Microsoft Visual Basic 2010: Reloaded Fourth Edition Chapter Three Memory 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 Edition2

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 Microsoft Visual Basic 2010: Reloaded, Fourth Edition3

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 Microsoft Visual Basic 2010: Reloaded, Fourth Edition4

Internal Memory (cont'd.) Microsoft Visual Basic 2010: Reloaded, Fourth Edition5 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 Microsoft Visual Basic 2010: Reloaded, Fourth Edition6

7 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 Microsoft Visual Basic 2010: Reloaded, Fourth Edition8

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 Microsoft Visual Basic 2010: Reloaded, Fourth Edition9

10 Figure 3-3: Basic data types in Visual Basic

Microsoft Visual Basic 2010: Reloaded, Fourth Edition11 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

Microsoft Visual Basic 2010: Reloaded, Fourth Edition12 Figure 3-4: How to declare a variable Declaring a Variable in Code (cont’d.)

Microsoft Visual Basic 2010: Reloaded, Fourth Edition13 Figure 3-4: How to declare a variable (cont’d.) Declaring a Variable in Code (cont’d.)

Microsoft Visual Basic 2010: Reloaded, Fourth Edition14 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 Microsoft Visual Basic 2010: Reloaded, Fourth Edition15

Microsoft Visual Basic 2010: Reloaded, Fourth Edition16 Figure 3-5: How to assign a value to a variable Assigning Data to an Existing Variable (cont'd.)

Microsoft Visual Basic 2010: Reloaded, Fourth Edition17 Figure 3-5: How to assign a value to a variable (cont’d.) Assigning Data to an Existing 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 Microsoft Visual Basic 2010: Reloaded, Fourth Edition18

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 Microsoft Visual Basic 2010: Reloaded, Fourth Edition19

Microsoft Visual Basic 2010: Reloaded, Fourth Edition20 Figure 3-6: How to use the basic syntax of the TryParse method

Microsoft Visual Basic 2010: Reloaded, Fourth Edition21 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 Microsoft Visual Basic 2010: Reloaded, Fourth Edition22

Microsoft Visual Basic 2010: Reloaded, Fourth Edition23 Figure 3-8: How to use the Convert class methods Using the Convert Class Methods (cont’d.)

Microsoft Visual Basic 2010: Reloaded, Fourth Edition24 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

Microsoft Visual Basic 2010: Reloaded, Fourth Edition25 Including Variables in Arithmetic Expressions (cont'd.) Figure 3-9: Most commonly used arithmetic operators

Microsoft Visual Basic 2010: Reloaded, Fourth Edition26 Including Variables in Arithmetic Expressions (cont'd.) Figure 3-10: How to use the integer division and Mod operators

Microsoft Visual Basic 2010: Reloaded, Fourth Edition27 Including Variables in Arithmetic Expressions (cont'd.) Figure 3-11: Expressions containing more than one operator having the same precedence

Microsoft Visual Basic 2010: Reloaded, Fourth Edition28 Figure 3-12: How to use variables and arithmetic operators

Microsoft Visual Basic 2010: Reloaded, Fourth Edition29 Figure 3-12: How to use variables and arithmetic operators (cont’d.) Including Variables in Arithmetic Expressions (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 Microsoft Visual Basic 2010: Reloaded, Fourth Edition30

Microsoft Visual Basic 2010: Reloaded, Fourth Edition31 Arithmetic Assignment Operators (cont’d.) Figure 3-13: How to use the arithmetic assignment operators

Microsoft Visual Basic 2010: Reloaded, Fourth Edition32 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 scope Microsoft Visual Basic 2010: Reloaded, Fourth Edition33

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 Microsoft Visual Basic 2010: Reloaded, Fourth Edition34

Microsoft Visual Basic 2010: Reloaded, Fourth Edition35 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.) Microsoft Visual Basic 2010: Reloaded, Fourth Edition36 Figure 3-15: Examples of using procedure-level variables (cont’d.)

Variables with Class Scope Microsoft Visual Basic 2010: Reloaded, Fourth Edition37 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

Microsoft Visual Basic 2010: Reloaded, Fourth Edition38 Figure 3-17: Example of using a class-level variable

Microsoft Visual Basic 2010: Reloaded, Fourth Edition39 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

Microsoft Visual Basic 2010: Reloaded, Fourth Edition40 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 Microsoft Visual Basic 2010: Reloaded, Fourth Edition41

Microsoft Visual Basic 2010: Reloaded, Fourth Edition42 Figure 3-19: How to declare a named constant

Microsoft Visual Basic 2010: Reloaded, Fourth Edition43 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 Microsoft Visual Basic 2010: Reloaded, Fourth Edition44

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 Microsoft Visual Basic 2010: Reloaded, Fourth Edition45

Microsoft Visual Basic 2010: Reloaded, Fourth Edition46 Figure 3-22: Rules and examples of type conversions

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 Microsoft Visual Basic 2010: Reloaded, Fourth Edition47

Option Explicit, Option Infer, and Option Strict (cont'd.) Microsoft Visual Basic 2010: Reloaded, Fourth Edition48 Figure 3-23: Option statements entered in the General Declarations section

Coding the Sunshine Cellular Application Microsoft Visual Basic 2010: Reloaded, Fourth Edition49 Figure 3-24: Sunshine Cellular interface from Chapter 2

Coding the Sunshine Cellular Application (cont'd.) Microsoft Visual Basic 2010: Reloaded, Fourth Edition50 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 Microsoft Visual Basic 2010: Reloaded, Fourth Edition51

Using Pseudocode to Plan a Procedure (cont’d.) Microsoft Visual Basic 2010: Reloaded, Fourth Edition52 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 Microsoft Visual Basic 2010: Reloaded, Fourth Edition53

Microsoft Visual Basic 2010: Reloaded, Fourth Edition54 Figure 3-27: Flowcharts for the Sunshine Cellular application

Microsoft Visual Basic 2010: Reloaded, Fourth Edition55 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.) Microsoft Visual Basic 2010: Reloaded, Fourth Edition56 Figure 3-29: Named constants and variables for the calcButton’s Click event procedure

Coding the Calculate Order Button’s Click Event Procedure (cont'd.) Microsoft Visual Basic 2010: Reloaded, Fourth Edition57 Figure 3-25: Declaration statements entered in the procedure

Coding the Calculate Order Button’s Click Event Procedure (cont'd.) Microsoft Visual Basic 2010: Reloaded, Fourth Edition58 Figure 3-31: User input stored in variables

Microsoft Visual Basic 2010: Reloaded, Fourth Edition59 Figure 3-32: Completed calcButton’s Click event procedure

Completing the Sunshine Cellular Application Microsoft Visual Basic 2010: Reloaded, Fourth Edition60 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

Microsoft Visual Basic 2010: Reloaded, Fourth Edition61 Figure 3-34: Sunshine Cellular application’s code

Microsoft Visual Basic 2010: Reloaded, Fourth Edition62 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 Microsoft Visual Basic 2010: Reloaded, Fourth Edition63

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 Microsoft Visual Basic 2010: Reloaded, Fourth Edition64

Microsoft Visual Basic 2010: Reloaded, Fourth Edition65 Testing and Debugging the Application (cont'd.) Figure 3-35: Sample test data for the Sunshine cellular application

Microsoft Visual Basic 2010: Reloaded, Fourth Edition66 Testing and Debugging the Application (cont'd.) Figure 3-36: Result of using the data from Set 1 in Figure 35

Microsoft Visual Basic 2010: Reloaded, Fourth Edition67 Testing and Debugging the Application (cont'd.) Figure 3-37: Result of using the data from Set 2 in Figure 3-35

Microsoft Visual Basic 2010: Reloaded, Fourth Edition68 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

Microsoft Visual Basic 2010: Reloaded, Fourth Edition69 Figure 3-38: How to format a number

Microsoft Visual Basic 2010: Reloaded, Fourth Edition70 Figure 3-39: The calcButton’s modified Click event procedure

Microsoft Visual Basic 2010: Reloaded, Fourth Edition71 Formatting Numeric Output (cont'd.) Figure 3-40: Formatted output shown in the interface

Microsoft Visual Basic 2010: Reloaded, Fourth Edition72 Programming Tutorial 1 Figure 3-51: Result of entering 255, 255, and 0 as the RGB values

Microsoft Visual Basic 2010: Reloaded, Fourth Edition73 Programming Tutorial 2 Figure 3-55: MainForm for the Vans & More Depot application

Microsoft Visual Basic 2010: Reloaded, Fourth Edition74 Programming Example Figure 3-63: Pseudocode

Microsoft Visual Basic 2010: Reloaded, Fourth Edition75 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 Microsoft Visual Basic 2010: Reloaded, Fourth Edition76

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 Microsoft Visual Basic 2010: Reloaded, Fourth Edition77

Microsoft Visual Basic 2010: Reloaded, Fourth Edition78 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 Edition79