3-13: How to use the arithmetic assignment operators (cont’d.)

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

procedure scope or block scopeMicrosoft Visual Basic 2010: Reloaded, Fourth Edition 33

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



Figure 3-14: The MainForm in the Sales Tax application

Figure 3-15: Examples of using procedure-level variables

Variables with Procedure Scope (cont’d.)


Figure 3-15: Examples of using procedure-level variables (cont’d.)

Variables with Class Scope


• 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


Figure 3-17: Example of using a class-level variable


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


Figure 3-18: Example of using a static variable

Static Variables (cont’d.)

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



Figure 3-19: How to declare a named constant


Figure 3-20: Area Calculator application’s interface

Figure 3-21: Example of using a named constant

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


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



Figure 3-22: Rules and examples of type conversions


• Option Strict On statement: ensures that values cannot be converted from one data type to a narrower data type, resulting in lost precision




Figure 3-23: Option statements entered in the General Declarations section

Coding the Sunshine Cellular Application


Figure 3-24: Sunshine Cellular interface from Chapter 2

Coding the Sunshine Cellular Application (cont'd.)


Figure 3-25: Sunshine Cellular TOE chart from Chapter 2

Using Pseudocode to Plan a Procedure

• Pseudocode: short phrases that describe the steps a procedure needs to take to accomplish its goal


Using Pseudocode to Plan a Procedure (cont’d.)


Figure 3-26: Pseudocode for the Sunshine Cellular application

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



Figure 3-27: Flowcharts for the Sunshine Cellular application


Coding the Calculate Order Button’s Click Event Procedure

Figure 3-28: Pseudocode for the calcButton’s Click event procedure

Coding the Calculate Order Button’s Click Event Procedure (cont'd.)


Figure 3-29: Named constants and variables for the calcButton’s Click event procedure



Figure 3-25: Declaration statements entered in the procedure



Figure 3-31: User input stored in variables

Microsoft Visual Basic 2010: Reloaded, Fourth Edition 59Figure 3-32: Completed calcButton’s Click event procedure

Completing the Sunshine Cellular Application


• 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


Figure 3-34: Sunshine Cellular application’s code


Figure 3-34: Sunshine Cellular application’s code (cont'd.)

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


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




Figure 3-35: Sample test data for the Sunshine cellular application



Figure 3-36: Result of using the data from Set 1 in Figure 35



Figure 3-37: Result of using the data from Set 2 in Figure 3-35


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


Figure 3-38: How to format a number


Figure 3-39: The calcButton’s modified Click event procedure


Formatting Numeric Output (cont'd.)

Figure 3-40: Formatted output shown in the interface


Programming Tutorial 1

Figure 3-51: Result of entering 255, 255, and 0 as the RGB values


Programming Tutorial 2

Figure 3-55: MainForm for the Vans & More Depot application


Programming Example

Figure 3-63: Pseudocode


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

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


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



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

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


microsoft visual basic 2010: reloaded fourth edition chapter three memory locations and calculations

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