Download Chapter 5

Chapter 5.
Looping
1
5.1 The Increment and Decrement
Operators
• ++ and -- are operators that add and subtract
one from their operands.
Num = Num + 1;
Num += 1;
Num++;
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Program 5-1
// This program demonstrates the increment and decrement
// operators.
#include <iostream.h>
void main(void)
{
int BigVal = 10, SmallVal = 1;
cout << "BigVal is " << BigVal
<< " and SmallVal is " << SmallVal << endl;
SmallVal++;
BigVal--;
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Program continues
cout << "BigVal is " << BigVal
<< " and SmallVal is " << SmallVal << endl;
++SmallVal;
--BigVal;
cout << "BigVal is " << BigVal
<< " and SmallVal is " << SmallVal << endl;
}
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Program Output
BigVal is 10 and SmallVal is 1
BigVal is 9 and SmallVal is 2
BigVal is 8 and SmallVal is 3
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Program 5-2
#include <iostream.h>
void main(void)
{
int BigVal = 10, SmallVal = 1;
cout << "BigVal starts as " << BigVal;
cout << " and SmallVal starts as " << SmallVal << endl;
cout << "BigVal--: " << BigVal-- << endl;
cout << "SmallVal++: " << SmallVal++ << endl;
cout << "Now BigVal is: " << BigVal << endl;
cout << "Now SmallVal is: " << SmallVal << endl;
cout << "--BigVal: " << --BigVal << endl;
cout << "++SmallVal: " << ++SmallVal << endl;
}
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Program Output
BigVal starts as 10 and SmallVal starts as 1
BigVal--: 10
SmallVal++: 1
Now BigVal is: 9
Now SmallVal is: 2
--BigVal: 8
++SmallVal: 3
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Using ++ and -- in Mathematical
Expressions
A = 2;
B = 5;
C = A * B++;
cout << A << “ “ << B << “ “ << C;
Results: 2 6 10
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Using ++ and -- in Relational
Expressions
X = 10;
if ( X++ > 10)
cout << “X is greater than 10.\n”;
• Two operations are happening:
• the value in X is tested to determine if it is
greater than 10
• then X is incremented
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5.2 Introduction to Loops - The while
Loop
• A loop is part of a program that repeats.
• A while loop is a “pre test” loop - the
expression is tested before the loop is
executed
while (expression)
statement;
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Program 5-3
// This program demonstrates a simple while loop.
#include <iostream.h>
void main(void)
{
int Number = 0;
cout << "This program will let you enter number after\n";
cout << "number. Enter 99 when you want to quit the ";
cout << "program.\n";
while (Number != 99)
cin >> Number;
}
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Program Output with Example Input
This program will let you enter number after number. Enter
99 when you want to quit the program.
1 [Enter]
2 [Enter]
30 [Enter]
75 [Enter]
99 [Enter]
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Terminating a Loop
A loop that does not have a way of stopping is called
an infinite loop
int Test = 0;
while (Test < 10)
cout << “Hello\n”;
A null statement is also an infinite loop, but it does
nothing forever:
while (Test < 10);
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Programming Style and the while Loop
• If there is only one statement repeated by
the loop, it should appear on the line after
the while statement and be indented one
additional level
• If the loop repeats a block, the block should
begin on the line after the while statement
and each line inside the braces should be
indented
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5.3 Counters
• A counter is a variable that is incremented
or decremented each time a loop iterates.
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Program 5-4
// This program displays the numbers 1 through 10 and
// their squares.
#include <iostream.h>
void main(void)
{
int Num = 1;
cout << "Number Number Squared\n";
cout << "-------------------------\n";
while (Num <= 10)
{
cout << Num << "\t\t" << (Num * Num) << endl;
Num++;
}
}
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Program Output
Number
Number Squared
------------------------1
1
2
4
3
9
4
16
5
25
6
36
7
49
8
64
9
81
10
100
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Program 5-5
// This program displays the numbers 1 through 10 and
// their squares.
#include <iostream.h>
void main(void)
{
int Num = 0;
cout << "Number Number Squared\n";
cout << "-------------------------\n";
while (Num++ < 10)
cout << Num << "\t\t" << (Num * Num) << endl;
}
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Program Output
Number
Number Squared
------------------------1
1
2
4
3
9
4
16
5
25
6
36
7
49
8
64
9
81
10
100
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5.4 Letting the User Control the Loop
• Loops can be designed to repeat until the
user enters a particular value.
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Program 5-6
// This program averages a set of test scores for multiple
// students. It lets the user decide how many.
#include <iostream.h>
void main(void)
{
int NumStudents, Count = 0;
cout << "This program will give you the average of three\n";
cout << "test scores per student.\n";
cout << "How many students do you have test scores for? ";
cin >> NumStudents;
cout << "Enter the scores for each of the students.\n";
cout.precision(2);
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Program continues
while (Count++ < NumStudents)
{
int Score1, Score2, Score3;
float Average;
cout << "\nStudent " << Count << ": ";
cin >> Score1 >> Score2 >> Score3;
Average = (Score1 + Score2 + Score3) / 3.0;
cout << "The average is " << Average << ".\n";
}
}
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Program Output with Example Input
This program will give you the average of three test scores per student.
How many students do you have test scores for? 3 [Enter]
Enter the scores for each of the students.
Student 1: 75 80 82 [Enter]
The average is 79.
Student 2: 85 85 90 [Enter]
The average is 86.67.
Student 3: 60 75 88 [Enter]
The average is 74.33.
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5.5 Keeping a Running Total
• A running total is a sum of numbers that
accumulates with each iteration of a loop.
The variable used to keep the running total
is called an accumulator.
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Program 5-7
// This program takes daily sales figures over a period of time
// and calculates their total.
#include <iostream.h>
void main(void)
{
int Days, Count = 0;
float Total = 0.0;
cout << "For how many days do you have sales figures? ";
cin >> Days;
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Program continues
while (Count++ < Days)
{
float Sales;
cout << "Enter the sales for day " << Count << ": ";
cin >> Sales;
Total += Sales;
}
cout.precision(2);
cout.setf(ios::fixed | ios::showpoint);
cout << "The total sales are $" << Total << endl;
}
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Program Output with Example Input
For how many days do you have sales figures? 5 [Enter]
Enter the sales for day 1: 489.32 [Enter]
Enter the sales for day 2: 421.65 [Enter]
Enter the sales for day 3: 497.89 [Enter]
Enter the sales for day 4: 532.37 [Enter]
Enter the sales for day 5: 506.92 [Enter]
The total sales are $2448.15
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5.6 Sentinels
• A sentinel is a special value that marks the
end of a list of values.
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Program 5-8
// This program calculates the total number of points a
// soccer team has earned over a series of games. The user
// enters a series of point values, then -1 when finished.
#include <iostream.h>
void main(void)
{
int Count = 0, Points = 0, Total = 0;
cout << "Enter the number of points your team has earned\n";
cout << "so far in the season, then enter -1 when\n";
cout << "finished.\n";
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Program continues
while (Points != -1)
{
Count++;
cout << "Enter the points for game " << Count << ": ";
cin >> Points;
if (Points != -1)
Total += Points;
}
cout << "The total points are " << Total << endl;
}
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Program Output with Example Input
Enter the number of points your team has earned so far in the
season, then enter -1 when you are finished.
Enter the points for game 1: 7 [Enter]
Enter the points for game 2: 9 [Enter]
Enter the points for game 3: 4 [Enter]
Enter the points for game 4: 6 [Enter]
Enter the Points for game 5: 8 [Enter]
Enter the points for game 6: -1 [Enter]
The total points are 34
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5.7 The do-while Loop and for
Loops
• In addition to the while loop, C++ also
offers the do-while and for loops.
• A do-while loop is similar to a while loop,
but in post-test format:
do
statement;
while (expression);
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Program 5-9
#include <iostream.h>
void main(void)
{
int Score1, Score2, Score3;
float Average;
char Again;
do
{
cout << "Enter 3 scores and I will average them: ";
cin >> Score1 >> Score2 >> Score3;
Average = (Score1 + Score2 + Score3) / 3.0;
cout << "The average is " << Average << ".\n";
cout << "Do you want to average another set? (Y/N) ";
cin >> Again;
} while (Again == 'Y' || Again == 'y');
}
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Program Output with Example Input
Enter 3 scores and I will average them: 80 90 70 [Enter]
The average is 80.
Do you want to average another set? (Y/N) y [Enter]
Enter 3 scores and I will average them: 60 75 88 [Enter]
The average is 74.333336.
Do you want to average another set? (Y/N) n [Enter]
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Program 5-10
// This program displays a menu and asks the user to make a
// selection. A switch statement determines which item the
// user has chosen. A do-while loop repeats the program until
// the user selects item 4 from the menu.
#include <iostream.h>
void main(void)
{
int Choice, Months;
float Charges;
cout.setf(ios::fixed | ios::showpoint);
cout.precision(2);
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Program continues
do
{
cout << "\n\t\tHealth Club Membership Menu\n\n";
cout << "1. Standard Adult Membership\n";
cout << "2. Child Membership\n";
cout << "3. Senior Citizen Membership\n";
cout << "4. Quit the Program\n\n";
cout << "Enter your choice: ";
cin >> Choice;
if (Choice != 4)
{
cout << "For how many months? ";
cin >> Months;
}
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Program continues
switch (Choice)
{
case 1: Charges = Months * 40.00;
cout << "The total charges are $";
cout << Charges << endl;
break;
case 2: Charges = Months * 20.00;
cout << "The total charges are $";
cout << Charges << endl;
break;
case 3: Charges = Months * 30.00;
cout << "The total charges are $";
cout << Charges << endl;
break;
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Program continues
case 4: cout << "Thanks for using this ";
cout << "program.\n";
break;
default: cout << "The valid choices are 1-4. ";
cout << "Try again.\n";
}
} while (Choice != 4);
}
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Program Output with Example Input
Health Club Membership Menu
1. Standard Adult Membership
2. Child Membership
3. Senior Citizen Membership
4. Quit the Program
Enter your choice: 1 [Enter]
For how many months 12 [Enter]
The total charges are $480.00
Health Club Membership Menu
1. Standard Adult Membership
2. Child Membership
3. Senior Citizen Membership
4. Quit the Program
Enter your choice: 4 [Enter]
Thanks for using this program.
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The for Loop
• Ideal for situations that require a counter
because it has built-in expressions that
initialize and update variables.
for (initialization; test; update)
statement;
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Program 5-11
// This program displays the numbers 1 through 10 and
// their squares.
#include <iostream.h>
void main(void)
{
int Num;
cout << "Number Number Squared\n";
cout << "-------------------------\n";
for (Num = 1; Num <= 10; Num++)
cout << Num << "\t\t" << (Num * Num) << endl;
}
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Program Output
Number
Number Squared
------------------------1
1
2
4
3
9
4
16
5
25
6
36
7
49
8
64
9
81
10
100
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Omitting the for Loop’s Expressions
int Num = 1;
for ( ; Num <= 10; Num++)
cout << Num << “\t\t” << (Num * Num) << endl;
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Program 5-12
// This program takes daily sales figures for one week
// and calculates their total.
#include <iostream.h>
void main(void)
{
const int Days = 7;
int Count;
float Total;
for (Count = 1, Total = 0.0; Count <= Days; Count++)
{
float Sales;
cout << "Enter the sales for day " << Count << ": ";
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Program continues
cin >> Sales;
Total += Sales;
}
cout.precision(2);
cout.setf(ios::fixed | ios::showpoint);
cout << "The total sales are $" << Total << endl;
}
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Program Output with Example Input
Enter the sales for day 1: 489.32 [Enter]
Enter the sales for day 2: 421.65 [Enter]
Enter the sales for day 3: 497.89 [Enter]
Enter the sales for day 4: 532.37 [Enter]
Enter the sales for day 5: 506.92 [Enter]
Enter the sales for day 6: 489.01 [Enter]
Enter the sales for day 7: 476.55 [Enter]
The total sales are $3413.71
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5.8 Focus on Software Engineering:
Deciding Which Loop to Use
•
•
•
The while Loop
• A pre-test loop.
• Use when you do not want the loop to iterate if the condition is false from
the beginning.
• Ideal if you want to use a sentinel.
The do-while Loop
• A post-test loop.
• Use if you always want the loop to iterate at least once.
The for Loop
• A pre-test loop.
• Automatically executes an update expression at the end of each iteration.
• Ideal for situations where a counter variable is needed.
• Used when the exact number of required iterations is known.
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5.9 Focus on Software Engineering:
Nested Loops
• A loop that is inside another loop is called a
nested loop.
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Program 5-13
// This program averages test scores. It asks the user for the
// number of students and the number of test scores per student.
#include <iostream.h>
void main(void)
{
int NumStudents, NumTests, Total;
float Average;
cout << "This program averages test scores.\n";
cout << "For how many students do you have scores? ";
cin >> NumStudents;
cout << "How many test scores does each student have? ";
cin >> NumTests;
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Program continues
for (int Count1 = 1; Count1 <= NumStudents; Count1++)
{
Total = 0;
for (int Count2 = 1; Count2 <= NumTests; Count2++)
{
int Score;
cout << "Enter score " << Count2 << " for ";
cout << "student " << Count1 << ": ";
cin >> Score;
Total += Score;
}
Average = Total / NumTests;
cout << "The average score for student " << Count1;
cout << " is " << Average << ".\n\n";
}
}
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Program Output with Example Input
This program averages test scores.
For how many students do you have scores? 2 [Enter]
How many test scores does each student have? 3 [Enter]
Enter score 1 for student 1: 84 [Enter]
Enter score 2 for student 1: 79 [Enter]
Enter score 3 for student 1: 97 [Enter]
The average for student 1 is 86.
Enter score 1 for student 2: 92 [Enter]
Enter score 2 for student 2: 88 [Enter]
Enter score 3 for student 2: 94 [Enter]
The average for student 2 is 91.
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5.10 Breaking Out of a Loop
• The break statement causes a loop to
terminate early.
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Program 5-14
// This program raises the user's number to the powers
// of 0 through 10.
#include <iostream.h>
#include <math.h>
void main(void)
{
int Value;
char Choice;
cout << "Enter a number: ";
cin >> Value;
cout << "This program will raise " << Value;
cout << " to the powers of 0 through 10.\n";
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Program continues
for (int Count = 0; Count < 10; Count++)
{
cout << Value << " raised to the power of ";
cout << Count << " is " << pow(Value, Count);
cout << "\nEnter Q to quit or any other key ";
cout << "to continue. ";
cin >> Choice;
if (Choice == 'Q' || Choice == 'q')
break;
}
}
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Program Output
Enter a number: 2 [Enter]
This program will raise 2 to the powers of 0 through 10.
2 raised to the power of 0 is 1
Enter Q to quit or any other key to
continue. C [Enter]
2 raised to the power of 1 is 2
Enter Q to quit or any other key to continue. C [Enter]
2 raised to the power of 2 is 4
Enter Q to quit or any other key to continue. Q [Enter]
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5.11 The continue Statement
• The continue statement causes a loop to
stop its current iteration and begin the next
one.
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Program 5-15
// This program calculates the charges for video rentals.
// Every third video is free.
#include <iostream.h>
#include <iomanip.h>
void main(void)
{
int VideoCount = 1, NumVideos;
float Total = 0.0;
char Current;
cout << "How many videos are being rented? ";
cin >> NumVideos;
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Program continues
do
{
if ((VideoCount % 3) == 0)
{
cout << "Video #" << VideoCount << " is free!\n";
continue;
}
cout << "Is video #" << VideoCount;
cout << " a current release? (Y/N)";
cin >> Current;
if (Current == 'Y' || Current == 'y')
Total += 3.50;
else
Total += 2.50;
} while (VideoCount++ < NumVideos);
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Program continues
cout.precision(2);
cout.setf(ios::fixed | ios::showpoint);
cout << "The total is $" << Total;
}
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Program Output with Example Input
How many Videos are being rented? 6 [Enter]
Is video #1 a current release? y [Enter]
Is video #2 a current release? n [Enter]
Video #3 is free!
Is video #4 a current release? n [Enter]
Is video #5 a current release? y [Enter]
Video #6 is free!
The total is $12.00
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5.12 Using Loops for Input Validation
• Loops can be used to create input routines
that repeat until acceptable data is entered.
61
Program 5-16
// This program calculates the number of soccer teams
// that a youth league may create from the number of
// available players. Input validation is demonstrated
// with do-while loops.
#include <iostream.h>
void main(void)
{
int Players, Max, NumTeams, LeftOver;
do
{
cout << "What is the maximum number of players ";
cout << "per team? ";
cin >> Max;
62
Program continues
if (Max < 9 || Max > 15)
{
cout << "You should have at least 9 but no\n";
cout << "more than 15 per team.\n";
}
} while (Max < 9 || Max > 15);
do
{
cout << "How many players are available? ";
cin >> Players;
if (Players < 0)
cout << "Please enter a positive number.\n";
} while (Players < 0);
NumTeams = Players / Max;
LeftOver = Players % Max;
cout << "There will be " << NumTeams << " teams with\n";
cout << LeftOver << " players left over.\n";
}
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Program Output with Example Input
What is the maximum number of players per team? 4 [Enter]
You should have at least 9 but no more than 15 per team.
What is the maximum number of players per team? 12 [Enter]
How many players are available? -142 [Enter]
Please enter a positive number
How many players are available? 142 [Enter]
There will be 11 teams with 10 players left over.
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