Computer Programming Concepts and Visual Basic David I. Schneider

preceded with the word ByVal. Built-in functions have all their arguments
passed by value. Some programmers feel that “by value” should have been the
default for Function procedures, rather than “by reference.”
2. Function procedures can perform the same tasks as Sub procedures. For
instance, they can request input and display text; however, they are primarily
used to calculate a single value. Normally, Sub procedures are used to carry out
other tasks.
3. Function procedures differ from Sub procedures in the way they are accessed.
Sub procedures are invoked with Call statements, whereas functions are
invoked by placing them where you would otherwise expect to find a constant,
variable, or expression. Unlike a Function procedure, a Sub procedure can’t be
used in an expression.
4. Function procedures can invoke other Function procedures or Sub procedures.
5. Function procedures, like Sub procedures need not have any parameters. Unlike
Sub procedures, when a parameterless function is used, the function name may
be followed by an empty set of parentheses. The following program uses a
“parameterless” function.
Private Sub cmdButton_Click()
‘Request and display a saying
picBox.Cls
picBox.Print Saying() ‘() is optional
End Sub
Private Function Saying() As String
‘Retrieve a saying from the user
Saying = InputBox(“What is your favorite saying?”)
End Function
[Run, click the command button, and then type Less is more. into the message
box.]
The saying Less is more. is displayed in the picture box.
6. An alternative method of creating a Function procedure is to move the cursor to
a blank line outside of any procedure, type Private Function FunctionName, and
press the Enter key.
3.4 MODULAR DESIGN
■ TOP-DOWN DESIGN
Large problems usually require large programs. One method programmers use to make a large
problem more understandable is to divide it into smaller, less complex subproblems. Repeatedly
using a “divide-and-conquer” approach to break up a large problem into smaller subproblems
is called stepwise refinement. Stepwise refinement is part of a larger methodology of writing
programs known as top-down design. The term top-down refers to the fact that the more
general tasks occur near the top of the design and tasks representing their refinement occur
below. Top-down design and structured programming emerged as techniques to enhance programming
productivity. Their use leads to programs that are easier to read and maintain. They
also produce programs containing fewer initial errors, with these errors being easier to find
and correct. When such programs are later modified, there is a much smaller likelihood of introducing
new errors.
The goal of top-down design is to break a problem into individual tasks, or modules, that
can easily be transcribed into pseudocode, flowcharts, or a program. First, a problem is
restated as several simpler problems depicted as modules. Any modules that remain too complex
are broken down further. The process of refining modules continues until the smallest
Modular Design 93