Additional Questions Arising in MIS 210

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Ongoing assistance from John M. Dennis, a colleague and Visual Basic(c) expert, in building and maintaining this site, is gratefully acknowledged.

Q: How do I insert comments in code?
A: Type a single quote or apostrophe at the beginning of the comment; the single quote should be preceded and followed by a blank space for readability. An entire-line comment may begin with an apostrophe or the keyword "Rem" (short for "remark").
Q: How do I break one VB statement across lines?
A: The last character of the line must be an underscore ("_"), and that underscore must be preceded by at least one blank space. The line cannot be broken in the middle of a "word" (e.g., an identifier).
Q: How do I put multiple statements on one line?
A: In general, you don't, or shouldn't; doing so can make code very difficult to read. However, statements that are very short and closely related may reasonably be put on the same line. To do so, separate them with a colon. Example:
sngX = sngX1 : sngY = sngY1 ' copy Cartesian co-ordinates
Q: What are some good techniques for controlling the movement of the focus when the user presses the "Tab" key?
A: When the user presses the tab key, the focus goes to the next control (in TabIndex sequence) capable of receiving the focus (for example, a Label control cannot receive the focus, but a Text Box control can). TabIndex sequence numbers are, by default, assigned in the order controls are added to a Form. Therefore, I recommend placing the controls on the Form in the general order in which the user will traverse them. For example, if a Label specifies what the user should type into a matching Text Box, place the Label control on the Form and then place the matching Text Box on the form (rather than placing all Labels on the Form, and then all Text Boxes). Near the end of Form layout, reassign TabIndex sequence numbers as needed.
Q: What are some comments on "when to use which data type" in addition to the text material on pages 166-167?
A: The Byte data type is useful for counters which assume only reasonably small values (must stay below 256) and cannot possibly be negative (e.g., number of salespeople in a small store).
Although the Single type can assume an enormous range of values, it carries fewer significant digits than does the Double type. The Double type is occasionally needed for long engineering or scientific calculations during which round-off errors gradually accumulate.
Q: What are the three levels of scope?
A: The narrowest scope is "local" -- variable is accessible to only one procedure pertinent to one control. The variable vanishes when the procedure completes execution (unless it is Static -- see Exercise 20 page 189). The next level of scope is "form" -- the variable is accessible to every procedure in the form. The broadest level of scope is "global," in which a variable is accessible across the entire project.
If variables of different scope have identical names, a procedure will use the variable of narrowest scope accessible to itself.
General principle: use the narrowest scope that will get the job done (a variety of information hiding).
Additionally:
If a variable is declared with a "Public" statement instead of a "Dim" statement, it acquires "global" scope. The "Public" declaration can be used only at the Form level, not the Procedure level.
The "Private" statement has replaced the "Dim" statement for "form" scope of variables declared at the Form level; however, the "Private" statement, like the "Public" statement, is illegal in declarations within a Procedure.
For backward compatibility, "Dim" can be used instead of "Private" at the Form level (bad practice). At the Procedure level, declarations must begin with "Dim" or "Static".
Q: What is an additional reason to use symbolic constants?
A: The textbook points out (page 184) that these constants make a program easier to read and understand. Here is another reason: suppose the tax rate is 5%, and the literal constant 0.05 is sprinkled through the procedures in a project. Now the legislature raises the tax rate to 6%. The maintenance programmer must find all instances of 0.05 and change them to 0.06 -- OOPS -- one of those 0.05 wasn't really the tax rate, it was the commission for the referring agent, and now we have two new bugs -- the place where the tax rate wasn't changed, and the place where the commission rate was changed wrongly. Better: we had the line
Const conTaxRate as Single = 0.05
and we changed it to:
Const conTaxRate as Single = 0.06
Q: What is the difference between the operator "+" and "&"?
A: They overlap: "&" can be used to concatenate strings; "+" can be used to concatenate strings or to add numbers. For clarity, use "+" only to add numbers.
Q: What is the distinction between modal and modeless in the context of describing a form?
A: A modal form demands that the user devote attention to it before using any other form or dialog box, by doing something that causes code to execute either hiding or unloading the form. A modeless form does not make this demand of the user.
Q: What is the distinction between a syntax error and a logic error?
A: A syntax error is a violation of the language Visual Basic can detect. A logic error is a flaw in translating the user's and programmer's intentions into Visual Basic; Visual Basic cannot detect a logic error. Here's an example of a syntax error in English:
The dog bited the mail carrier. (since "to bite" is a strong verb)
Here's an example of a logic error:
The mail carrier bit the dog.
Q: Page 250 says Visual Basic has 6 logical operators and describes 3. What are the other 3?
A: One is "Xor", meaning eXclusive OR. This operator is True if exactly one of its operands is True; otherwise, False.
Another is "Eqv", meaning EQuiValent. This operator is True if both of its operands are True or if both of its operands are False. If its operands have different Boolean values, Eqv returns False.
The last logical operator is "Imp", which stands for IMPlication. Imp returns True unless its first operand is True and its second operand False, in which case Imp returns False.
Q: What is the difference between sentence capitalization and book capitalization?
A: Sentence capitalization capitalizes only the first word and any other words always capitalized by fiat (for example, the names of the months are always capitalized in English). Book capitalization capitalizes the first word and all "important" words, except for short articles and prepositions. Examples:
Sentence capitalization:
This sentence was written in March.
Book capitalization:
This Title Was Written in April.
Q: What is the distinction between Load and Show?
A: Load brings a Form into memory, but does not make it visible. Show makes a Form visible, performing a preparatory Load if necessary. Unload is the exact antonym of Load. Hide is the exact antonym of Show.
Q: We skipped over pages 224-225 of the text about changing the mouse pointer. Since reference is made there to value 13, there must be many mouse pointer options. What are the others?
A: There are many; see the following table. Note that changing the mouse pointer does not, in and of itself, undertake actions such as permitting or preventing resizing. Changing the mouse pointer simply communicates to the user -- no small feat in itself.
Q: How does the "Switch" statement, which is not in our text, work?
A: "Switch" is an alternative to "If" with many "Else" clauses, or to a "Case" statement. It is less bulky than either, but does not provide Else functionality. As an example, here's Exercise 5, page 276, implemented with a Switch statement:
curCharge = Switch(intCode <= 0, 0, _ intCode <= 3, 25, _ intCode <= 6, 20, _ intCode <= 10, 15, _ intCode > 10, 10) Hence, Switch is "activated" by the first True condition it finds.
Q: How does the "ElseIf" keyword work?
A: This keyword works much like "Switch". A skeleton of code would look like:
If condition-1 thencode-block-1 ElseIf condition-2 thencode-block-2 ElseIf condition-3 thencode-block-3 Else code-block-4End If
"ElseIf" must be written as one word.
The "Else" block may be omitted, but must be the last block if it appears. The first block of code whose header condition is True will be executed. At most 1 block of code will be executed. If the "Else" block is present, exactly 1 block of code will be executed.
Q: What is the difference between a function and a subprocedure?
A: A function is invoked and returns a value. A typical function invocation looks like:
return_value = function_name(argument_one, argument_two, argument_three)
A subprocedure is called. A typical subprocedure call looks like:
Call subprocedure_name(argument_one, argument_two, argument_three)or
subprocedure_name argument_one, argument_two, argument_three
Ucase is a function which returns the upper-case version of its one argument, the character string sent to it. RandomNumbers (we wrote it on page 296) is a subprocedure taking zero arguments.
Q: What are some additional repetition-of-code capabilities in Visual Basic?
A: Visual Basic has 6 looping structures, 3 of which are covered in Tutorial 5, Lesson A. They are:
1. For/Next (pp. 340 ff.), to be used when the analyst knows, on loop entry, how many times the loop needs to be executed. The loop may be executed zero or more times.
2. Do While/Loop (pp. 344 ff.), to be used when the analyst does not know, on loop entry, how many times the loop needs to be executed, but wishes the loop to be executed zero or more times while a condition remains true.
3. Do/Loop Until (pp 344 ff.), to be used when the analyst does not know, on loop entry, how many times the loop needs to be executed, but wishes the loop to be executed one or more times while a condition remains false.
4. While/Wend, to be used when the analyst does not know, on loop entry, how many times the loop needs to be executed, but wishes the loop to be executed zero or more times while a condition remains true. This structure is a clone of Do While/Loop.
  It is provided for backward compatibility with earlier versions of Visual Basic(c). I recommend disdaining its use because (a) plans are to phase it out of the language, and (b) it cannot be exited "from the side" (see below).
5. Do/Loop While, to be used when the analyst does not know, on loop entry, how many times the loop needs to be executed, but wishes the loop to be executed one or more times while a condition remains true.
6. Do Until/Loop, to be used when the analyst does not know, on loop entry, how many times the loop needs to be executed, but wishes the loop to be executed zero or more times while a condition remains false
An Exit For statement may be placed inside a For/Next loop to provide premature exit "from the side" of the loop as the analyst deems necessary (e.g., if an unforeseen error such as faulty data or failure of convergence in a mathematical algorithm manifests itself). The Exit Do statement may be similarly used within the other looping structures, except for the While/Wend (#4 above).
Q: What if I want things to be done when a control loses the focus, instead of when it gains the focus?
A: Write code describing the things to be done within, not the GotFocus event procedure, but the LostFocus event procedure.
Q: What was the specific error and its effect relative to page 318 (and derivatively page 320) of the textbook?
A: The symbolic constant conBtns matches the "buttons" argument in the table on page 314. It must be declared As Integer, not As String. Indeed, it is defined as the sum of 4 integers, one from each of the groups shown in the table on page 315.
Q: What are some properties of control arrays?
A: The Count property retrieves the number of control array elements.
The LBound and UBound properties retrieve the lower and upper bounds, respectively, of indices pertinent to the array. By default, these indices are consecutive from zero to Count-1, but that is not required.
In Tutorial 5, lesson B, chkDone.Count = 5, chkDone.LBound = 0, and chkDone.UBound = 4.
Q: What is the difference between the functions Ucase and Ucase$?
A: Ucase returns a type Variant, presumably to be placed in a string, whereas Ucase$ returns a type String. Consider the lines of code
strMyString = Ucase(strMyString) versus
strMyString = Ucase$(strMyString)
The first line returns a Variant which is automatically converted to a String for assignment to strMyString, whereas the second returns a String for immediate assignment to strMyString. Therefore, the second (using the "$") is more efficient and recommended.
Q: Aside from twips (page 46 of text), what other coordinate systems are available?
A: There are 8 coordinate system scales accessible through the ScaleMode property of a Form. They are:
0 (vbUser) scale defined by the programmer
1 (vbTwips) twips
2 (vbPoints) points (a point is 1/72^nd inch)
3 (vbPixels) pixels (each pixel is one dot of screen resolution)
4 (vbCharacters) 120 twips horizontally, 240 twips vertically
5 (vbInches) physical inch
6 (vbMillimeters) physical millimeter
7 (vbCentimeters) physical centimeter
Pixels are a device-dependent unit of measurement, whereas twips are a device-independent unit of measurement.
Q: How do I make sure that, for example, only digits can be entered into a text box?
A: A text box has a KeyPress event which returns an integer KeyAscii specifying which key the user pressed. Every key the user presses has an ASCII code, integer-valued. For example, the backspace key has ASCII code 8. A built-in VB function Asc returns the ASCII code for any character. The following lines of code will ensure that only numbers are typed:
If (KeyAscii < Asc("0") Or KeyAscii > Asc("9")) And KeyAscii <> 8) Then KeyAscii = 0 ' setting to zero cancels the user's key press End if
This code would appear in the KeyPress event of the text box control.
Q: How do I put a double-quote mark into a character string, inasmuch as that character is used to delimit a string fore and aft?
A: Use the "Chr$" function, which is the inverse of the "Asc" function described in the immediately preceding Q&A. The double-quote mark has ASCII code integer 34. So, for example, the line
txtMyTextBox.text = "Test" & Chr$(34) & "String"will set the content of txtMyTextBox.text to the eleven-character string Test"String
Q: I saw code reading:
lblMyTextBox = strMyString
It actually seemed to work! How?
A: That line of code defaults to
lblMyTextBox.Caption = strMyString
inasmuch as the most frequently used property of a Text Box control is the Caption property. If the "." and the property name are omitted thus, the default is to the most commonly used property. Such omission is a horrible programming practice.
Q: Isn't there an easier way to ensure that a form appears centered on the screen than to do the fussing with co-ordinates at the time of the form's Load event?
A: Yes. In the Properties list for the form, change the StartUpPosition property from 3 ("Windows Default") to 2 ("CenterScreen"). This method works reliably in the default MDI (Multiple Document Interface). In the SDI (Single Document Interface), the reset value of the StartUpPosition property will be lost if the user subsequently moves or resizes the form window in design mode.
Q: How does the user toggle between MDI (Multiple Document Interface) and SDI (Single Document Interface)?
A: On the Tools menu, select Options; then select the Advanced tab. Check the "SDI" check box. The change will take effect next time Visual Basic is started.
Q: What are similarities and differences between Image controls and PictureBox controls?
A: The basic similarity: both support graphic image presentation. However, Image controls can stretch (recall our first lesson with the corporate logo); PictureBox controls can't. Image controls use less memory than do PictureBox controls. Both can be bound to fields in a database to support DDE (dynamic data exchange). Graphics can't be drawn (at execution time) on top of an Image, but they can be drawn on top of a PictureBox. The perhaps most significant difference: PictureBox, unlike Image, is a container object. For example, a set of command buttons can be placed physically on a PictureBox, and become contained in the PictureBox. If, at run time, the PictureBox Visible property is set to False, the command buttons, as well as the picture itself, disappear from the user's screen.
Q: Can controls be created at run time?
A: Certainly! For example, clicking on a command button can create new command buttons, each with different click-on actions. A common approach is to create the initial command button, at design time, as the zero-th element of an array (set its Index property, by default blank, to 0). Then write code for the click event as illustrated in this example (download and unzip).