C - Programming - C - Program - Chapter 05.pdf - po ang - Januszek66

CHAPTER

C ONTROL S TRUCTURES II

(R EPETITION )

IN THIS CHAPTER, YOU WILL:

n Learn about repetition (looping) control structures

n Explore how to construct and use counter-controlled,

sentinel-controlled, flag-controlled, and EOF-controlled

repetition structures

n Examine break and continue statements

n Discover how to form and use nested control structures

232

Chapter 5: Control Structures II (Repetition)

In Chapter 4, you saw how decisions are incorporated in programs. In this chapter, you

learn how repetitions are incorporated in programs.

Why Is Repetition Needed?

Suppose you want to add five numbers to find their average. From what you have learned

so far, you could proceed as follows (assume that all variables are properly declared):

cin >> num1 >> num2 >> num3 >> num4 >> num5;

//read five numbers

sum = num1 + num2 + num3 + num4 + num5;

//add the numbers

average = sum / 5;

//find the average

But suppose you want to add and average 100, or 1000, or more numbers. You would

have to declare that many variables, and list them again in cin statements and, perhaps,

again in the output statements. This takes an exorbitant amount of space and time. Also, if

you want to run this program again with different values, or with a different number of

values, you have to rewrite the program.

Suppose you want to add the following numbers:

5 3 7 9 4

Consider the following statements, in which sum and num are variables of type int :

1. sum = 0;

2. cin >> num;

3. sum = sum + num;

The first statement initializes sum to 0 . Let us execute statements 2 and 3. Statement 2

stores 5 in num ; statement 3 updates the value of sum by adding num to it. After statement

3, the value of sum is 5 .

Let us repeat statements 2 and 3. After statement 2 (after the programming code reads the

next number):

num = 3

After statement 3:

sum = sum + num = 5 + 3 = 8

At this point, sum contains the sum of the first two numbers. Let us again repeat

statements 2 and 3 (third time). After statement 2 (after the code reads the next number):

num = 7

After statement 3:

sum = sum + num = 8 + 7 = 15

Now sum contains the sum of the first three numbers. If you repeat statements 2 and 3

two more times, sum will contain the sum of all five numbers.

while Looping (Repetition) Structure

233

Ifyouwanttoadd10numbers,youcanrepeatstatements2and3tentimes.Andifyouwantto

add 100 numbers, you can repeat statements 2 and 3 one hundred times. In either case, you do not

have to declare any additional variables, as you did in the first code. You can use this C++ code to

add any set of numbers, whereas the earlier code requires you to drastically change the code.

There are many other situations where it is necessary to repeat a set of statements. For

example, for each student in a class, the formula for determining the course grade is the same.

C++ has three repetition, or looping, structures that let you repeat statements over and over

until certain conditions are met. This chapter introduces all three looping (repetition)

structures. The next section discusses the first repetition structure, called the while loop.

while Looping (Repetition) Structure

In the previous section, you saw that sometimes it is necessary to repeat a set of statements

several times. One way to repeat a set of statements is to type the set of statements in the

program over and over. For example, if you want to repeat a set of statements 100 times,

you type the set of statements 100 times in the program. However, this solution of

repeating a set of statements is impractical, if not impossible. Fortunately, there is a better

way to repeat a set of statements. As noted earlier, C++ has three repetition, or looping,

structures that allow you to repeat a set of statements until certain conditions are met.

This section discusses the first looping structure, called a while loop.

The general form of the while statement is:

while (expression)

statement

In C++, while is a reserved word. Of course, the statement can be either a simple

or compound statement. The expression acts as a decision maker and is usually a

logical expression. The statement is called the body of the loop. Note that the

parentheses around the expression are part of the syntax. Figure 5-1 shows the flow

of execution of a while loop.

expression

true

statement

false

FIGURE 5-1 while loop

234

Chapter 5: Control Structures II (Repetition)

The expression provides an entry condition. If it initially evaluates to true ,the

statement executes. The loop condition—the expression —is then reevaluated. If it again

evaluates to true ,the statement executes again. The statement (body of the loop)

continues to execute until the expression is no longer true . A loop that continues to

execute endlessly is called an infinite loop. To avoid an infinite loop, make sure that the loop’s

body contains statement(s) that assure that the exit condition—the expression in the while

statement—will eventually be false .

EXAMPLE 5-1

Consider the following C++ program segment:

i = 0;

//Line 1

while (i <= 20)

//Line 2

{

cout << i << " ";

//Line 3

i = i + 5;

//Line 4

}

cout << endl;

Sample Run:

0 5 10 15 20

In Line 1, the variable i is set to 0 . The expression in the while statement (in Line

2), i < = 20 , is evaluated. Because the expression i < = 20 evaluates to true , the body of

the while loop executes next. The body of the while loop consists of the statements in

Lines 3 and 4. The statement in Line 3 outputs the value of i , which is 0 . The statement

in Line 4 changes the value of i to 5 . After executing the statements in Lines 3 and 4, the

expression in the while loop (Line 2) is evaluated again. Because i is 5 ,the

expression i < = 20 evaluates to true and the body of the while loop executes again.

This process of evaluating the expression and executing the body of the while loop

continues until the expression , i < = 20 (in Line 2), no longer evaluates to true .

The variable i (in Line 2, Example 5-1 ) in the expression is called the loop control variable.

Note the following from Example 5-1:

Within the loop i becomes 25 , but is not printed because the entry

condition is false .

If you omit the statement:

i = i + 5;

from the body of the loop, you will have an infinite loop, continually

printing rows of zeros.

while Looping (Repetition) Structure

235

You must initialize the loop control variable i before you execute the

loop. If the statement:

i = 0;

(in Line 1) is omitted, the loop may not execute at all. (Recall that

variables in C++ are not automatically initialized.)

In Example 5-1, if the two statements in the body of the loop are

interchanged, it may drastically alter the result. For example, consider

the following statements:

i = 0;

while (i <= 20)

{

i = i + 5;

cout << i << " ";

}

cout << endl;

Here the output is :

5 10 15 20 25

Typically, this would be a semantic error because you rarely want a

condition to be true for i < = 20 , and yet produce results for i > 20 .

If you put a semicolon at the end of the while loop, (after the logical

expression), then the action of the while loop is empty or null. For

example, the action of the following while loop is empty.

i = 0;

while (i <= 20);

{

i = i + 5;

cout << i <<

" ";

}

cout << endl;

The statements within the braces do not form the body of the while loop.

Designing while loops

As in Example 5-1, the body of a while executes only when the expression ,inthe

while statement, evaluates to true . Typically, the expression checks whether a

variable(s), called the loop control variable (LCV), satisfies certain conditions. For

example, in Example 5-1, the expression in the while statement checks whether

i < = 20 . The LCV must be properly initialized before the while loop and it should

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