Computer Programming Concepts and Visual Basic David I. Schneider

3. In Appendix D, the section “Stepping Through a Program Containing a Procedure:
Section 3” uses the Visual Basic debugger to trace the flow through a program
and observe the interplay between arguments and parameters.
4. You can use the Print method in a Form_Load event procedure. If so, you must
set the AutoRedraw property of the picture box to True. Otherwise the contents
of the picture box will be erased when the event procedure terminates.
3.3 FUNCTION PROCEDURES
Visual Basic has many built-in functions. In one respect, functions are like miniature programs.
They use input, they process the input, and they have output. Some functions we encountered
earlier are listed in Table 3.1.
TABLE 3.1
Some Visual Basic Built-In Functions
Function Example Input Output
Int Int(2.6) is 2 number number
Chr Chr(65) is “A” number string
Len Len(“perhaps”) is 7 string number
Mid Mid(“perhaps”,4,2) is “ha” string, string
number,
number
InStr InStr(“to be”,“ ”) is 3 string,string number
Although the input can involve several values, the output always consists of a single
value. The items inside the parentheses can be constants (as in Table 3.1), variables, or
expressions.
In addition to using built-in functions, we can define functions of our own. These new
functions, called Function procedures or user-defined functions, are defined in much the
same way as Sub procedures and are used in the same way as built-in functions. Like builtin
functions, Function procedures have a single output that can be string or numeric. Function
procedures can be used in expressions in exactly the same way as built-in functions.
Programs refer to them as if they were constants, variables, or expressions. Function procedures
are defined by function blocks of the form
Private Function FunctionName(var1 As Type1, var2 As Type2, ...) As dataType
statement(s)
FunctionName = expression
End Function
The variables in the top line are called parameters, and variables inside the function block that
are not parameters have local scope. Function names should be suggestive of the role performed
and must conform to the rules for naming variables. The type dataType, which specifies
the type of the output, will be one of String, Integer, Single, and so on. In the preceding
general code, the next-to-last line assigns the output, which must be of type dataType, to the
function name. Two examples of Function procedures are as follows:
Private Function FtoC(t As Single) As Single
‘Convert Fahrenheit temperature to Celsius
FtoC = (5 / 9) * (t - 32)
End Function
Private Function FirstName(nom As String) As String
Dim firstSpace As Integer
‘Extract the first name from the full name nom
firstSpace = InStr(nom, “ “)
Function Procedures 87