Chapter 4: Looping. Resource: Starting Out with C++, Third Edition, Tony Gaddis 5.1 The Increment...

Chapter 4: Looping

Resource: Starting Out with C++, Third Edition, Tony Gaddis

5.1 The Increment and Decrement Operators

• ++ and -- are operators that add and subtract one from their operands.

num = num + 1;

num += 1;

num++;


Using ++ and -- in Mathematical Expressions

a = 2;

b = 5;

c = a * b++;

cout << a << “ “ << b << “ “ << c;

Results: 2 6 10


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


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;


5-3 Program

// 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;

}


5-3 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]


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);


5.3 Counters

• A counter is a variable that is incremented or decremented each time a loop iterates.


5-4 Program// This program displays the numbers 1 through 10// and their squares.#include <iostream.h>

void main(void){

int num = 1; // Initialize counter

cout << "number number Squared\n";cout << "-------------------------\n";while (num <= 10){

cout << num << "\t\t" << (num * num) << endl;num++; // Increment counter

}}


5-4 Program Output

number number Squared-------------------------1 12 43 94 165 256 367 498 649 8110 100


5.4 Letting theUser Control the Loop

• We can let the user indicate the number of times a loop should repeat.


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";

}

5-6 Program



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.


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.


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;

}

5-7 Program Continued



For how many days do you have sales figures? 5 [Enter]

Enter the sales for day 1: 489.32 [Enter]





The total sales are $2448.15


5.6 Sentinels

• A sentinel is a special value that marks the end of a list of values.


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;

}




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 6: -1 [Enter]

The total points are 34


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);


5-9 Program// This program averages 3 test scores. It repeats as many// times as the user wishes#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');}



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]


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;


5-11 Program// This program displays the numbers 1 through 10

and

// their squares.

#include <iostream.h>

void main(void)

{

cout << “Number Number Squared\n";

cout << "-------------------------\n";

for (int num = 1; num <= 10; num++){

cout << num << "\t\t" << (num * num) << endl;}

}


5-11 Program Output

Number Number Squared-------------------------1 12 43 94 165 256 367 498 649 8110 100


5.8 Other Forms of the Update Expression

• Incrementing the counter by something besides 1:for(number = 2; number <= 100; number +=2)

cout << number << endl; // print the even numbers 2 – 100

• Going backwards:for(number = 10; number >= 0; number--)

cout << number << endl; //count from 10 to 0

• A stand-alone for loop://This one prints the integers from 1 to 10

for(number = 1; number <= 10; cout << number++)

• There are quite a few variations, try some of your own!


5.8 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.


5.9 Nested Loops

• A loop that is inside another loop is called a nested loop.


for (int count1 = 1; count1 <= numStudents; count1++)

{

total = 0; // Initialize accumulator

for (int count2 = 1; count2 <= numTests; count2++)

{

int score;

cout << "Enter score " << count2 << " for ";

cout << "student " << count1 << ": ";

cin >> score;

total += score; // accumulate running total

}

average = total / numTests;

cout << "The average score for student " << count1;

cout << " is " << average << ".\n\n";

}

}




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.


5.10 Breaking Out of a Loop

• The break statement causes a loop to terminate early.


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;

}

}



5-14 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]


Enter Q to quit or any other key to continue.

C [Enter]


Enter Q to quit or any other key to continue.

Q [Enter]


5.11 Using break in a nested loop

• The break statement below breaks out of the inner loop but NOT the outer loop.

for(int row = 0; row < 5; row++)

{ //begin outer loop

for(star = 0; star < 20; star++)

{ //begin inner loop

…………// some statements

break;

…………// some more statements

} //end inner loop

} //end outer loop


5.11 The continue Statement

• The continue statement causes a loop to stop its current iteration and begin the next one.


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);

cout.precision(2);

cout.setf(ios::fixed | ios::showpoint);

cout << "The total is $" << total;

}


Chapter 4: Looping. Resource: Starting Out with C++, Third Edition, Tony Gaddis 5.1 The Increment...

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