Visual Basic.NET How to Program (PDF)

Chapter 4 Control Structures: Part 1 115
It is crucial to employ a sentinel value that cannot be confused with an acceptable input
value. Grades on a quiz are normally nonnegative integers, thus –1 is an acceptable sentinel
value for this problem. A run of the class-average program might process a stream of inputs
such as 95, 96, 75, 74, 89 and –1. The program would then compute and print the class
average for the grades 95, 96, 75, 74 and 89. The sentinel value, –1, should not enter into
the averaging calculation.
Common Programming Error 4.5
Choosing a sentinel value that is also a legitimate data value could result in a logic error
that would cause a program to produce incorrect results. 4.5
When solving more complex problems, such as that contained in this example, the
pseudocode representation might not appear obvious. For this reason we approach the
class-average program with top-down, stepwise refinement, a technique for developing
well-structured algorithms. We begin with a pseudocode representation of the top:
Determine the class average for the quiz
The top is a single statement that conveys the overall function of the program. As such, the
top is a complete representation of a program. Unfortunately, the top rarely conveys a sufficient
amount of detail from which to write the Visual Basic algorithm. Therefore, we conduct
the refinement process. This involves dividing the top into a series of smaller tasks that are
listed in the order in which they must be performed resulting in the following first refinement:
Initialize variables
Input, sum and count the quiz grades
Calculate and print the class average
Here, only the sequence structure has been used—the steps listed are to be executed in order,
one after the other.
Software Engineering Observation 4.2
Each refinement, including the top, is a complete specification of the algorithm; only the level
of detail in each refinement varies. 4.2
To proceed to the next level of refinement (i.e., the second refinement), we commit to
specific variables. We need a running total of the numbers, a count of how many numbers
have been processed, a variable to receive the value of each grade and a variable to hold the
calculated average. The pseudocode statement
Initialize variables
can be refined as follows:
Initialize total to zero
Initialize counter to zero
Notice that only the variables total and counter are initialized before they are used; the variables
average and grade (the program in Fig. 4.16 uses these variables for the calculated
average and the user input, respectively) need not be initialized because the assignment of
their values does not depend on their previous values, as is the case for total and counter.
The pseudocode statement
Input, sum and count the quiz grades