Visual Basic.NET How to Program (PDF)

114 Control Structures: Part 1 Chapter 4
Lines 7–10 declare variables total, gradeCounter, and grade to be of type
Integer and average to be of type Double. In this example, total accumulates the
sum of the grades entered, and gradeCounter counts the number of grades entered.
Variable grade stores the value entered (line 21).
Good Programming Practice 4.6
Always place a blank line between declarations and executable statements. This makes the
declarations stand out in a program and contributes to program readability. 4.6
Notice from the output that although each grade entered is an integer, the averaging
calculation is likely to produce a number with a decimal point (i.e., a floating-point
number). The type Integer cannot represent floating-point numbers, so this program
uses data type Double, which stores double-precision floating-point numbers. Visual
Basic also provides data type Single for storing single-precision floating-point numbers.
Data type Double requires more memory to store a floating-point value, but is more accurate
than type Single. Type Single is useful in applications that need to conserve
memory and do not require the accuracy provided by type Double.
Lines 13–14 initialize total to 0 and gradeCounter to 1. Line 17 indicates that
the While structure should iterate while the value of gradeCounter is less than or
equal to 10. Lines 20–21 correspond to the pseudocode statement “Input the next grade.”
The statement on line 20 displays the prompt Enter integer grade: in the command
window. The second statement (line 21) reads the value entered by the user, and stores that
value in the variable grade.
Next, the program updates the total with the new grade entered by the user—line
23 adds grade to the previous value of total and assigns the result to total—using
the += assignment operator. Variable gradeCounter is incremented (line 25) to indicate
that a grade has been processed. Line 25 adds 1 to gradeCounter, so the condition in
the While structure eventually becomes false, terminating the loop. Line 29 assigns the
results of the average calculation to variable average. Line 32 writes a blank line to
enhance the appearance of the output. Line 33 displays a message containing the string
"Class average is " followed by the value of variable average.
4.12 Formulating Algorithms with Top-Down, Stepwise
Refinement: Case Study 2 (Sentinel-Controlled Repetition)
Let us generalize the class-average problem. Consider the following problem:
Develop a class-averaging program that averages an arbitrary number of grades each time
the program is run.
In the first class-average example, the number of grades (10) was known in advance. In this
example, no indication is given of how many grades are to be input. The program must process
an arbitrary number of grades. How can the program determine when to stop the input
of grades? How will it know when to calculate and print the class average?
One way to solve this problem is to use a special value called a sentinel value (also
called a signal value, a dummy value or a flag value) to indicate “end of data entry.” The
user inputs all grades and then types the sentinel value to indicate that the last grade has
been entered. Sentinel-controlled repetition is called indefinite repetition because the
number of repetitions is not known before the loop begins its execution.