4.7. Sentinel-Controlled Repetition

Let us generalize the class average problem. Consider the following problem:

Develop a class average program that processes grades for an arbitrary number of students each time it is run.

In the previous class average example, the problem statement specified the number of students, so the number of grades (10) was known in advance. In this example, no indication is given of how many grades the user will enter during the program's execution. 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 types grades in until all legitimate grades have been entered. The user then types the sentinel value to indicate that the last grade has been entered.

Clearly, the sentinel value must be chosen so that it cannot be confused with an acceptable input value. Grades on a quiz are normally nonnegative integers, so –1 is an acceptable sentinel value for this problem. Thus, 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. Since –1 is the sentinel value, it should not enter into the averaging calculation.

Implementing Sentinel-Controlled Repetition in Class GradeBook

Figures 4.9 and 4.10 show the C++ class GradeBook containing member function determineClassAverage that implements the class average algorithm with sentinel-controlled repetition. Although each grade entered is an integer, the averaging calculation is likely to produce a number with a decimal point. The type int cannot represent such a number, so this class must use another type to do so. C++ provides several data types for storing floating-point numbers, including float and double. The primary difference between these types is that, compared to float variables, double variables can typically store numbers with larger magnitude and finer detail (i.e., more digits to the right of the decimal point—also known as the number's precision). This program introduces a special operator called a cast operator to force the averaging calculation to produce a floating-point numeric result. These features are explained in detail as we discuss the program.

 1   // Fig. 4.9: GradeBook.h
 2   // Definition of class GradeBook that determines a class average.
 3   // Member functions are defined in GradeBook.cpp
 4   #include <string> // program uses C++ standard string class
 5   using std::string;
 6
 7   // GradeBook class definition
 8   class GradeBook
 9   {
10   public:
11      GradeBook( string ); // constructor initializes course name
12      void setCourseName( string ); // function to set the course name
13      string getCourseName(); // function to retrieve the course name
14      void displayMessage(); // display a welcome message
15      void determineClassAverage(); // averages grades entered by the user
16   private:
17      string courseName; // course name for this GradeBook
18   }; // end class GradeBook

					  

 1   // Fig. 4.10: GradeBook.cpp
 2   // Member-function definitions for class GradeBook that solves the
 3   // class average program with sentinel-controlled repetition.
 4   #include <iostream>
 5   using std::cout;
 6   using std::cin;
 7   using std::endl;
 8   using std::fixed; // ensures that decimal point is displayed
 9
10   #include <iomanip> // parameterized stream manipulators  
11   using std::setprecision; // sets numeric output precision
12
13   // include definition of class GradeBook from GradeBook.h
14   #include "GradeBook.h"
15
16   // constructor initializes courseName with string supplied as argument
17   GradeBook::GradeBook( string name )
18   {
19      setCourseName( name ); // validate and store courseName
20   } // end GradeBook constructor
21
22   // function to set the course name;
23   // ensures that the course name has at most 25 characters
24   void GradeBook::setCourseName( string name )
25   {
26      if ( name.length() <= 25 ) // if name has 25 or fewer characters
27         courseName = name; // store the course name in the object
28      else // if name is longer than 25 characters
29      { // set courseName to first 25 characters of parameter name
30         courseName = name.substr( 0, 25 ); // select first 25 characters
31         cout << "Name \"" << name << "\" exceeds maximum length (25).\n"
32            << "Limiting courseName to first 25 characters.\n" << endl;
33      } // end if...else
34   } // end function setCourseName
35
36   // function to retrieve the course name
37   string GradeBook::getCourseName()
38   {
39      return courseName;
40   } // end function getCourseName
41
42   // display a welcome message to the GradeBook user
43   void GradeBook::displayMessage()
44   {
45      cout << "Welcome to the grade book for\n" << getCourseName() << "!\n"
46         << endl;
47   } // end function displayMessage
48
49   // determine class average based on 10 grades entered by user
50   void GradeBook::determineClassAverage()
51   {
52      int total; // sum of grades entered by user
53      int gradeCounter; // number of grades entered
54      int grade; // grade value
55      double average; // number with decimal point for average
56
57      // initialization phase
58      total = 0; // initialize total
59      gradeCounter = 0; // initialize loop counter
60
61      // processing phase
62      // prompt for input and read grade from user  
63      cout << "Enter grade or -1 to quit: ";        
64      cin >> grade; // input grade or sentinel value
65
66      // loop until sentinel value read from user   
67      while ( grade != -1 ) // while grade is not -1
68      {
69         total = total + grade; // add grade to total
70         gradeCounter = gradeCounter + 1; // increment counter
71
72         // prompt for input and read next grade from user
73         cout << "Enter grade or -1 to quit: ";           
74         cin >> grade; // input grade or sentinel value   
75      } // end while
76
77      // termination phase
78      if ( gradeCounter != 0 ) // if user entered at least one grade...
79      {
80         // calculate average of all grades entered              
81         average = static_cast< double >( total ) / gradeCounter;
82
83         // display total and average (with two digits of precision)
84         cout << "\nTotal of all " << gradeCounter << " grades entered is "
85            << total << endl;
86         cout << "Class average is " << setprecision( 2 ) << fixed << average
87            << endl;
88      } // end if
89      else // no grades were entered, so output appropriate message
90         cout << "No grades were entered" << endl;
91   } // end function determineClassAverage

					  

In this example, we see that control statements can be stacked. The while statement (lines 67–75 of Fig. 4.10) is immediately followed by an if...else statement (lines 78–90) in sequence. Much of the code in this program is identical to the code in Fig. 4.7, so we concentrate on the new features and issues.

Line 55 (Fig. 4.10) declares the double variable average. Recall that we used an int variable in the preceding example to store the class average. Using type double in the current example allows us to store the class average calculation's result as a floating-point number. Line 59 initializes the variable gradeCounter to 0, because no grades have been entered yet. Remember that this program uses sentinel-controlled repetition. To keep an accurate record of the number of grades entered, the program increments variable gradeCounter only when the user enters a valid grade value (i.e., not the sentinel value) and the program completes the processing of the grade. Finally, notice that both input statements (lines 64 and 74) are preceded by an output statement that prompts the user for input.

Good Programming Practice 4.2

Prompt the user for each keyboard input. The prompt should indicate the form of the input and any special input values. For example, in a sentinel-controlled loop, the prompts requesting data entry should explicitly remind the user what the sentinel value is.

Floating-Point Number Precision and Memory Requirements

Variables of type float represent single-precision floating-point numbers and have seven significant digits on most 32-bit systems. Variables of type double represent double-precision floating-point numbers. These require twice as much memory as floats and provide 15 significant digits on most 32-bit systems—approximately double the precision of floats. For the range of values required by most programs, float variables should suffice, but you can use double to "play it safe." In some programs, even variables of type double will be inadequate—such programs are beyond the scope of this book. Most programmers represent floating-point numbers with type double. In fact, C++ treats all floating-point numbers you type in a program's source code (such as 7.33 and 0.0975) as double values by default. Such values in the source code are known as floating-point constants. See Appendix C, Fundamental Types, for the ranges of values for floats and doubles.

Converting Between Fundamental Types Explicitly and Implicitly

The variable average is declared to be of type double (line 55 of Fig. 4.10) to capture the fractional result of our calculation. However, total and gradeCounter are both integer variables. Recall that dividing two integers results in integer division, in which any fractional part of the calculation is lost (i.e., truncated). In the following statement:

average = total / gradeCounter;

the division calculation is performed first, so the fractional part of the result is lost before it is assigned to average. To perform a floating-point calculation with integer values, we must create temporary values that are floating-point numbers for the calculation. C++ provides the unary cast operator to accomplish this task. Line 81 uses the cast operator static_cast< double >( total ) to create a temporary floating-point copy of its operand in parentheses—total. Using a cast operator in this manner is called explicit conversion. The value stored in total is still an integer.

The calculation now consists of a floating-point value (the temporary double version of total) divided by the integer gradeCounter. The C++ compiler knows how to evaluate only expressions in which the data types of the operands are identical. To ensure that the operands are of the same type, the compiler performs an operation called promotion (also called implicit conversion) on selected operands. For example, in an expression containing values of data types int and double, C++ promotes int operands to double values. In our example, we are treating total as a double (by using the unary cast operator), so the compiler promotes gradeCounter to double, allowing the calculation to be performed—the result of the floating-point division is assigned to average. In Chapter 6, Functions and an Introduction to Recursion, we discuss all the fundamental data types and their order of promotion.

Common Programming Error 4.7

The cast operator can be used to convert between fundamental numeric types, such as int and double, and between related class types (as we discuss in Chapter 13, Object-Oriented Programming: Polymorphism). Casting to the wrong type may cause compilation errors or runtime errors.

Common Programming Error 4.8

An attempt to divide by zero normally causes a fatal runtime error.

Error-Prevention Tip 4.2

When performing division by an expression whose value could be zero, explicitly test for this possibility and handle it appropriately in your program (such as by printing an error message) rather than allowing the fatal error to occur.

Cast operators are available for use with every data type and with class types as well. The static_cast operator is formed by following keyword static_cast with angle brackets (< and >) around a data-type name. The cast operator is a unary operator—an operator that takes only one operand. In Chapter 2, we studied the binary arithmetic operators. C++ also supports unary versions of the plus (+) and minus (-) operators, so that you can write such expressions as -7 or +5. Cast operators have higher precedence than other unary operators, such as unary + and unary -. This precedence is higher than that of the multiplicative operators *, / and %, and lower than that of parentheses. We indicate the cast operator with the notation static_cast< type >() in our precedence charts (see, for example, Fig. 4.18).

Formatting for Floating-Point Numbers

The formatting capabilities in Fig. 4.10 are discussed here briefly and explained in depth in Chapter 15, Stream Input/Output. The call to setprecision in line 86 (with an argument of 2) indicates that double variable average should be printed with two digits of precision to the right of the decimal point (e.g., 92.37). This call is referred to as a parameterized stream manipulator (because of the 2 in parentheses). Programs that use these calls must contain the preprocessor directive (line 10)

#include <iomanip>

Line 11 specifies the name from the <iomanip> header file that is used in this program. Note that endl is a nonparameterized stream manipulator (because it is not followed by a value or expression in parentheses) and does not require the <iomanip> header file. If the precision is not specified, floating-point values are normally output with six digits of precision (i.e., the default precision on most 32-bit systems today), although we'll see an exception to this in a moment.

The stream manipulator fixed (line 86) indicates that floating-point values should be output in so-called fixed-point format, as opposed to scientific notation. Scientific notation is a way of displaying a number as a floating-point number between the values of 1.0 and 10.0, multiplied by a power of 10. For instance, the value 3,100.0 would be displayed in scientific notation as 3.1 x 10³. Scientific notation is useful when displaying values that are very large or very small. Formatting using scientific notation is discussed further in Chapter 15. Fixed-point formatting, on the other hand, is used to force a floating-point number to display a specific number of digits. Specifying fixed-point formatting also forces the decimal point and trailing zeros to print, even if the value is a whole number amount, such as 88.00. Without the fixed-point formatting option, such a value prints in C++ as 88 without the trailing zeros and without the decimal point. When the stream manipulators fixed and setprecision are used in a program, the printed value is rounded to the number of decimal positions indicated by the value passed to setprecision (e.g., the value 2 in line 86), although the value in memory remains unaltered. For example, the values 87.946 and 67.543 are output as 87.95 and 67.54, respectively. Note that it also is possible to force a decimal point to appear by using stream manipulator showpoint. If showpoint is specified without fixed, then trailing zeros will not print. Like endl, stream manipulators fixed and showpoint are nonparameterized and do not require the <iomanip> header file. Both can be found in header <iostream>.

Lines 86 and 87 of Fig. 4.10 output the class average. In this example, we display the class average rounded to the nearest hundredth and output it with exactly two digits to the right of the decimal point. The parameterized stream manipulator (line 86) indicates that variable average's value should be displayed with two digits of precision to the right of the decimal point—indicated by setprecision( 2 ). The three grades entered during the sample execution of the program in Fig. 4.11 total 257, which yields the average 85.666666.... The parameterized stream manipulator setprecision causes the value to be rounded to the specified number of digits. In this program, the average is rounded to the hundredths position and displayed as 85.67.

 1   // Fig. 4.11: fig04_14.cpp
 2   // Create GradeBook object and invoke its determineClassAverage function.
 3
 4   // include definition of class GradeBook from GradeBook.h
 5   #include "GradeBook.h"
 6
 7   int main()
 8   {
 9      // create GradeBook object myGradeBook and
10      // pass course name to constructor
11      GradeBook myGradeBook( "CS101 C++ Programming" );
12
13      myGradeBook.displayMessage(); // display welcome message
14      myGradeBook.determineClassAverage(); // find average of 10 grades
15      return 0; // indicate successful termination
16   } // end main

					  

Welcome to the grade book for
CS101 C++ Programming

Enter grade or -1 to quit: 97
Enter grade or -1 to quit: 88
Enter grade or -1 to quit: 72
Enter grade or -1 to quit: -1

Total of all 3 grades entered is 257
Class average is 85.67