Bug Proofing Visual Basic: A Guide to Error Handling and Prevention
(Publisher: John Wiley & Sons, Inc.)
Author(s): Rod Stephens
ISBN: 0471323519
Publication Date: 11/01/98

Function hit test statistics provide a useful sanity check. If you discover that one routine is being called far more often than you expected it to be, you should look at the code and figure out why. You should also examine routines that are never called. Either modify the code so they are called properly, expand your test cases so they exercise these routines, or remove them if they are unnecessary.

Figure 15.6  Function hit count statistics sorted by hits.

Figure 15.7  Program Bad15 showing text and its encryption.

Self-Test

Program Bad15, shown in Figure 15.7, uses some relatively slow code to perform simple encryption. Enter some text in the left text box and a numeric value in the Key field. Then click the Encrypt button to make the program encrypt the text.

Enter encrypted text in the right text box and the key that was used to encrypt the text in the Key field. Then click the Decrypt button to make the program decrypt the text.

This program is reasonably responsive for moderate amounts of text. If you enter a lot of text, however, the program slows down considerably. If you enter 1000 characters, the program takes 6 or 7 seconds to encrypt or decrypt the text. It should be able to run much faster.

The code for program Bad15 follows. Enter the code or download it from the book’s Web page at www.vb-helper.com/err.htm. Run it using the Visual Basic Code Profiler to see where the program spends most of its time. Then make the fewest changes possible to improve the program’s performance. You should be able to make the program much faster with only a few changes.

    Option Explicit

    ‘ Encrypt the text by adding the key number
    ‘ to each character's ASCII code.
    Private Sub cmdEncrypt_Click()
        txtCiphertext.Text = “”
        MousePointer = vbHourglass
        DoEvents

    Crypt txtPlaintext, txtCiphertext, Int(txtKey.Text)
     MousePointer = vbDefault
 End Sub

    ‘ Encrypt the text by subtracting the key number
    ‘ from each character's ASCII code.
    Private Sub cmdDecrypt_Click()
        txtPlaintext.Text = “”
        MousePointer = vbHourglass
        DoEvents

        Crypt txtCiphertext, txtPlaintext, -Int(txtKey.Text)

        MousePointer = vbDefault
    End Sub

    ‘ Encrypt the input text.
    Private Sub Crypt(ByVal txtIn As TextBox, ByVal txtOut As TextBox, _
        ByVal key As Integer)
    Const MIN_ASC = 32  ‘ ASCII code for space.
    Const MAX_ASC = 126 ‘ ASCII code for ~.
    Const NUM_ASC = MAX_ASC - MIN_ASC + 1

    Dim i As Integer
    Dim ch As String
    Dim ch_num As String

         txtOut.Text = “”
         For i = 1 To Len(txtIn.Text)
             ch = Mid$(txtIn.Text, i, 1)
             ch_num = Asc(ch)
	        ‘ Translate only printable characters.
             If ch_num >= MIN_ASC And ch_num <= MAX_ASC Then
                 ch_num = ch_num + key - MIN_ASC
                 ch_num = (ch_num Mod NUM_ASC)
                 If ch_num < 0 Then ch_num = ch_num + NUM_ASC
                 ch_num = ch_num + MIN_ASC
             End If
             ch = Chr$(ch_num)
             txtOut.Text = txtOut.Text & ch
         Next i
     End Sub

Summary

Use the Visual Basic Code Profiler to make sure all of the code is executed by your tests. For any line that does not execute, fix the code, expand the tests, or remove the line.

Also use VBCP to identify the routines that would benefit most from optimization. Wait until the code is working correctly before you search for these routines. Then optimize only those that will provide significant improvement to the application’s overall performance.

The following list of Bug Stoppers summarizes the tests you can perform using VBCP.

	BUG STOPPERS: Profiling
	Use line timing statistics to see which lines take the longest.
	Use function timing statistics to see which routines take the longest. These are candidates for optimization.
	Use line hit statistics to find code that is never executed. Fix the code, extend the tests, or remove the lines.
	Use function hit statistics to get an idea of which routines are used the most and which are never used.