Microsoft Visual Basic 2005: Reloaded Second Edition Chapter 3 Variables, Constants, Methods, and...

Microsoft Visual Basic 2005: Reloaded

Second Edition

Chapter 3Variables, Constants, Methods, and

Calculations

Microsoft Visual Basic 2005: Reloaded, Second Edition 2

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


Objectives (continued)

• Understand the purpose of the Option Explicit, Option Strict, and Imports statements

• Use a TOE chart, pseudocode, and a flowchart to code an application

• Clear the contents of a control’s Text property while an application is running


Objectives (continued)

• Send the focus to a control while the application is running

• Explain the difference between syntax errors and logic errors

• Format an application’s numeric output


Variables

• Variables: computer memory locations used to store data while an application is running

• Every variable has a:– Name– Data type– Scope– Lifetime


Selecting a Data Type for a Variable

• Each variable must be assigned a data type

• Data type: the type of data the variable can store

• Each data type is a class• Unicode:

– Universal coding scheme for characters– Assigns a unique numeric value to each character


Selecting a Data Type for a Variable (continued)


Selecting a Name for a Variable

• Identifier: descriptive name given to a variable• Use a meaningful name that reflects the purpose of

the variable• Use camel casing for variable identifiers• Variable names must conform to naming rules


Selecting a Name for a Variable (continued)


Declaring a Variable

• Declaration statement: used to declare, or create, a variable

• Declaration statement includes– Scope keyword: Dim or Private or Static– Name of the variable– Data type– Initial value (optional)


Declaring a Variable (continued)


Assigning Data to an Existing Variable

• Assignment statement:– Used to assign values to properties of controls– Used to assign values to variables

• Assignment operator: (=)– Value on the right of the = operator is assigned to

the variable on the left of the = operator


Assigning Data to an Existing Variable (continued)



• String: group of characters enclosed in quotation marks

• 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 with a decimal place is treated as a Double type

• Literal type character: forces a literal constant to assume a specific data type


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

• TryParse method has 4 arguments– String: string value to be converted– Variable: location to store the result – IFormatProvider (optional): specifies formatting– NumberStyles (optional): allows formatting

characters to be in the data to be converted


Using the TryParse Method (continued)

• IFormatProvider argument formats numbers, dates, and times

• NumberFormatInfo.CurrentInfo value:– Uses the formatting characters specified in the

Windows Customize Regional Options dialog box



• Assign the TryParse method’s return value to a Boolean variable– If True, the conversion was successful– If False, the value could not be converted

• Line continuation character: the underscore (_)– Breaks up a long instruction into two or more lines– Must appear at end of line, preceded by a space

• Must have an Imports statement in the General Declarations section of code to use NumberStyles and NumberformatInfo.CurrentInfo:– Imports System.Globalization


Using the Convert Class

• Convert class:– Contains methods for converting numeric values to

specific data types

• Use the dot member access operator to separate the class name from the method name


Using the Convert Class (continued)


Writing Arithmetic Expressions• 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


Writing Arithmetic Expressions (continued)


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 are usually declared in two places:– Within a procedure– In the form’s Declarations section

• Procedure-level variable: declared within a procedure

• Procedure scope: only the procedure can use the variable


The Scope and Lifetime of a Variable (continued)



• 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



•Module scope: variable can be used by all procedures in the form•Module-level variable:

–Declared in the form’s Declarations section–Use Private keyword in declaration

•Module-level variables retain their values until the application ends



• Block scope: variable can be used within a specific block of code

• Block-level variable: declared within a specific block of code


Static Variables

• Static variable: – Procedure-level variable that retains its value even after

the procedure ends– Retains its value until the application ends– Can be used instead of a module-level variable

• A static variable has:– Same lifetime as a module-level variable– Narrower scope than a module-level variable

• Declared using the Static keyword


Static Variables (continued)


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


Named Constants (continued)


Option Explicit and Option Strict

• Option Explicit: – When on, all variables used must first be declared– Protects against misspelled variable names in code– Placed in the General Declarations section of code

editor

• Implicit type conversion: can occur if the value on the right side of an assignment statement is not the same data type as the variable on the left side


Option Explicit and Option Strict (continued)

• Promoting: when a value is converted to another data type that stores larger numbers

• Demoting: when a value is converted to another data type that stores only smaller numbers– Data loss can occur with demoting

• Option Strict:– Can be used to enforce correct data typing– Placed in the General Declarations section of the code

editor



• Option Strict On follows these conversion rules:– Strings are not implicitly converted to numbers or

vice versa– Narrower data types are implicitly promoted to wider

data types– Wider data types are not implicitly demoted to

narrower data types


Coding the Skate-Away Sales Application


Coding the Skate-Away Sales Application (continued)


Using Pseudocode to Plan a Procedure

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


Using Pseudocode to Plan a Procedure (continued)


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


Using a Flowchart to Plan a Procedure (continued)


Coding the clearButton’s Click Event Procedure


Clearing the Contents of a Control’s Text Property


Clearing the Contents of a Control’s Text Property (continued)

• Zero-length string (or empty string): – Removes the contents in the Text property of a

control– Use empty set of quotation marks: “”

• String.Empty: used to assign an empty string to a control’s Text property

• For TextBox control, use the Clear method


Setting the Focus

• Focus method: moves the focus to a specified control at runtime


Setting the Focus (continued)


Coding the calcButton’s Click Event Procedure


Coding the calcButton’s Click Event Procedure (continued)


Testing and Debugging the Application

• Valid data: data that the application is expecting

• Invalid data: data that is unexpected

• Debugging: locating errors in a program

• Syntax errors: usually caused by mistyping

• Logic errors: occur when you enter an instruction that does not give the expected results

• Test a program with both valid and invalid data


Testing and Debugging the Application (continued)


Formatting Numeric Output

• Formatting: specifying the number of decimal places and any special characters to display

• Format specifier: specifies the type of formatting to use

• Precision specifier: controls the number of significant digits or zeros to the right of the decimal point


Formatting Numeric Output (continued)


Programming Tutorial


Programming Example


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, scope, and lifetime

• Use Dim to declare a variable at block or procedure level

• Use Private to declare a variable at module level


Summary (continued)

• Assignment statement is used to assign values to an existing variable

• Literals are constant items of data

• Use the TryParse method to convert a string to a number

• Use the Imports statement to import a namespace

• The Convert class contains methods to convert values to a specified data type


Summary (continued)

• A procedure-level variable is usable only by the procedure in which it is declared

• A module-level variable is usable by all procedures in the form

• A block-level variable is usable only within the block in which it is declared

• A static variable is a procedure-level variable that retains its value when the procedure ends

• Option Explicit On forces declaration of all variables before use


Summary (continued)

• 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

• Use the Clear method or empty string to clear a textbox

• The Focus method moves the focus to a control

• Test a program with both valid and invalid data

Microsoft Visual Basic 2005: Reloaded Second Edition Chapter 3 Variables, Constants, Methods, and...

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