BACS 287

BACS 287

Programming Fundamentals 1

BACS 287

Programming Fundamentals

This lecture introduces the following topics:– Variables

State Scope Lifetime

– Constants– User-Defined Data Structures

BACS 287

Visual Basic Programming

Programs are explicit instructions to the computer telling it how to solve a problem.

Programs use the structured logic that you create with pseudocode or flowcharts.

Programs differ from pseudocode in that you must write your program according to the rules of the language (syntax). Thus, you must convert your pseudocode to VB statements.

You can see the full VB reference documentation at: http://msdn.microsoft.com/en-us/library/2x7h1hfk.aspx

BACS 287

Visual Basic Programming

The general hierarchical structure of a VB program is as follows:– Solution– Project– Forms & Modules– Procedures (functions & subroutines)– Structures (constructs)– Statements– Variables/Constants/Functions/...

Highest

Lowest

BACS 287

Variables

Variables are named memory locations that hold temporary data values during the execution of a program.

There are 4 key characteristics of variables:– Name – what it is called– Type – the kind of data it is designed to hold– Size – how much memory does it take to store it– Value – the current value of the variable

When you define a variable you determine its name, type, and size.

You can determine its value either when you define it or later in the program.

Memory and Variables

4721GOD

1 2 3 4 5

Name: ANIMAL, Type: string, Size: from 0 for 3, Value: "DOG"

Name: COST, Type: number, Size: from 3 for 4, Value: 1274

0 6

CPU/Memory Interaction

BACS 287

Variable Definition

Name: Assigns a name to memory location– Begins with a letter or an _ (underscore)– No periods, dashes, spaces, or special characters allowed

in name (underscore ‘_’ is ok)– If it begins with underscore, it must have at least one

alphabetic character or number– <= 1,023 characters in length– Unique within the variable scope– Cannot be a “reserve word” (like ‘print’, ‘true’,…)

The variable can be defined explicitly by the programmer or implicitly by Visual Basic. You should always use the explicit method.

BACS 287

Variable Name Prefixes

There are several naming schemes for variables in Visual Basic.

A popular one uses a 3 or 4-letter prefix before a “camel case” descriptive name. It’s called “Hungarian notation”

The main prefixes are:bol – Boolean lng – Long byt - Byte

int – Integer sng – Single dec - Decimal

sho – Short dbl – Double dat – Date

str – String obj – Object uint – unsigned integer

This is the “Hungarian notation”

BACS 287

Variable Definition

Variables can be define explicitly by using the DIM statement.

The DIM statement allows you to define the variable’s name, type, size, and value. For example:

Dim intClassSize as Integer

Dim sngGPA as Single = 3.21

BACS 287

Variable State

The state of the variable is determined by:– Type – Length– Current value

The state of the variable can change during the execution of the program.

BACS 287

Variable Definition

Type: Determines what kind of data can be stored in the variable. These are common VB data types.

– Byte - whole numbers (0 - 255) (1 byte)– Short - whole numbers (-32,768 – 32,767) (2 bytes)– Integer - whole numbers ( 4 bytes) (-2,147,483,648 to 2,147,483,647)– Long – big whole numbers (8 bytes) (apx. -9 septillion to +9 septillion)– Single – floating point numbers (4 bytes) (apx. 1.4 X 10 -45 to 3.4 X 10 38)– Double – big floating point numbers (8 bytes)– Decimal - high precision decimal numbers (16 bytes)– String - text information (1 byte per character) (up to apx. 2 billion chars)– Boolean - logical values (True or False) (2 bytes)– Date - date and time information (8 bytes)– Object - any data type (4 bytes) (pointer to object)– User-Defined Type – defined by programmer (built from base types)

BACS 287

Variable Definition

It is best to use the smallest data type that can hold the anticipated data.

It is best to use one of the integer data types unless you need decimal values.

It is best to explicitly define a variable’s data type. Otherwise it will be the object type by default. You should only use the object type when necessary.

BACS 287

Variable Definition

Size: The amount of memory used by the variable. This is fixed by the data type selected (or the data value entered). For example:

Dim intValue as integer uses 4 bytes

Dim strName as string = “My Name” uses 7 bytes

Dim datDate as date = #01/05/2002# uses 8 bytes

Dim objForm as object = frmMyForm uses 4 bytes

BACS 287

Variable Definition

Value: The value assigned to the memory location named by the variable.

You can assign values to variables at the time they are defined or with an assignment statement later in the program.Dim strName as String = “Jay Lightfoot”

-OR-Dim strName as StringstrName = “Jay Lightfoot”

This example assign literal values (i.e., literals) to the variable.

BACS 287

Variable Definition

You can also assign other variables and constants to variables.strName = strProfessor

intClassSize = MAX_CLASS These examples assume strProfessor and MAX_CLASS were previously defined and assigned a value.

BACS 287

Variable Definition Examples

Dim bytAge as byte = 30

Dim sngTorque as single = 124.33

Dim dblMass as double = 1.243342E+12

Dim lngNationalDebt as long = 4842532334000

Dim strAddress as string = “123 W. 10th St.”

Dim datValue as date = #09/14/2003#

Dim blnResident as boolean = true

BACS 287

Special Object Values

Value of “Nothing” Object variables are initially created with a value of “Nothing”

(unless you specify another value). “Nothing” means no value and is converted to 0 for number data types and the empty string for strings.

Null Value Object variables may be assigned a value of “null”. Null

means that the data is unknown or missing. It is different from 0 or blank. It is also different from “nothing”.

BACS 287

Variable Scope

The scope of the variable is the range of program instructions over which the variable is known, and thus capable of being manipulated.

A variable is visible within its scope and invisible outside of it.

In Visual Basic, the scope of a variable is determined by where it is defined.

BACS 287

Variable Scope

Variables can be declared (defined) at different levels in Visual Basic. The place they are declared determines the variable scope. The common levels are:– Public-Level (Global level) Highest– Friend-level– Form / Module-Level– Procedure-level– Block-level Lowest

Each level has different scoping characteristics.

BACS 287

Variable Scope – Block Level

A “block” is a sub-part of a statement. This statement has 2.

If A = true then dim B as byte Sum = Sum + 1 Block

Cnt = Cnt + 1Else MsgBox(“Error”, “Input Error”) Block

EndIf

Variable Scope

Form1 Form2

Procedure1 Procedure2 Procedure1 Procedure2

PROJECT

Public-Level

Module-Level

Form-Level Form-Level

Block-Level Block-Level Block-Level Block-Level

Friend-Level

BACS 287

Variable Scope

Block-Level - Only visible inside the code block where it is defined.

Dim or Static - Used to define block-level variables

Procedure-Level - Only visible inside the procedure where it is defined. Blocks within the procedure can see it also.

Dim or Static - Used to define procedure-level variables

Form-Level - Visible to all procedures within the form. Invisible outside the form.

Dim - Used to define form-level variables.

BACS 287

Variable Scope

Module-Level - Visible inside the module and invisible outside the module.

Private - Used to define module-level variables. Only in modules.Private intCount as Integer

Friend-Level - Visible inside the project and invisible outside the project.

Friend - Used to define module-level variables. Only in modules.Friend sngCount as single

Public-Level (Global) - Visible to the entire project and in other projects while application is running.

Public - Used to define Public-Level variables. Only in modules.Public strName as String

Variable Scope

Project

Form1 Form2

Procedure A

Procedure B

Procedure C

Procedure D

Public X as Integer

Dim Y as Integer Dim Z as Integer

Dim L as Integer

Dim M as Integer

Dim N as Integer

Dim O as Integer

BACS 287

Variable Lifetime

The lifetime (or extent) of the variable is the interval of time in which the memory storage area is bound to the variable.

Variables defined locally exist only while the procedure or block in which they are declared is executing.

Variables defined as Form-level exist as long as the form is loaded in memory.

Variables defined as module-level or public exist as long as the application is executing.

BACS 287

Variable Lifetime

Block-Level – Only exists as long as the lexical block where it is defined is executing. Normally the ‘End’ terminator of the block indicates the end of the variable lifetime.

Procedure-Level - Only exists as long as the procedure where it is defined is executing. Static variables are an exception to this. They exist as long as the program is executing. Their scope is the same.

Form-Level – Only exist as long as the form is loaded in memory.

BACS 287

Variable Lifetime

Module-Level – Exist as long as the program is executing.

Friend-Level - Exist as long as the program is executing.

Public-Level (Global) - Exist as long as the program is executing.

BACS 287

Variable Conversion

The data type of a variable can be changed while the program is running.

This is often done to modify a string to a number so you can do calculations or to take a number and convert it to a string with special formatting characters (like $ and commas).

Visual Basic has two main ways to do this:– Build-in conversion functions – only work in VB– Conversion methods – work in any .NET language

BACS 287

Variable Conversion Functions

CDec(expression) – convert to decimal CByte(expression) – convert to byte CChar(expression) – convert to character CDbl(expression) – convert to double CSng(expression) – convert to single CInt(expression) – convert to integer CStr(expression) – convert to string CDate(expression) – convert to date/time

Example: strResult = Cstr(123) strResult = Cstr(2 + 6)

BACS 287

Variable Conversion Methods

Convert.ToDecimal(value) – convert to decimal Convert.ToByte(value) – convert to byte Convert.ToChar(value) – convert to character Convert.ToDouble(value) – convert to double Convert.ToSingle(value) – convert to single Convert.ToInt32(value) – convert to 4 byte integer Convert.ToInt16(value) – convert to 2 byte integer Convert.ToString(value) – convert to string Convert.ToDateTime(value) – convert to date/time

Example: strResult = Convert.ToString(123)

BACS 287

Variable Conversion – TryParse

A special method called TryParse exists to perform conversions with formatting.

This method also returns a value of True or False; thus, it can be used as the condition of an IF-Then statement.

Example:

blnResult = Decimal.TryParse(textbox.text, sales)

Options also exist to add formatting characters to the resulting output.

BACS 287

Constants

A constant is similar to a variable except that its value cannot be changed once it is set.

This is useful for situations where you have a value that may change over time (but is static while the program runs).

It also helps document the program and improves readability.

BACS 287

Constant Definition

There are 2 types of constants:– User defined constants (symbolic)Public Const PI as Double = 3.141592653Private Const mdatDUE_DATE as Date = #6/16/99#

– System defined constants (intrinsic)If vbYes then Print “yes”...If “red” then color = Color.Red...

BACS 287

Constants

Constants can be defined at all the scope levels that variables can be.

Constants share the same lifetime rules as variables.

Good programming practices dictate that all constants be declared at the module level (or above).

BACS 287

Constants

Constants often follow these naming conventions:– First character – scope indicator (m for module)– Next 3 characters – date type indicator (int,dec,…)– Rest of name capitalized

Public Const mintMIN_AGE as integer = 18Const decINTEREST as decimal = 3.22332

The second example is a local constant as would be defined in a procedure.

BACS 287

User-Define Data Structures

User-Defined data structures allow you to combine several different types of data into a single structure.

They are useful when you need a single variable that records several related pieces of information.

Use the STRUCTURE statement to declare them. The definition must be made in a module (i.e., not local scope).

BACS 287

User-Define Data Structures

Public Structure StudentInfoPublic SSN as StringPublic Name as StringPublic EnrollDate as DatePublic CurrentStatus as BytePublic GPA as single

End Structure...Dim stuLocal as StudentInfo...If stuLocal.Name = “John Smith” then...

BACS 287

User-Define Data Structure

Public Structure ProjectPackPublic frmInput as formPublic frmCalc as formPrivate dbProject as database

End Structure...Dim Project1 as ProjectPack...Project1.frmCalc = frmMyForm

BACS 287

User-Define Data Structures

You can nest user-defined data types so that one type can be made up of other user-defined data types.

You can also include other complex structures (arrays, objects, ...).

Structures support most of the features of classes (including methods). Thus, structures can contain procedures that act upon local data.

User-Define Data StructuresPublic Structure Employee ' Public members, accessible throughout declaration region.

Public FirstName As String Public MiddleName As String Public LastName As String

' Friend members, accessible anywhere within the same assembly. Friend EmployeeNumber As Integer Friend BusinessPhone As Long

' Private members, accessible only within the structure itself. Private HomePhone As Long Private Salary As Double Private Bonus As Double

' Procedure member, which can access structure's private members. Friend Sub CalculateBonus(ByVal Rate As Single)

Bonus = Salary * CDbl(Rate) End Sub

End Structure

BACS 287

Review

State– Name, type, value, size

Scope– Block, procedure, form/module, public

Lifetime– Block, procedure, form/module, public

Conversions Constants User-defined data structures