Chapter 3: Using Variables and Constants

Previewing the Modified Play It Again Movies Application New screen for salesclerk’s name Sales tax added to order Figure 3-1 Name Entry dialog box Figure 3-2 Completed sales receipt Programming with Microsoft Visual Basic 2012

Lesson A Objectives After studying Lesson A, you should be able to: Declare variables and named constants Assign data to an existing variable Convert string data to a numeric data type using the TryParse method Convert numeric data to a different data type using the Convert class methods Explain the scope and lifetime of variables and named constants Explain the purpose of Option Explicit, Option Infer, and Option Strict Programming with Microsoft Visual Basic 2012

Using Variables to Store Information Controls and variables temporarily store data Variable A temporary storage location in main memory Specified by data type, name, scope, and lifetime Reasons to use variables To hold information that is not stored in a control on the form To allow for more precise treatment of numeric data To enable code to run more efficiently Programming with Microsoft Visual Basic 2012

Using Variables to Store Information (cont.) Selecting a Data Type for a Variable Data type Specifies the type of data a variable can store Provides a class template for creating variables Unicode A universal coding scheme for characters Assigns a unique numeric value to each character in the written languages of the world Programming with Microsoft Visual Basic 2012

Using Variables to Store Information (cont.) Figure 3-3 Basic data types in Visual Basic Programming with Microsoft Visual Basic 2012

Using Variables to Store Information (cont.) The textbook uses: The Integer data type for all integers Either the Decimal or Double data type for numbers containing decimal places or numbers used in calculations The String data type for text or numbers not used in calculations The Boolean data type for Boolean values Programming with Microsoft Visual Basic 2012

Using Variables to Store Information (cont.) Selecting a Name for a Variable Names must begin with a letter or underscore Names can contain only letters, numbers, or underscores No punctuation, special characters, or spaces are allowed The recommended length for a name variable is 32 characters Variable names cannot be reserved words (such as Sub or Double) Figure 3-4 Three-character IDs and examples Programming with Microsoft Visual Basic 2012

Using Variables to Store Information (cont.) Valid names: intFeb_Income decSales2014 dblEastRegion strName Invalid Names: 4thQuarter The name must begin with a letter or underscore dblWest Region The name cannot contain a space strFirst.Name The name cannot contain punctuation decSales$East The name cannot contain a special character Programming with Microsoft Visual Basic 2012

Using Variables to Store Information (cont.) Declaring a Variable Declaration statement Used to declare (create) a variable and reserve space in memory for it If no initial value is given to a variable when declaring it, the computer stores a default value Numeric variables are set to 0 Boolean variables are set to False Object and String variables are set to Nothing Date variables are set to 1/1/0001 12:00:00AM Programming with Microsoft Visual Basic 2012

Using Variables to Store Information (cont.) Figure 3-6 Syntax and examples of a variable declaration statement Programming with Microsoft Visual Basic 2012

Assigning Data to an Existing Variable Assignment statement Assigns a value to a variable at run time Syntax: variablename = expression An expression may include literal constants, object properties, variables, keywords, and arithmetic operators Literal constant A data item whose value does not change while the application is running Example: The string “Mary” Literal type character Forces a literal constant to change its data type Programming with Microsoft Visual Basic 2012

Assigning Data to an Existing Variable (cont.) Figure 3-7 Syntax and examples of assigning a value to a variable during run time Programming with Microsoft Visual Basic 2012

Assigning Data to an Existing Variable (cont.) The TryParse Method Converts a string to a number Is preferred over Val Allows the programmer to specify the data type Val only returns a Double number Arguments dataType: A numeric data type, such as Integer String: A string to be converted Variable: A variable that receives the numeric value Programming with Microsoft Visual Basic 2012

Assigning Data to an Existing Variable (cont.) Figure 3-9 Basic syntax and examples of the TryParse method Programming with Microsoft Visual Basic 2012

Assigning Data to an Existing Variable (cont.) Figure 3-10 Results of the TryParse method for the Double, Decimal and Integer data types Programming with Microsoft Visual Basic 2012

Assigning Data to an Existing Variable (cont.) The Convert Class Can be used to convert a number from one type to another Methods include ToDecimal, ToDouble, ToInt32, and ToString TryParse is recommended for converting strings to numeric data types Will not produce an error if the conversion fails Programming with Microsoft Visual Basic 2012

Assigning Data to an Existing Variable (cont.) Figure 3-11 Syntax and examples of the Convert class methods Programming with Microsoft Visual Basic 2012

The Scope and Lifetime of a Variable Indicates where a variable can be used Lifetime How long a variable remains in memory Scope and lifetime are determined by where a variable is declared: either the General Declarations section or the form’s Declaration section Three types of scope: Class: The variable can be used by all procedures in a form Procedure: The variable can be used within a procedure Block: The variable can be used within a specific code block Programming with Microsoft Visual Basic 2012

The Scope and Lifetime of a Variable (cont.) Variables with Procedure Scope Can be used only by that procedure Declared at the beginning of the procedure Removed from memory when the procedure ends Declared using the Dim keyword Most variables used in this course will be procedure-level variables Figure 3-12 User Interface for the Discount Calculator application Programming with Microsoft Visual Basic 2012

The Scope and Lifetime of a Variable (cont.) Figure 3-13 Click event procedures using procedure-level variables Programming with Microsoft Visual Basic 2012

The Scope and Lifetime of a Variable (cont.) Figure 3-14 Code Editor window for the Discount Calculator application Programming with Microsoft Visual Basic 2012

The Scope and Lifetime of a Variable (cont.) Variables with Class Scope Can be used by all procedures in the form Declared in the form’s Declarations section Will remain in memory until the application ends Declared using the Private keyword Figure 3-16 User Interface for the Total Scores application Programming with Microsoft Visual Basic 2012

The Scope and Lifetime of a Variable (cont.) Figure 3-17 Total Scores application code using a class-level variable Programming with Microsoft Visual Basic 2012

Static Variables Static variable A procedure-level variable with an extended lifetime Remains in memory between procedure calls Retains its value even when the procedure ends Static keyword Used to declare a static variable Static variables act like class-level variables but have narrower scope They can only be used by the procedure in which they are declared Programming with Microsoft Visual Basic 2012

Static Variables (cont.) Figure 3-19 Total Scores application’s code using a static variable Programming with Microsoft Visual Basic 2012

Named Constants Named constant Const statement A memory location inside the computer whose contents cannot be changed at run time Const statement Creates named constant Stores value of expression in a named constant expression: Can be a literal constant, another named constant, or an arithmetic operator Cannot contain a variable or method Programming with Microsoft Visual Basic 2012

Named Constants (cont.) Figure 3-20 Syntax and examples of the Const statement Programming with Microsoft Visual Basic 2012

Named Constants (cont.) Figure 3-21 User interface for the Area Calculator application Figure 3-22 Calculate Area button’s Click event procedure Programming with Microsoft Visual Basic 2012

Option Statements Option Explicit and Option Infer Prevent you from using undeclared variables Option Strict Implicit type conversion Converts the right-side value to the data type on the left side Promotion Data is converted to a greater precision number (e.g., Integer to Decimal) Demotion Data is truncated (e.g., Decimal to Integer) Data loss can occur when demotion occurs Infer Ensures that every variable is declared with a data type Programming with Microsoft Visual Basic 2012

Option Statements (cont.) Option Strict (cont.) Disallows implicit conversions Type conversion rules are applied when this option is on Figure 3-24 Rules and examples of type conversions Programming with Microsoft Visual Basic 2012

Option Statements (cont.) Figure 3-25 Option statements entered in the General Declarations section Programming with Microsoft Visual Basic 2012

Lesson A Summary Declare a variable using {Dim | Private | Static} An assignment statement assigns a value to a variable Three levels of scope: block, procedure, class The TryParse method converts strings to numeric data Use Const to declare a named constant Avoid programming errors by using Option Explicit On, Option Infer Off, and Option Strict On Programming with Microsoft Visual Basic 2012

Lesson B Objectives After studying Lesson B, you should be able to: Include procedure-level and class-level variables in an application Concatenate strings Get user input using the InputBox function Include the ControlChars.NewLine constant in code Designate the default button for a form Format numbers using the ToString method Programming with Microsoft Visual Basic 2012

Modifying the Play It Again Movies Application Modifications needed Calculate and display the sales tax Display salesperson’s name Revise the TOE chart to reflect the new tasks You must modify btnCalc button’s Click event and the form’s Load event Figure 3-26 Revised TOE chart for the Play It Again Movies application Programming with Microsoft Visual Basic 2012

Modifying the Play It Again Movies Application (cont.) Figure 3-27 Modified user interface for the Play It Again Movies application Programming with Microsoft Visual Basic 2012

Modifying the Calculate Button’s Code General strategy Remove the existing code from the Click event procedure Recode the procedure using variables in equations Use the Option Explicit On statement Enforces full variable declaration Use the Option Infer Off statement Ensures that variables are declared with data types Use the Option Strict On statement Suppresses implicit type conversions Programming with Microsoft Visual Basic 2012

Modifying the Calculate Button’s Code (cont.) Figure 3-28 A jagged blue line indicates a syntax error Figure 3-29 Lines to delete from the procedure Programming with Microsoft Visual Basic 2012

Modifying the Calculate Button’s Code (cont.) Figure 3-30 Revised pseudocode and flowchart for the btnCalc control’s Click event procedure Programming with Microsoft Visual Basic 2012

Modifying the Calculate Button’s Code (cont.) Figure 3-31 List of named constants and variables Figure 3-32 Const and Dim statements entered in the procedure Programming with Microsoft Visual Basic 2012

Modifying the Calculate Button’s Code (cont.) Figure 3-34 Calculated amounts shown in the interface Figure 3-33 Code entered in the btnCalc control’s Click event procedure Programming with Microsoft Visual Basic 2012

Concatenating Strings Concatenate To connect strings together Concatenation operator The ampersand (&) Include a space before and after the ampersand Numeric values used with the & operator are converted to strings Figure 3-35 Examples of string concatenation Programming with Microsoft Visual Basic 2012

Concatenating Strings (cont.) Figure 3-36 String concatenation included in the assignment statement Figure 3-37 Concatenated strings displayed in the lblMessage control Programming with Microsoft Visual Basic 2012

The InputBox Function InputBox function Arguments Displays an input dialog box and retrieves user input Arguments prompt: Contains the message to display inside the dialog box title: Controls the text that appears in the dialog box’s title bar defaultResponse: Controls the text that appears in the input field The returned value most often assigned to String variable The syntax is shown in Figure 3-39 on the next slide Programming with Microsoft Visual Basic 2012

The InputBox Function (cont.) Figure 3-38 Example of an input dialog box Figure 3-39 Basic syntax and examples of the InputBox function Programming with Microsoft Visual Basic 2012

The InputBox Function (cont.) Figure 3-42 Dialog box created by the InputBox function Figure 3-40 Class-level variable declared in the form’s Declarations section Figure 3-41 frmMain Load event procedure Programming with Microsoft Visual Basic 2012

The ControlChars.Newline Constant Advances the insertion point to the next line in a control Also used to advance the insertion point in file or on the printer To use, type ControlChars.NewLine in the appropriate location Can be used with string concatenation Line continuation character (_) Used to break up a long line of code into two or more lines Programming with Microsoft Visual Basic 2012

The ControlChars.Newline Constant (cont.) Figure 3-43 Modified assignment statement Figure 3-45 Salesclerk’s name shown on the sales receipt Programming with Microsoft Visual Basic 2012

Designating a Default Button Activated by pressing the Enter key Not required to have the focus Only one per form The default button should be the button used most often by the user Except if button’s task is destructive and irreversible, such as deleting data Set the form’s AcceptButton property to the button name Programming with Microsoft Visual Basic 2012

Using the ToString Method to Format Numbers Formatting Specifying decimal places and special characters to display The ToString method replaces the Format function Syntax: variablename.ToString(formatString) variablename: The name of a numeric variable formatString: The string specifying the format you want to use formatString must take the form Axx, where A is the format specifier and xx is the precision specifier Programming with Microsoft Visual Basic 2012

Using the ToString Method to Format Numbers (Cont.) Figure 3-46 Syntax and examples of the ToString method Programming with Microsoft Visual Basic 2012

Using the ToString Method to Format Numbers (Cont.) Figure 3-48 Play It Again Movies application’s code at the end of Lesson B (continues…) Programming with Microsoft Visual Basic 2012

Using the ToString Method to Format Numbers (Cont.) Figure 3-48 Play It Again Movies application’s code at the end of Lesson B Programming with Microsoft Visual Basic 2012

Lesson B Summary The concatenation operator (&) is used to link strings The InputBox function displays an interactive dialog box Use ControlChars.NewLine to move the insertion point to a new line Set the default button in the form’s AcceptButton property The ToString method formats a number for string output Programming with Microsoft Visual Basic 2012

Lesson C Objectives After studying Lesson C, you should be able to: Include a static variable in code Code the TextChanged event procedure Create a procedure that handles more than one event Programming with Microsoft Visual Basic 2012

Modifying the Load and Click Event Procedures Figure 3-51 Revised TOE Chart for the Play It Again Movies application in Lesson C Programming with Microsoft Visual Basic 2012

Modifying the Load and Click Event Procedures (Cont.) Figure 3-52 Revised pseudocode for the Calculate button in Lesson C Programming with Microsoft Visual Basic 2012

Coding the TextChanged Event Procedure Occurs when the Text property value of a control changes Can occur when: The user enters data into the control The code assigns data to the control’s Text property Example: A change is made to the number of items ordered Programming with Microsoft Visual Basic 2012

Coding the TextChanged Event Procedure (Cont.) Associating a Procedure with Different Objects and Events Handles clause Appears in an event procedure’s header Indicates the object and event associated with the procedure You can associate an event procedure with more than one object and/or event In the Handles section of procedure header, list each object and event separated by commas Procedure names are entered in Pascal case Capitalize the first letter in the name and the first letter of each subsequent word in the name Programming with Microsoft Visual Basic 2012

Coding the TextChanged Event Procedure (cont.) Figure 3-53 TextChanged event procedure associated with the txtDvds control Figure 3-54 Completed ClearLabels procedure Programming with Microsoft Visual Basic 2012

Lesson C Summary The TextChanged event procedure responds to a change in the value of a control’s Text Property The Handles clause determines which objects and events are associated with the event procedure To create a procedure for more than one object or event, list each object and event after the Handles keyword Programming with Microsoft Visual Basic 2012