Lab Sheet 4.doc. Visual Basic. Lab Sheet 4: Non Object-Oriented Programming Practice

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1 Visual Basic Lab Sheet 4: Non Object-Oriented Programming Practice This lab sheet builds on the basic programming you have done so far, bringing elements of file handling, data structuring and information retrieval. By the end of this lab, you will have had experience in: Saving information to a file Retrieving information from a file Sorting data using a simple sorting algorithm Specific actions you are perform in this lab session will be indicated with a symbol. Assignment 1: Set up a simple (array-based) data structure In this exercise we will create a simple application core with a data structure of student information. We can think of a data structure as simply the way that programming language facilities allow us to organise information for easier retrieval, management etc. In this case, our data structure will be an array of user-defined types (UDTs), each element storing the name, matric number and course code for a single student. This data structure could be pictured as a set of card index entries: To begin this project: Joe Bloggs PGDITO-98 Start Visual Basic Rename the Project to Students and the Form to frmstudents. Add a new code module to the project (right click in the project explorer, choose New Module) and name this modstudents Save the project in a new folder (called Students) We now need to define the data structure for holding the student information. There will be two components to this a UDT for a structure to hold the data for a single student, and an array declaration for holding a number of student records. Double click on the code module entry in the project explorer, and enter the following code into the code window: Option Explicit Type Student Name As String Matric As String CourseCode As String End Type A.McMonnies, CIS Page 1 02/10/2000

2 Const MAXSTUDENT = 20 Private Students(1 To MAXSTUDENT) As Student Private LastStudent As Integer The above structure definitions give us the facility to store the records of 20 students in out program. The next stage in this process is to add Subs and/or Functions to allow us to add information to this structure, look up information and make changes to it. Note that since the StudentList structure has been declared as private, we will need to use code to manipulate this information within a project. Define a subroutine to enter a student s details in the code module: Public Sub () LastStudent = LastStudent+1 If LastStudent > MAXSTUDENT Then MsgBox No more room Else Students(LastStudent).Name = InputBox( Enter name ) Students(LastStudent).Matric = InputBox( Enter matric ) Students(LastStudent).CourseCode = _ InputBox( Enter code ) Note you should enter the student name as Surname, Initial e.g. Smith, J. we need a way to report on a particular student. A subroutine that will return a descriptive string, given a number that corresponds to the position in the array (e.g. 5 => 5 th entry in the array, Students(5)) would suit). Enter a function to return student information Public Function GetStudent(n As Integer) As String Dim result as String If n<1 Or n>laststudent Then MsgBox No entry at this number result = Else result = Students(n).Name & vbcrlf result = result & Students(n).Matric & vbcrlf result = result & Students(n).CourseCode GetStudent = result End Function Using this function, a call such as: will result in the information in the second entry in the array being packed into a string and displayed by the MsgBox statement, or an error message if there is not a second entry. A.McMonnies, CIS Page 2 02/10/2000

3 Test the data subsystem developed so far, by entering the details for 3 or 4 students from the Immediate Window (simply type 3 or 4 times in the Immediate Window), and then retrieving one of these (say the second) by typing. A short Sub can be added to the module so that the entire list can be displayed. This will simplify testing and debugging your project: Exercise: Public Sub DisplayList() Dim i As Integer, d As String For i = 1 To LastStudent d = d & Students(i).Name & ", " & _ Students(i).Matric & ", " & _ Students(i).CourseCode & vbcrlf MsgBox d 1. Add command buttons to the form and code their _Click( ) event handlers to enable (a) a new student to be entered and (b) the list of students to be displayed. Assignment 2 Making the Data Structure Persistent If you needed to test the data subsystem developed so-far more than once, you will probavbly be well aware of the annoyance of having to re-enter the same information over and over. We can get around this problem by adding functions to store and retrieve the array information in a disk file. Lets assume that we wish the disk file to be structured as shown below: <Number of entries> <First entry name> <First entry matric> <First entry course code> <Second entry name> <Second entry matric> < There are a number of possible formats that could be used, but the above is simple and efficient. The algorithm for storing the data is therefore: Get file handle Open file for output Send LastStudent to file For entry From 1 To LastStudent Send entry name to file Send entry matric to file Send entry coursecode to file Close file Create the SaveList and LoadList subroutines Public Sub SaveList(FileName As String) A.McMonnies, CIS Page 3 02/10/2000

4 Dim f As Integer, entry As Integer F = FreeFile Open FileName For Output As #f Write #f, LastStudent For entry = 1 To LastStudent Write #f, Students(entry).Name, _ Students(entry).matric, _ Students(entry).CourseCode Close #f The other side of this coin is that we need to be able to retrieve the information from disk again. The code to do this is almost a mirror of the SaveList sub: Public Sub LoadList(FileName As String) Dim f As Integer, entry As Integer F = FreeFile Open FileName For Input As #f Input #f, LastStudent For entry = 1 To LastStudent Input #f, Students(entry).Name, _ Students(entry).Matric, _ Students(entry).CourseCode Close #f By this stage, we should be able to test the system by entering some sets of data, saving them to file and retrieving them. If for any reason, a test causes the program to terminate, there will be no need to re-enter the data since it is saved on disk. Enter some data and store it to disk from the immediate window: SaveList \Students.dat ***** NOTE Note that in the call to SaveList, you will need to provide your own filename. I would suggest saving the information to the folder in your personal network folder that the project files are stored in. This will make it much easier to connect the program with the data at a later date. See the following exercise for a programming hint relating to this. You can now try to retrieve the information from the program. First, you should terminate the program by making sure the project is not in a running state (press the stop button on the toolbar, or Run/End from the menus) From the immediate window, enter: LoadList \Students.dat MsgBox GetStudent(1) MsgBox GetStudent(3) etc A.McMonnies, CIS Page 4 02/10/2000

5 If all is well, the array of information should have been restored. You can add data to the array at this point, re-save the structure and, the next time you load the list, all entries, including the new ones, should be restored. Exercise: 1. Add command buttons to the form to cause the list of students to be stored and retrieved. You can use the following expression to provide a full file name in both cases: App.Path & \Students.dat Note that this file name specifies that the file will be loaded from and saved to the same folder that the project files are stored in. App.Path is simply the project folder. Assignment 3: Sorting the information into order Information can be searched more easily if it is stored in some order. For example, we could store student records in alphabetical order of name. In this exercise, you will add routines to the code module to allow the information in the array to be sorted into order. Enter the following Sub definition: Private Sub Swap(S1 As Student, S2 As Student) Dim temp As Student temp = S1 S1 = S2 S2 = temp By defining a routine to swap two entries in the array, the sort routine will be reduced in apparent complexity. There are a number of possible sorting strategies that could be used: Bubblesort : keep swapping adjacent elements I the array until it is in order Selection-sort : sort by finding smallest, then next smallest etc., moving them up to correct place in the list Quicksort : an advanced algorithm that minimises the sort time for large lists We will implement the Bubblesort algorithm: Public Sub BSortList() Dim did_a_swap As Boolean, pos As Integer did_a_swap = True ' kick start the algorithm While did_a_swap did_a_swap = False For pos = 1 To LastStudent - 1 If Students(pos).Name > Students(pos + 1).Name Then Swap Students(pos), Students(pos + 1) did_a_swap = True Wend To test this, we can load a list of student entries, sort them and then determine if they are in the right A.McMonnies, CIS Page 5 02/10/2000

6 order. The following immediate window sequence will help to determine this: LoadList <FileName> MsgBox GetStudent(1) MsgBox GetStudent(3) BSortList MsgBox GetStudent(1) MsgBox GetStudent(3) etc. to display all entries. etc. to display all entries. The second time the list is displayed, entries should appear in alphabetical order. Additional Exercises: 1. Add a button to the form to cause the list of students to be sorted. 2. (Advanced) Write a second sort routine, SSortList, using the Selection sort algorithm (listed below in pseudocode). For pos From first to last min = pos For cmp = pos+1 To last If Students(cmp).Name < Students(min).Name Then Swap Students(cmp) with Students (min) Summary You should now be able to write programs in which simple data is well structured and can be made persistent by saving to disk files. You may wish to return to previous exercises and try to add persistence to them, and to try expending them by making use of arrays and UDTs. END OF LAB SHEET A.McMonnies, CIS Page 6 02/10/2000

(Subroutines in Visual Basic)

Ch 7 Procedures (Subroutines in Visual Basic) Visual Basic Procedures Structured Programs To simplify writing complex programs, most Programmers (Designers/Developers) choose to split the problem into