- A Simple Temperature Conversion Program
- Building a Temperature Class
- Putting the Decisions into the Temperature Class
- Using Classes for Format and Value Conversion
- A String Tokenizer Class
- Classes as Objects
- Class Initialization
- Classes and Properties
- Another Interface Example·The Voltmeter
- A vbFile Class
- Programming Style in Visual Basic
Using Classes for Format and Value Conversion
It is convenient in many cases to have a method for converting between formats and representations of data. You can use a class to handle and hide the details of such conversions. For example, you might enter an elapsed time in minutes and seconds with or without the colon.
315.20 3:15.20 315.2
Since all styles are likely, you'd like a class to parse the legal possibilities and keep the data in a standard format within. Figure 3-2 shows how the entries “112” and “102.3” are parsed.
Figure 3-2. A simple parsing program that uses the Times class
The accessor functions for our Times class include the following.
setText (tx as String) setSingle (t as Single) getSingle as Single getFormatted as String getSeconds as Single
Parsing is quite simple and depends primarily on looking for a colon. If there is no colon, then values greater than 99 are treated as minutes.
Public Function setText(ByVal tx As String) As Boolean Dim i As Integer, mins As Long, secs As Single errflag = False i = InStr(tx, ":") If i > 0 Then mins = Val(Left$(tx, i - 1)) secs = Val(Right$(tx, Len(tx) - i)) If secs > 59.99 Then errflag = True End If t = mins * 100 + secs Else mins = Val(tx) \ 100 secs = Val(tx) - (100 * mins) If secs > 59.99 Then errflag = True t = NT Else setSingle Val(tx) End If End If setText = errflag End Function
Since illegal time values might also be entered, we test for cases like 89.22 and set an error flag.
Depending on the kind of time measurements these represent, you might also have some non-numeric entries such as NT for no time or in the case of athletic times, SC for scratch or DQ for disqualified. All of these are best managed inside the class. Thus, you never need to know what numeric representations of these values are used internally.
Private Const tmNT As Integer = 10000, tmDQ As Integer = 20000 Private Const tmSCRATCH As Integer = 30000
Some of these are processed in the code represented by Figure 3-3.
Figure 3-3. The time entry interface, showing the parsing of symbols for No Time, Scratch, and Disqualification
Handling Unreasonable Values
A class is also a good place to encapsulate error handling. For example, it might be that times greater than some threshold value are unlikely and might actually be times that were entered without a decimal point. If large times are unlikely, then a number such as 123473 could be assumed to be 12:34.73.
Public Sub setSingle(tv As Single) t = tv If tv > minVal And tv <> tmNT Then t = tv / 100 End If End Sub
The cutoff value minVal may vary with the domain of times being considered and thus should be a variable. While classes do not have a Form_Load event like Forms do, they do have and initialize events where you can set up default values for variables.
Private Sub Class_Initialize() minVal = 10000 End Sub
To set up the Initialize event in the IDE, click on the left drop-down in the editor title bar so that Class is selected and select Initialize from the right drop-down as shown in Figure 3-4.
Figure 3-4. Selecting the Class Initialize method