Lecture Set 12 Sequential Files and Structures Part D - Structures

Slide 2 Objectives Use structures to store and group data together Structures versus structure variables Structure declarations Accessing structures The with statement More complex data structures

Slide 3 Introduction to Structures A structure is used to store related data items, and groups them together logically A structure can take the place of multiple individual variables The Structure keyword is used to declare a structure Once declared, variables can be declared having the declared data type

Slide 4 Structures (Syntax) [Public | Friend] Structure name variableDeclarations [procedureDeclarations] End Structure

Slide 5 Structures (Syntax, continued) The Structure and End Structure keywords mark the beginning and end of a structure block The Public and Friend keywords mark the accessibility of the structure name defines the structure name Use Pascal case for structure names variableDeclarations contains the structure members procedureDeclarations contains procedures appearing in the structure

Slide 6 Structures (Example 1) Declare a structure named Contact with four members Public Structure Contact Public FirstName As String Public LastName As String Public TelephoneNumber As String Public DateAdded As DateTime End Structure

Slide 7 Structures (Example 2) Declare a structure for cell in a Sudoku Puzzle Grid Public Structure CellType Public Val As Integer Public Potvals As String Public isOriginal as Boolean End Structure

Slide 8 Declaring Structures Structures can be declared in a Class or Module block A Structure block can contain a nested Structure block A Structure block cannot appear inside of a procedure block A structure can have members that are themselves structures

Slide 9 Declaring a Structure Variable Declaring a Structure block does not declare a variable It declares a new data type The syntax to declare a structure variable (of a structured type) is the same as the syntax to declare any other variable Examples: Dim CurrentContact As Contact Dim PreviousContact As Contact Dim SudokuGrid(9,9) As cellType

Slide 10 Declaring a Structure Variable (continued) It's also possible to declare an array of structures Examples: Public ContactList() As Contact ReDim ContactList(9) As Contact

Slide 11 Storing and Retrieving Data From a Structure Assignment statements are used to store and retrieve data from a structure A period (.) separates the structure variable name and the member name Examples: CurrentContact.FirstName = "Joe" CurrentContact.LastName = "Smith" CurrentContact.Telephone = " " CurrentContact.DateAdded = #3/22/2006# SudokuGrid(i,j).Val = newval

Slide 12 Assignment Statements Using Structures 1 Structures can be used on both the left and right side of assignment statements As always, the data types of each side of the expression must match Examples: PreviousContact.FirstName = _ CurrentContact.FirstName PreviousContact.LastName = _ CurrentContact.LastName PreviousContact.Address = _ CurrentContact.Address PreviousContact.DateAdded = _ CurrentContact.DateAdded

Slide 13 Assignment Statements Using Structures 2 Unlike arrays, entire structures may be accessed (all fields accessed together) Example: Dim CurrentContact As Contact Dim PreviousContact As Contact PreviousContact = CurrentContact In the third line, the entire four-component structure CurrentContact is assigned to the PreviousContact Structure The structures of both Structures must be identical

Slide 14 The With Statement (Introduction) The With statement supplies a shorthand way of referencing structure and class members A With block begins with the With statement and ends with the End With statement

Slide 15 The With Statement (Example) With CurrentContact.FirstName = "Joe".LastName = "Smith".Telephone = " ".DateAdded = #3/22/2006# End With

Slide 16 Restrictions on the With Statement Decision-making statements cannot break up a With block Repetition structures cannot break up a With block

Slide 17 Using Arrays of Structures It's possible to declare arrays of structures Following the structure variable name, the array subscript appears Following the array subscript, the structure member appears

Slide 18 Using Arrays of Structures (Example) Store data in the first element of the array named ContactList Private ContactList(99) As Contact ContactList(0).FirstName = "Joe" ContactList(0).LastName = "Smith" ContactList(0).Telephone = " " ContactList(0).DateAdded = #3/22/2006#

Slide 19 Processing Delimited Files Using Arrays of Structures A sequential file can be read into an array of structures Steps: Read the first record performing the priming read In a loop: Parse and process the current record Read the next record until there are no more records

Slide 20 Reading One Record from a File to a Structure