chapter 4 control structures - repetition

8/13/2019 Chapter 4 Control Structures - Repetition

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CHAPTER 4

CONTROL STRUCTURES

(REPETITION


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Objectives

In this chapter, you will:

Learn about repetition (looping) control

structures

Explore how to construct and use count-controlled, sentinel-controlled, flag-controlled,

and EOF-controlled repetition structures

Examine breakand continuestatements Discover how to form and use nested control

structures


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whileLooping (Repetition)

Structure

The general form of the whilestatement is:

whileis a reserved word

Statement can be simple or compound

Expression acts as a decision maker and is

usually a logical expression Statement is called the body of the loop

The parentheses are part of the syntax


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Structure (continued)

Infinite loop: continues to execute endlessly Avoided by including statements in loop body that

assure exit condition is eventually false


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Example:


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Case 1: Counter-Controlled while

Loops

If you know exactly how many pieces of dataneed to be read, the whileloop becomes acounter-controlled loop:


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Case 2: Sentinel-Controlled while

Loops

Sentinel variable is tested in the condition and

loop ends when sentinel is encountered


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Case 3: Flag-Controlled while

Loops

A flag-controlled whileloop uses a bool

variable to control the loop

The flag-controlled whileloop takes the form:


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Case 4: EOF-Controlled while

Loops

Use an EOF (End Of File)-controlled whileloop

The logical value returned by cincan determine

if the program has ended input


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More on Expressions in while

Statements

The expression in a whilestatement can

be complex

For example:while((noOfGuesses < 5) && (!isGuessed))

{

}


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forLooping (Repetition) Structure

The general form of the forstatement is:

The initial statement, loopcondition, and update statementare

called forloop control statements

initial statementusually initializes a variable

(called the forloop control, or forindexed,

variable)

In C++, foris a reserved word


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(continued)


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(continued)


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(continued)

The output will be five lines of Hello and a line of

*

Without the loop block (curly braces), only the first

statement will be considered for the loop.


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(continued)

C++ allows you to use fractional values for loopcontrol variables of the doubletype

Results may differ

The following is a semantic error:

The following is a legal forloop:for(;;)

cout


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(continued)


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dowhileLooping (Repetition)

Structure General form of a do...while:

The statementexecutes first, and then the

expressionis evaluated

To avoid an infinite loop, body must contain a

statement that makes the expression false The statementcan be simple or compound

Loop always iterates at least once


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Choosing the Right Looping

Structure

All three loops have their place in C++

If you know or can determine in advance thenumber of repetitions needed, the forloop is

the correct choice If you do not know and cannot determine in

advance the number of repetitions needed,and it could be zero, use a whileloop

If you do not know and cannot determine inadvance the number of repetitions needed,and it is at least one, use a do...whileloop


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breakand continueStatements

breakand continuealter the flow of control

breakstatement is used for two purposes:

To exit early from a loop

Can eliminate the use of certain (flag) variables

To skip the remainder of the switchstructure

After the breakstatement executes, the

program continues with the first statement after

the structure


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break& continueStatements

(continued)

continueis used in while, for, and

dowhilestructures

When executed in a loop

It skips remaining statements and proceeds

with the next iteration of the loop


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Nested Control Structures

To create the following pattern:*

**

***

*********

We can use the following code:for(i = 1; i


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Nested Control Structures

(continued)

What is the output of the following loops?

for(i = 5; i >= 1; i--)

{

for(j = 1; j


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Summary

C++ has three looping (repetition) structures:

while, for, and dowhile

while, for, and doare reserved words

whileand forloops are called pretest loops

do...whileloop is called a posttest loop

whileand formay not execute at all, but

do...whilealways executes at least once while: expression is the decision maker, and

the statement is the body of the loop


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Summary (continued)

A whileloop can be: Counter-controlled

Sentinel-controlled

EOF-controlled

In the Windows console environment, the end-of-file

marker is entered using Ctrl+z forloop: simplifies the writing of a counter-controlled

while loop

Executing a breakstatement in the body of a

loop immediately terminates the loop Executing a continuestatement in the body ofa loop skips to the next iteration

Source:

C++ Programming: From Problem Analysis to Program Design, Fourth

Edition

chapter 4 control structures - repetition

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