variables, constants, methods, and calculations chapter microsoft visual basic.net: reloaded 1

Variables, Constants, Methods, and Calculations

Chapter Microsoft Visual Basic .Net: Reloaded

1

2Microsoft Visual Basic .NET: Reloaded

Objectives

• Declare both a variable and a named constant

• Write an assignment statement

• Use the Convert class methods to convert data to an appropriate type

• Write arithmetic expressions

• Understand the scope of both a variable and a named constant


Objectives (continued)

• Include internal documentation in the code

• Use the Option Strict and Option Explicit statements

• Use a TOE chart to code an application

• Use pseudocode and a flowchart to plan an object’s code

• Send focus to a control


Objectives (continued)

• Explain the difference between syntax errors and logic errors

• Format an application’s numeric output


Variables

• Computer memory locations where users can temporarily store data

• Must be assigned a data type by programmer

• Data type determines type of data variable can store

• Contents can change as application runs


Selecting a Data Type for a Variable


Selecting a Data Type for a Variable (continued)

• Integers whole numbers• Integer, Long, Short

• Floating-point numbers real numbers• Single, Double – use exponential notation

• Example: 32000 = 3.2E4

• Decimal uses fixed decimal point

• Character Types• Char – one character

• String – 0 to 2 billion characters


Selecting a Name for a Variable

• Name should be descriptive

• Use three-letter prefix

• Use “m” for “module-scope” variables

• Punctuate using Pascal-case

• Type m and 3 letter prefix using lowercase, then capitalize the first letter of each word in the variable name


Selecting a Name for a Variable (continued)


Declaring a Variable


Declaring a Variable (continued)• accessibility variablename as [datatype = initialvalue]

• accessibility - sets scope of variable

• Dim, Public, Private

• variablename = programmer chosen name

• datatype = data type of the variable

• Anything in brackets is optional

• initialvalue = default value at start of program

• If no value specified numeric types = 0, character types = Nothing, boolean = false


Assigning Data to an Existing Variable

• assignment statement

• strName = “Mary”

• intAge = 35

• “=“ sign is called assignment operator

• Literal constant – value does not change

• Number 35 is numeric literal constant

• “Mary” is a string literal constant

• String - a group of characters enclosed in quotes


Assigning Data to an Existing Variable (continued)


Assigning Data to an Existing Variable (continued)

• Literal type characters force literal constants to assume new data type


Using the Convert Class

• Convert class creates object that is either a number or a string by converting value to a specified data type and returning result strAge = Convert.ToString(intAge)


Writing Arithmetic Expressions

• Precedence number indicates order in which VB performs operation in an expression


Writing Arithmetic Expressions (continued)

• Integer division operator (\)

• Returns integer portion of division discarding remainder

• 8\3 results in answer of 2

• Modulus arithmetic operator (Mod)

• Returns remainder portion of division discarding integer portion

• 7 Mod 3 results in an answer of 1

• Often used to determine Leap years

• Valid only with division of integer values


How To…


The Scope and Lifetime of a Variable

• Scope

• Indicates where in the application’s code the variable can be used

• Lifetime

• Indicates how long the variable remains in the computer’s internal memory


The Scope and Lifetime of a Variable (continued)

• Procedure Scope

• Procedure-level variable

• Variable declared within a procedure

• Variable can only be used in procedure in which declared

• Module scope

• Module-level variable

• Variable declared in General Declarations section of the form

• Variable can be used within all procedures in the form


Procedure-level variables example


Module-level variable example


The Scope and Lifetime of a Variable (continued)

• Block scope

• Block-level variable

• Declared inside specific blocks of code

• If…Then…Else or For….Next

• Can be used only inside the block in which declared


Named Constants• A memory location inside a computer whose

value cannot be changed while the application is running


Named Constants (continued)


Internally Documenting the Program Code

• Internal documentation:

• Term used by programmers for comments placed in code

• Place an apostrophe (‘) before text you want treated as internal documentation (comment)

• VB ignores everything after apostrophe on line

• Place comments at beginning of each procedure


Internally Documenting the Program Code (continued)

• Include comments:

• as necessary to explain various sections of code

• at beginning of application explaining application’s:

• Name

• Purpose

• Author

• Date of creation or modification


Internally Documenting the Program Code (continued)


Option Explicit and Option Strict

• Option Explicit On• Tells computer to warn you if your code

contains name of an undeclared variable• Prevents unintentional declaration of variables

which automatically are of type Object• Option Strict On

• Tells computer not to perform any implicit type conversions which may lead to loss of data

• Implicit type conversions are automatic conversions from one data type to another performed by the computer to fit data into an assigned memory location


Option Explicit and Option Strict (continued)


Coding the SKATE-AWAY SALES Application

• Application calculates and displays total number of skateboards ordered and total price of skateboards including 5% sales tax

• Planning steps:• Identify tasks the application needs to perform

• Identify objects to which you will assign those tasks

• Identify events required to trigger an object into performing its assigned tasks

• Design the user interface


Coding the SKATE-AWAY SALES Application (continued)


Using Pseudocode to Plan a Procedure

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


Using a Flowchart to Plan a Procedure

• Flowchart – uses standardized symbols to show steps procedure must follow to reach its goal

• Standard symbols utilized:• Flowlines - connection lines between symbols• Start/stop – ovals indicating starting and

ending points of procedure• Process – rectangles designating tasks such

as calculations• Input/Output – parallelogram designating input

and output tasks such as getting information from user


Using a Flowchart to Plan a Procedure (continued)


Using a Flowchart to Plan a Procedure (continued)

• Coding the one task of bntExit_Click event using either pseudocode or flowchart tools


Coding the btnClear control’s Click Event Procedure


Assigning a Value to the Property of a Control

• Zero-length string - “”• Also called empty string

• Set of quotation marks with nothing between them

• Assigned to the text property of a control at run-time removes contents of control

• Me.txtname.Text• Me refers to the current form

• txtName refers to a textbox control on the form

• Text refers to a property of the txtName control


HOW TO…


Using the Focus Method

• Focus indicates the control that can accept input and/or is awaiting an action from user


btnClear_Click Event Procedure code


Coding the btnCalc Control’s Click Event Procedure


Coding the btnCalc Control’s Click Event Procedure (continued)


Completed Code for Application


Completed Code for Application (continued)


Testing and Debugging the Application

• Invalid data - data the application is not expecting

• Debugging:• Process of locating syntax and logic errors in

the program • Syntax errors

• Typographical errors that violate rules governing valid syntax of the language

• Me.Colse() instead of Me.Close()• Logic errors

• Instruction that does not give expected results • decAverage = decNum1 + decNum2 / 2


Testing and Debugging the Application (continued)

• First test with valid data


Testing and Debugging the Application (continued)

• Then test with invalid data


Formatting Numeric Output

• Formatting:

• Specifying number of decimal places and special characters to display in a number

• Format string:

• String used to specify format

• Example: “Axx” must be enclosed in quotes

• Format specifier - A

• Alphabetic character that specifies format you wish to use

• Precision specifier – xx

• xx is the sequence of digits desired


Formatting Numeric Output (continued)


Programming Example – Currency Calculator

• Application allows user to enter number of American dollars that he or she wants to convert to British pounds and Mexican pesos

• Make appropriate calculations and then display results on screen


TOE Chart


User Interface


Objects, Properties, and Settings


Tab Order


Pseudocode

btnExit Click event procedureclose application

btnCalc Click event procedure1. assign input value to a variable2. calculate number of British pounds by multiplying American dollars by .6268813. calculate number of Mexican pesos by multiplying American dollars by 10.3924.display number of British pound and Mexican pesos in lblBritish and lblMexican5.send the focus to the txtAmerican text box


Code


Code (continued)


Summary

• Variables and named constants are computer memory locations that store data

• Variables are declared with a name and data type

• Scope refers to where a program can access a variable or named constant• Module scope variables can be accessed

anywhere in the module• Procedure scope variables can only be

accessed in the procedure in which declared• Block scope variables can only be accessed

within the block in which declared


Summary (continued)

• Lifetime refers to how long a variable remains in the computer’s memory

• Literal constants are items of data• Convert class contains methods to convert

values to a specified data type• Integer division operator divides 2 integers

and returns the result discarding any remainder

• Modulus division operator divides 2 integers and returns the remainder


Summary (continued)

• Option Explicit warns if undeclared variables are used

• Option Strict prevents implicit type conversions that may result in loss of data

• Use pseudocode (short phrases) or flowcharts (standardized symbols) to plan the application

• Test application with both valid and invalid data

• Format numeric output to display special characters

variables, constants, methods, and calculations chapter microsoft visual basic.net: reloaded 1

Documents

variable slide

type of data variable

reloaded assigning data

new data type slide

reloaded objectives

control slide

false slide

variable continued integers