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BACS 287

BACS 287. Programming Fundamentals 1. Programming Fundamentals. This lecture introduces the following topics: Variables State Scope Lifetime Constants User-Defined Data Structures. Visual Basic Programming.

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BACS 287

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  1. BACS 287 Programming Fundamentals 1 BACS 287

  2. Programming Fundamentals • This lecture introduces the following topics: • Variables • State • Scope • Lifetime • Constants • User-Defined Data Structures BACS 287

  3. 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

  4. 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

  5. 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. BACS 287

  6. Memory and Variables

  7. CPU/Memory Interaction

  8. 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

  9. 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 BACS 287

  10. 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

  11. 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

  12. 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

  13. 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

  14. 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

  15. 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 String strName = “Jay Lightfoot” • This example assign literal values (i.e., literals) to the variable. BACS 287

  16. 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

  17. 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

  18. 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

  19. 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

  20. 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

  21. Variable Scope PROJECT Public-Level Friend-Level Module-Level Form2 Form1 Form-Level Form-Level Procedure1 Procedure2 Procedure2 Procedure1 Block-Level Block-Level Block-Level Block-Level

  22. 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

  23. 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 BACS 287

  24. Variable Scope Project Public X as Integer Form1 Form2 Dim Y as Integer Dim Z as Integer Procedure A Procedure C Dim L as Integer Dim N as Integer Procedure B Procedure D Dim M as Integer Dim O as Integer

  25. 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

  26. 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

  27. 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

  28. 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

  29. 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

  30. 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

  31. 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

  32. 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

  33. Constant Definition • There are 2 types of constants: • User defined constants (symbolic) Public Const PI as Double = 3.141592653 Private 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

  34. 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

  35. 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 = 18 Const decINTEREST as decimal = 3.22332 • The second example is a local constant as would be defined in a procedure. BACS 287

  36. 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

  37. User-Define Data Structures Public Structure StudentInfo Public SSN as String Public Name as String Public EnrollDate as Date Public CurrentStatus as Byte Public GPA as single End Structure ... Dim stuLocal as StudentInfo ... If stuLocal.Name = “John Smith” then... BACS 287

  38. User-Define Data Structure Public Structure ProjectPack Public frmInput as form Public frmCalc as form Private dbProject as database End Structure ... Dim Project1 as ProjectPack ... Project1.frmCalc = frmMyForm BACS 287

  39. 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. BACS 287

  40. User-Define Data Structures Public 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

  41. Review • State • Name, type, value, size • Scope • Block, procedure, form/module, public • Lifetime • Block, procedure, form/module, public • Conversions • Constants • User-defined data structures BACS 287

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