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Tutorial 10

The Repetition Structure

 The instructor’s notes for this tutorial will consist of the following:

Overview

This tutorial covers the repetition structure. Included is the proper design of a repetition structure using flowcharts and/or pseudocode, as well as the actual implementation of the repetition structure in C++ using the while statement. As with the selection structure, nesting repetition structures is also covered. Integrated with the repetition structure is the concept of initializing and updating counters and accumulators.

Objectives

After completing the tutorial, the student will be able to:

Lecture Notes

Introduction to the repetition structure

The repetition structure, also called a loop, is one of the three basic control structures used in programming. The other two control structures are sequence and the selection structure, which have been covered in previous tutorials. The repetition structure is used when you would like the program to repeatedly process one or more program instructions until some condition is met, at which time the repetition structure ends.

While the concept of a repetition structure is quite simple, it is important to properly design the repetition structure. Both flowcharts and pseudocode have been covered previously (review if necessary), and you will remember that the diamond symbol represents a decision and is called the selection/repetition symbol. (This symbol was introduced with the selection structure in the previous tutorials). Each diamond has one flowline entering the symbol, and two flowlines leaving the symbol. The two flowlines leaving the symbol represent the true and false paths and should be marked accordingly. See page 343 for an example of a flowchart containing a repetition structure.

Even though flowcharts are commonly used in representing repetition structures, many programmers still wish to use pseudocode. You should always use whichever design method is most effective for you. Many programmers will use a combination of flowcharts and pseudocode. Different programming structures may lend themselves to being designed in either a flowchart or pseudocode. See page 342 for an example of a repetition structure in pseudocode.

Components of the repetition structure

A repetition structure can either be a pretest loop or posttest loop. In a pretest loop, the loop condition is evaluated before the instructions within the loop are processed, while in a posttest loop, the evaluation occurs after the instructions within the loop are processed. Of the two types of loops, the pretest loop is the most commonly used. You will cover the pretest loop in this tutorial. The posttest loop will be covered in Appendix C.

With very rare exceptions, every loop has a loop condition and a loop body. The loop condition appears at the beginning of a pretest loop and determines the number of times the instructions within the loop, referred to as the loop body, will be processed. Similar to a selection structure condition, a loop condition must result in either a true or false value (a boolean value). When the loop condition evaluates to true, the one or more instructions listed in the loop body are processed. If the loop condition evaluates to false, the instructions listed in the loop body are skipped over and control is passed to the next statement after the loop.

Because the loop condition in a pretest loop is evaluated before any of the instructions within the loop are processed, it is possible that the loop body instructions may not be processed. This would be true if the loop condition is initially evaluated to false. Unless the loop has been properly primed (an initial value read to test the loop condition), more than likely this will be the case.

The input instruction(s) that appears just previous to the pretest loop is referred to as the priming read, because it is used to prime (prepare) the loop. The priming read gets only the first value(s) from the user. The input instruction(s) that appear within the loop get the remaining values. Some loops require the user to enter a special value, called a sentinel value, to end the loop. You should use a sentinel value that is easily distinguishable from the valid data the user will enter. Note that other loops may be terminated by the program itself, through the use of a counter.

Counters and accumulators are used within a repetition structure to calculate subtotals, totals, and averages. All counters and accumulators must be initialized and updated. Counters are updated by a constant value, whereas accumulators are updated by an amount that varies.

Coding the repetition structure

Most programming languages offer two forms of the repetition structure: while and do-while. While this tutorial will cover only the while form of the selection structure, the do-while form of the selection structure will be covered in Appendix C. The while statement is used to code the repetition structure in C++. The syntax of the C++ while statement is listed below:

while (loop condition)

one statement, or block of statements enclosed in braces,

to be processed as long as the loop condition is true

//end while

Notice following the keyword while, the loop condition is enclosed in parentheses. This is required in C++, unlike some other languages. Immediately following is the body of the loop, which is the one statement, or block of statements in braces, to be processed as long as the loop condition is true. As with the selection and decision structures, the //end while statement is not mandatory. However, it is very good programming practice and should be coded to aid in the readability and maintainability of the program.

See Figure 10-10 on page 346 which codes a C++ program, including a while statement, from an algorithm in an IPO chart.

Counters and accumulators

Counters and accumulators are used within a repetition structure to calculate subtotals, totals, and averages. A counter is a numeric variable used for counting something, while an accumulator is a numeric variable used for accumulating (adding together) something.

Two tasks are associated with counters and accumulators, initializing and updating. Initializing means to assign a beginning value to the counter or accumulator. Although the beginning value usually is zero, counters and accumulators can be initialized to any number (dependent upon your algorithm). The initialization is usually performed outside the loop body in a program, because it needs to be done only once.

Updating, also called incrementing, means adding a number to the value stored in the counter or accumulator (the number can be positive or negative). A counter is always incremented by a constant value, where an accumulator is incremented by a value that varies. The assignment statement that updates a counter or an accumulator is placed within the loop body in a program, because the update task must be performed each time the loop body instructions are processed.

See the example in Figure 10-11 on page 348 for an example program using counters and accumulators.

Infinite loops

An infinite loop (or endless loop) is a loop that processes its instructions indefinitely. When dealing with the repetition structure, one reason infinite loops occur is that the loop condition continues to be true. Remember that as long as the loop condition is true, the loop body is continually processed. Usually at the end of the loop body a variable is updated that will change the outcome of the loop condition. If this variable is not updated (the statement(s) are left out of the program), then the loop condition will not change. Therefore the loop will continue to be processed indefinitely.

Usually, you can stop a program that contains an infinite loop by pressing Ctrl-C. You can also use the DOS window’s close button.

Counter-controlled pretest loops

As mentioned previously, some loops are terminated by the program itself, through the use of a counter. In other words we can use a counter to count the number of times a loop is processed. For example, if you would like to print your name to the screen ten times, you could begin by initializing a loop counting variable to one. Your loop condition is this case would be to check to see when this loop counting variable would be greater than ten. The loop body would consist of printing your name, followed by incrementing the loop counting variable by one. The loop would continue until your name was printed the tenth time, and the loop counting variable being incremented to eleven. At this point the loop would terminate.

Another name for creating a loop that processes a set of code (loop body) a set amount of times is called iteration.

Nesting loops

You can nest repetition structures similar to the way you can nest selection structures. For repetition structures to be nested and work correctly, the entire inner loop must be contained within the outer loop. Remember from the discussion about nesting selection structures, to follow proper indentation rules, as well as being consistent with your programming style. Coding comments at the end of each loop can also make the code very readable and easy to understand.

Discussion Topics/Additional Activities

Solutions to Questions

1. c a pretest 9. d while (age >= 0)

2. a a diamond 10. a & b sequence and selection

3. d counter 11. a & c sequence and repetition

4. c updated 12. a & b sequence and selection

5. b Counters, accumulators 13. a, b, & c sequence, selection, & repetition

6. d numEmployees = numEmployees +1; 14. c sentinel

7. c total = total + sales; 15. c priming

8. d all of the above

Solutions to Concept Lesson Exercises

Note that the answers to exercises 5, 6, 13-16,and 18 may vary

 

1.

//T10ConE01.cpp - displays the number entered by the user

#include <iostream>

using namespace std;

int main()

{

//declare variable

int quantity = 0;

cout << "Type a number and press the Enter key. (Enter a negative number to stop.): ";

cin >> quantity;

//perform loop

while (quantity >= 0)

{

cout << "You typed the number " << quantity << endl;

cout << "Type a number and press the Enter key. (Enter a negative number to stop.): ";

cin >> quantity;

}//end while

cout << "Loop stopped because you typed the number " << quantity << endl;

return 0;

} //end of main function

 

 2.

//T10ConE02.cpp - displays the character entered by the user

#include <iostream>

using namespace std;

int main()

{

//declare variable

char more = ' ';

cout << "Type a character and press the Enter key. (Enter N or n to stop.): ";

cin >> more;

//perform loop

while (more != 'N' && more != 'n')

{

cout << "You typed the character " << more << endl;

cout << "Type a character and press the Enter key. (Enter N or n to stop.): ";

cin >> more;

}//end while

cout << "Loop stopped because you typed the character " << more << endl;

return 0;

} //end of main function

 

 

3.

//T10ConE03.cpp - displays the number entered by the user

#include <iostream>

using namespace std;

int main()

{

//declare variable

int number = 0;

cout << "Type a number and press the Enter key. (Enter a negative number or 0 to stop.): ";

cin >> number;

//perform loop

while (number > 0)

{

cout << "You typed the number " << number << endl;

cout << "Type a number and press the Enter key. (Enter a negative number or 0 to stop.): ";

cin >> number;

}//end while

cout << "Loop stopped because you typed the number " << number << endl;

return 0;

} //end of main function

 4.

//T10ConE04.cpp - displays the total sales

#include <iostream>

using namespace std;

int main()

{

//declare variables

char anotherSale = ' ';

int salesAmount = 0;

int totalSales = 0;

cout << "Do you want to enter a sales amount (Y or N)? ";

cin >> anotherSale;

//perform loop

while (anotherSale == 'Y' || anotherSale == 'y')

{

cout << "Enter the sales amount: ";

cin >> salesAmount;

totalSales = totalSales + salesAmount;

cout << "Do you want to enter another sales amount (Y or N)? ";

cin >> anotherSale;

}//end while

cout << "Total sales: " << totalSales << endl;

return 0;

} //end of main function

 

  

5.

//T10ConE05.cpp - displays Hello 10 times

#include <iostream>

using namespace std;

int main()

{

//declare variable

int count = 1;

//perform loop

while (count <= 10)

{

cout << "Hello" << endl;

count = count + 1;

}//end while

return 0;

} //end of main function

 

 

6.

//T10ConE06.cpp - displays a series of even numbers

#include <iostream>

using namespace std;

int main()

{

//declare variable

int count = 10;

//perform loop

while (count < 100)

{

cout << count * 2 << endl;

count = count + 10;

}//end while

return 0;

} //end of main function

 

 

7. The following numbers will appear on the screen when the code is processed: 0, 1, 2, 3, and 4. The temp

variable will contain the number 5 when the loop stops.

temp

0

1

2

3


4

5

 

8. The following numbers will appear on the screen when the code is processed: 0, 2, and 4. The totEmp variable

will contain the number 6 when the loop stops.

totEmp

0

2


4

6

9. numStudents = numStudents + 1;

 

10. quantity = quantity + -5;

(or)

quantity = quantity – 5;

11. total = total + sales;

12. total = total + gross;

13.

//T10ConE13.cpp - displays a pattern of asterisks

#include <iostream>

using namespace std;

int main()

{

//declare variables

int outer = 0;

int inner = 0;

//perform loops

outer = 2;

while (outer <= 12)

{

inner = 1;

while (inner <= outer)

{

cout << "*";

inner = inner + 1;

}//end while

outer = outer + 2;

cout << endl;

}//end while

return 0;

} //end of main function

14.

//T10ConE14.cpp - displays a pattern of asterisks

#include <iostream>

using namespace std;

int main()

{

//declare variables

int outer = 0;

int inner = 0;

//perform loops

outer = 9;

while (outer > 0)

{

inner = 1;

while (inner <= outer)

{

cout << "*";

inner = inner + 1;

}//end while

outer = outer - 1;

cout << endl;

}//end while

return 0;

} //end of main function

15.

//T10ConE15.cpp - displays a pattern of asterisks

#include <iostream>

using namespace std;

int main()

{

//declare variables

int outerEnd = 0;

int outerInc = 0;

int outer = 0;

int inner = 0;

//get maximum number of asterisks and increment value

cout << "Maximum number of asterisks: ";

cin >> outerEnd;

cout << "Increment value: ";

cin >> outerInc;

//perform loops

outer = outerInc;

while (outer <= outerEnd)

{

inner = 1;

while (inner <= outer)

{

cout << "*";

inner = inner + 1;

}//end while

outer = outer + outerInc;

cout << endl;

}//end while

return 0;

} //end of main function

 

 

16.

//T10ConE16.cpp - displays a pattern of asterisks

#include <iostream>

using namespace std;

int main()

{

//declare variables

int outer = 0;

int inner = 0;

int startWith = 0;

cout << "Start with 9 asterisks or 1 asterisk? ";

cin >> startWith;

if (startWith == 9)

{

outer = 9;

//perform loops

while (outer > 0)

{

inner = 1;

while (inner <= outer)

{

cout << "*";

inner = inner + 1;

}//end while

outer = outer - 1;

cout << endl;

inner = 1;

}//end while

}

else

{

outer = 1;

//perform loops

while (outer <= 9)

{

inner = 1;

while (inner <= outer)

{

cout << "*";

inner = inner + 1;

}//end while

outer = outer + 1;

cout << endl;

inner = 1;

}//end while

}

return 0;

} //end of main function

 

 

17. In this exercise, the student learns how to stop an endless (infinite) loop. The student does not make any

changes to the original T10ConE17.cpp file.

18.

//T10ConE18.cpp - displays a pattern of asterisks

#include <iostream>

using namespace std;

int main()

{

//declare variables

int outer = 0;

int inner = 0;

char process = 'Y';

//perform loops

while (process == 'Y' || process == 'y')

{

outer = 2;

while (outer <= 12)

{

inner = 1;

while (inner <= outer)

{

cout << "*";

inner = inner + 1;

}//end while

outer = outer + 2;

cout << endl;

}//end while

cout << "Process again? (Y or N) ";

cin >> process;

}//end while

return 0;

} //end of main function

19. Changes made to the code are shaded.

int num = 1;

while (num < 5)

{

cout << num << endl;

num = num + 1;

} //end while

 

20. Changes made to the code are shaded.

float sales = 0.0;

cout << "Enter a sales amount: ";

cin >> sales;

while (sales > 0)

{

cout << "Commission: " << sales * .1 << endl;

cout << "Enter a sales amount: ";

cin >> sales;

}//end while

Solutions to Application Lesson Exercises

 

1.

//T10AppE01.cpp - displays the grade for one or more students

#include <iostream>

using namespace std;

//function prototypes

int getPointsEarned();

char assignGrade(int);

int main()

{

//declare variables

int totalEarned = 0;

char grade = ' ';

char another = 'Y';

while (another == 'Y' || another == 'y')

{

//get total points earned

totalEarned = getPointsEarned();

//assign grade

grade = assignGrade(totalEarned);

//display grade

cout << "Grade: " << grade << endl;

cout << "Display another student's grade? ";

cin >> another;

}//end while

return 0;

} //end of main function

//*****program-defined functions*****

int getPointsEarned()

{

//gets and accumulates the scores, then returns the total

int score = 0;

int total = 0;

cout << "Enter the first score: ";

cin >> score;

while (score >= 0)

{

total = total + score;

cout << "Enter the next score: ";

cin >> score;

}//end while

return total;

} //end of getPointsEarned function

 

 

 

 

 

 

char assignGrade(int points)

{

//assigns the letter grade

char letterGrade = ' ';

if (points >= 360)

letterGrade = 'A';

else if (points >= 320)

letterGrade = 'B';

else if (points >= 280)

letterGrade = 'C';

else if (points >= 240)

letterGrade = 'D';

else letterGrade = 'F';

//end ifs

return letterGrade;

} //end of assignGrade function

 2.

IPO Chart:

Input

Processing

Output

sales

 

Processing items:

counter

Algorithm:

  1. assign the number 1 to the counter
  2. repeat while (the counter is less than or equal to 4)
  3. enter the sales

    add the sales to the total sales

    add 1 to the counter

    end repeat while

  4. display the total sales

total sales

 

Desk-check table:

sales

count

total

0


2000

3000

2500

1500

0

39000

45000

25000

56000

0

1

2

3


4

5

0

1

2

3


4

5

0

2000

5000

7500

9000

0

39000

84000

109000

165000

//T10AppE02.cpp - displays the total monthly sales

#include <iostream>

using namespace std;

int main()

{

//declare variables

int sales = 0;

int count = 0;

int total = 0;

//set counter to 1

count = 1;

//get and accumulate sales

while (count <= 4)

{

cout << "Enter sales: ";

cin >> sales;

total = total + sales;

count = count + 1;

}//end while

//display total sales

cout << "Total sales: $" << total << endl;

return 0;

} //end of main function

3.

//T10AppE03.cpp - displays the total monthly and quarterly sales

#include <iostream>

using namespace std;

int main()

{

//declare variables

int sales = 0;

int regionCount = 0;

int monthCount = 0;

int monthTotal = 0;

int qtrTotal = 0;

//set month counter to 1

monthCount = 1;

while (monthCount <= 3)

{

//set region counter to 1

regionCount = 1;

//get and accumulate sales

while (regionCount <= 4)

{

cout << "Enter sales: ";

cin >> sales;

monthTotal = monthTotal + sales;

regionCount = regionCount + 1;

}//end while

//display total monthly sales

cout << "Total sales for the month: $" << monthTotal << endl;

//update quarterly accumulator

qtrTotal = qtrTotal + monthTotal;

//update month counter

monthCount = monthCount + 1;

//reset month accumulator

monthTotal = 0;

}//end while

//display total quarterly sales

cout << "Total sales for the quarter: $" << qtrTotal << endl;

return 0;

} //end of main function

 4.

IPO Chart:

Input

Processing

Output

ID

 

Processing items: none

Algorithm:

  1. enter the ID
  2. repeat while (the ID is not equal to –1)

ID

1234 display Sue Nguyen

1345 display Janice Blackfeather

3456 display Allen Kraus

4567 display Margie O’Donnell

Other display "Incorrect ID – Please try again"

enter the ID

end repeat while

name

Desk-check table:

id

0

1345

4567

-1

//T10AppE04.cpp - displays a name

#include <iostream>

using namespace std;

int main()

{

//declare variable

int id = 0;

//get the ID

cout << "Enter an ID (enter -1 to stop the program): ";

cin >> id;

while (id != -1)

{

//display the name

switch (id)

{

case 1234: cout << "Sue Nguyen" << endl;

break;

case 1345: cout << "Janice Blackfeather" << endl;

break;

case 3456: cout << "Allen Kraus" << endl;

break;

case 4567: cout << "Margie O'Donnell" << endl;

break;

default: cout << "Incorrect ID - Please try again" << endl;

}//end switch

//get the ID

cout << "Enter an ID (enter -1 to stop the program): ";

cin >> id;

}//end while

return 0;

} //end of main function

5.

IPO Chart:

Input

Processing

Output

number registered

Processing items:

charge

Algorithm:

  1. enter the number registered
  2. while (the number registered > 0)

if (the number registered <= 3)

charge = 150 * number registered

else if (the number registered <= 9)

charge = 100 * number registered

else charge = 90 * number registered

end ifs

add charge to total charge

add number registered to total registered

enter the number registered

end repeat while

total registered

total charge

average charge

 

Desk-check table:

numReg

charge

totNumReg

totCharge

avgCharge

0

3

12

9

-1

0

450

1080

900

0

3

15

24

0

450

1530

2430

0.0

101.25

 

//T10AppE05.cpp - displays registration information

#include <iostream>

using namespace std;

int main()

{

//declare variables

int numReg = 0;

int charge = 0;

int totNumReg = 0;

int totCharge = 0;

float avgCharge = 0.0;

//get number registered

cout << "Enter number registered: ";

cin >> numReg;

while (numReg > 0)

{

//calculate charge

if (numReg <= 3)

charge = 150 * numReg;

else if (numReg <= 9)

charge = 100 * numReg;

else charge = 90 * numReg;

//end ifs

//update accumulators

totCharge = totCharge + charge;

totNumReg = totNumReg + numReg;

//get number registered

cout << "Enter number registered: ";

cin >> numReg;

}//end while

//display registration information

cout << fixed;

cout.precision(2);

cout << "Total registered: " << totNumReg << endl;

cout << "Total charge: $" << totCharge << endl;

cout << "Average charge: $" << float(totCharge) / totNumReg << endl;

return 0;

} //end of main function

 6.

//T10AppE06.cpp - displays the price of a concert ticket

#include <iostream>

using namespace std;

int main()

{

char location = ' ';

//enter input data

cout << "Enter the seat location (9 to stop the program): ";

cin >> location;

while (location != '9')

{

//display ticket price

if (location == 'B' || location == 'b')

cout << "Ticket price: $75" << endl;

else if (location == 'P' || location == 'p') cout << "Ticket price: $30" << endl;

else if (location == 'L' || location == 'l')

cout << "Ticket price: $21" << endl;

else cout << "Invalid location" << endl;

//end ifs

cout << "Enter the seat location (9 to stop the program): ";

cin >> location;

}//end while

return 0;

} //end of main function

 7.

//T10AppE07.cpp - displays the number of vacation weeks due an employee

#include <iostream>

using namespace std;

int main()

{

int years = 0;

int totEmp = 0;

//enter input data

cout << "Enter the years employed (negative number to stop the program): ";

cin >> years;

while (years >= 0)

{

//display vacation weeks

if (years == 0)

cout << "Vacation weeks: 0" << endl; else if (years <= 5)

cout << "Vacation weeks: 1" << endl;

else if (years <= 10)

cout << "Vacation weeks: 2" << endl;

else cout << "Vacation weeks: 3" << endl;

//end ifs

totEmp = totEmp + 1;

//enter input data

cout << "Enter the years employed (negative number to stop the program): ";

cin >> years;

}//end while

cout << "Total employees entered: " << totEmp << endl;

return 0;

} //end of main function

 

8.

//T10AppE08.cpp - displays a student's grade

#include <iostream>

using namespace std;

//function prototypes

void getPointsEarned(int &);

void assignGrade(int, char &);

int main()

{

//declare variables

int totalEarned = 0;

char grade = ' ';

//get total points earned

getPointsEarned(totalEarned);

//assign grade

assignGrade(totalEarned, grade);

//display grade

cout << "Grade: " << grade << endl;

return 0;

} //end of main function

//*****program-defined functions*****

void getPointsEarned(int &total)

{

//gets and accumulates the scores

int score = 0;

cout << "Enter the first score: ";

cin >> score;

while (score >= 0)

{

total = total + score;

cout << "Enter the next score: ";

cin >> score;

}//end while

} //end of getPointsEarned function

void assignGrade(int points, char &letterGrade)

{

//assigns the letter grade

if (points >= 360)

letterGrade = 'A';

else if (points >= 320)

letterGrade = 'B';

else if (points >= 280)

letterGrade = 'C';

else if (points >= 240)

letterGrade = 'D';

else letterGrade = 'F';

//end ifs

} //end of assignGrade function

 9.

//T10AppE09.cpp - gets 5 test scores, then displays number of students passing the test

#include <iostream>

using namespace std;

int main()

{

//declare variables

int score = 0;

int counter = 0;

int totalPass = 0;

counter = 5;

while (counter != 0)

{

//get score

cout << "Enter test score: ";

cin >> score;

//count passing score

if (score >= 70)

totalPass = totalPass++;

//end if

//update student counter

counter = counter--;

}//end while

//display total number of students passing the test

cout << "Total students passing test: " << totalPass << endl;

return 0;

} //end of main function

  

10. Changes to the original code are shaded.

//T10AppE10.cpp - displays the squares of the numbers from 1 to 5

#include <iostream>

#include <cmath>

using namespace std;

int main()

{

//declare variables

int number = 1;

while (number <= 5)

{

//display squares

cout << pow(number, 2) << endl;

number = number + 1;

}//end while

return 0;

} //end of main function

11. Changes to the original code are shaded.

//T10AppE11.cpp - displays the number of positive and negative integers

#include <iostream>

using namespace std;

int main()

{

//declare variables

int number = 0;

int positive = 0;

int negative = 0;

//get a number

cout << "Enter an integer: ";

cin >> number;

while (number != 0)

{

//update counters

if (number > 0)

positive = positive + 1;

else

negative = negative + 1;

//end if

//get a number

cout << "Enter an integer: ";

cin >> number;

}//end while

//display counters

cout << "Total positive numbers: " << positive << endl;

cout << "Total negative numbers: " << negative << endl;

return 0;

} //end of main function