Computer Programming Concepts and Visual Basic David I. Schneider

366 Computer Programming Concepts and Visual Basic
■ COLLECTIONS
A collection of objects is an ordered set of objects where the objects are identified by the
numbers 1, 2, 3, . . . A collection is declared with a statement of the form
Dim collectionName As New Collection
and initially contains no objects. (We say that the variable collectionName has type Collection.)
The statement
collectionName.Add objectName
adds the named object to the collection and automatically assigns it the next available number.
The numbers for the different objects will reflect the order they were added to the collection.
The statement
collectionName.Remove n
deletes the nth object from the collection and automatically reduces the object numbers from
n +1 on by 1 so that there will be no gap in the numbers. At any time, the value of
collectionName.Count
is the number of objects in the collection. The value of
collectionName.Item(n).propertyName
is the value of the named property in the nth object of the collection. The statement
collectionName.Item(n).methodName
runs the named method of the nth object of the collection.
EXAMPLE 1
In the following program, the user enters four pieces of data about a student into text boxes and selects a
type of registration. When the AddStudent button is pressed, the data is used to create and initialize an
appropriate object (either from class CStudent or class CPFStudent) and the object is added to a collection.
When the Calculate Grades button is pressed, the name, social security number, and semester grade
for each student in the collection is displayed in the picture box.
Object Property Setting
frm13_2_1 Caption Semester Grades
lblName Caption Name
txtName Text (blank)
lblSSN Caption SSN
txtSSN Text (blank)
lblMidterm Caption Midterm
txtMidterm Text (blank)
lblFinal Caption Final
txtFinal Text (blank)
fraType Caption Type of
Registration
optReg Caption Regular
optP Caption Pass/Fail
cmdAdd Caption &Add Student
cmdSemGrade Caption &Calculate Grades
cmdQuit Caption &Quit
picGrades