Structures
Arrays require that every element in the array be of the same type. This is adequate when dealing with customer names or student scores. However, there is usually more than one piece of data about a customer or a student. If all of the data is of the same type, then a multidimensional array can be used. If some of the data is of a different type, then parallel arrays can be used; however, it is sometimes cumbersome to maintain separate data structures, and it is very easy to forget to update one or more of the parallel arrays.
Structures, formerly called User Defined Types (UDTs), are another data structure provided by Visual Basic.
Structures are sometimes referred to as records.
They are similar to arrays in that both can represent a group of elements with a common name.
Although the elements of an array must all be of the same type, the elements of a structure can be of different types.
A structure can be thought of as a
"package" of related variables with a common name.
A structure in VB.NET is a composite data type consisting of a number of elements of other types. The structure in VB.NET can contain fields, methods, constants, constructors, properties, indexers, operators and even other structure types.
In grouping items describing an object, a structure may be a more logical description of the data and may be conceptually easier to handle than other data structures.
When the structure variable name and the element names reflect the meaning of the data, programs are both easier to read and write. Even when all the components are of the same type, a structure is sometimes the best choice of data structures if the components are related.
By using arrays of structures, data can be represented in a more meaningful manner.
Declaring a Structure
A structure can be
declared by using the Structure keyword in the General section of a module:
<modifier> Structure <struct_name>
<varmodifier> elementName [(subscripts)] As type
[<varmodifier> elementName [(subscripts)] As type]
. . .
End Structure
In this syntax,
<modifier> can be Public, Protected, Friend, Protected Friend, or Private (default is Friend)
Structure is the Visual Basic keyword that denotes the beginning of a structure block.
struct_name
is the name that you give to the structure.
<varmodifier> must be declared as Dim, Friend, Private, or Public. Members declared with Dim default to Public access, and members declared without the As clause default to the Object data type.
elementName
As type is the name of each member of the
structure.
Elements can be arrays (by
specifying subscripts).
End
Structure denotes the end of the Structure Statement.
The Structure statement can only be used at the module or class level.
Referencing Structure Elements
To access the elements of a
structure, the name of the structure must be specified, followed by a period, followed
by the name of the element. In many languages, this is called the field selector or member
selector. The field selector is treated and used as any other declared variable would be.
The field selector acts as the variable name for each field. It must be complete.
VarName.ElementName
Where
VarName is the name that you give to the variable (an instance of
the type).
ElementName is the name of the specific element in the type.
Private Structure PersonStructure
' Dim (Public) members, accessible throughout declaration region.
Dim Name As String
Dim Gender As String
Dim SSN As String
Dim MaritalStatus As String
Dim Degree( ) As String
End Structure 'PersonStructure
Dim Student as PersonStructure
Name, Gender, SSN, MaritalStatus, and Degree are the element names within the structure PersonStructure. These element names will be used to access the different components of the variable Student declared to be of type PersonStructure.
Student.Name = "Wallace Cleaver"
Arrays declared as structure members cannot be declared with an initial size.
Part of a structure cannot be
accessed outside the structure. Although Student.Name is declared as a String, Name is not an individual variable and cannot be referenced
directly in the program as Student.Name can.
Student.Name -- accesses the string that holds the name
Student.Gender -- accesses the gender
Student.SSN -- accesses the string that holds the social security number
Student.MaritalStatus -- accesses the marital status
Student.Degree -- accesses the degree(s) held
Student.Degree is an array, so it's size must be specified with a ReDim statement and an index will have to be specified to access individual elements:
ReDim Student.Degree(3)
Student.Degree(0) = "BA"
lblOutput.Text = Student.Degree(0)
Another Example
If each item in an inventory list is described by part number, description, cost, and quantity on hand, then a structure would be an ideal choice to describe it:
Private Structure PartStructure
Dim partNum As String
Dim description As String
Dim cost as Decimal
Dim quantity as Integer
End
Structure ' PartStructure
Dim part as PartStructure
The
following statements could be applied to part:
part.cost = 2.34
part.quantity = 36
part.quantity = part.quantity + 24
Hierarchical Structures
Hierarchical structures are structures that contain other structures as their components.
Private Structure dateStructure
Dim month as Integer ' assume 1-12
Dim day as Integer ' assume 1-31
Dim year as Integer
End Structure 'dateStructurePrivate Structure statisticsStructure
Dim failRate as Single
Dim lastServiced as dateStructure '*******
Dim downDays as Integer
End Structure ' statisticsStructurePrivate Structure machineStructure
Dim ID as Integer
Dim description as String
Dim maintHistory as statisticsStructure '*******
Dim purchaseDate as dateStructure '*******
Dim cost as Decimal
Dim depreciatedValue as Decimal
End Structure 'machineStructure
Dim machine as machineStructure
To access a hierarchical structure such as this one, build the accessing expression for the elements of the embedded structures from left to right, beginning with the structure variable name.
machine.maintHistory.downdays
machine.purchaseDate
machine.purchaseDate.month
machine.purchaseDate.year
machine.maintHistory.lastServiced.year
The figure below is a graphical representation of machine with values. Look carefully at how each component is accessed.
Arrays of Structures
Generally, more than a single structure is needed. In such a case an array of structures can be used.
Private Structure StudentStructure
Dim Name as String
Dim Course as String
Dim GPA as Single
Dim Major ( ) as String
End Structure 'StudentStructureDim studentRoll (50) as StudentStructure
An element of studentRoll is selected by a subscript: studentRoll(3) is the fourth element of studentRoll. Each element of studentRoll is a structure of type StudentType. In order to access the grade point average of the fourth student in studentRoll we type
studentRoll(3).GPA
To access the first major of the second student, we type