CHAPTER 5:Repetition Control Structure

Objectives

To develop algorithms that use DOWHILE and REPEAT.. UNTIL structures

Introduce a pseudocode for counted repetition

Develop algorithms using variations of repeat structure

Learn to avoid common loop mistakes

Repeat Loop

DOWHILE condition p is truestatement block

ENDDO

WHILE condition p is truestatement block

ENDWHILE

Advantages

Looping makes computer programming efficient and worthwhile

Write one set of instructions to operate on multiple, separate sets of data

Loop: structure that repeats actions while some condition continues

Controlling Loops

As long as a Boolean expression remains true, while loop’s body executes

Must control number of repetitions to avoid an infinite loop

Repetitions controlled by Counter Sentinel value

Controlling Loops with Counters and Sentinel Values As long as a Boolean expression

remains true, DOWHILE loop’s body executes

Must control number of repetitions to avoid an infinite loop

Repetitions controlled by Counter Sentinel value

Using a Definite while Loop with a Counter Three actions make a DOWHILE loop end

correctly: Loop control variable is initialized

Prior to entering the loop Loop control variable is tested

If result is true, loop body entered Loop control variable must be altered in loop body

• DOWHILE expression eventually evaluates to false Loop control variables altered by:

Incrementing Decrementing

Using a Definite DOWHILE Loop with a Counter (continued)

Programming Logic and Design, Fifth Edition, Comprehensive 9

Using a Definite while Loop with a Counter (continued) Definite loop: number of iterations

predetermined Also called counted loop

Counter: numeric variable used to count number of times an event occurs

Loop control variable may be altered by user input

Indefinite loop: loop iterates until some condition is true Number of iterations may vary

Using an Indefinite DOWHILE Loop with a Sentinel Value Indefinite loop: loop performed a different

number of times each time the program executes

Three crucial steps: Starting value to control the loop must be provided Comparison must be made using the value that

controls the loop Within the loop, value that controls the loop must

be altered Loop control variable: any variable that

determines whether the loop will continue

Using an Indefinite DOWHILE Loop with a Sentinel Value (continued)

Using an Indefinite DOWHILE Loop with a Sentinel Value (continued)

Avoiding Common Loop Mistakes Neglecting to initialize the loop control

variable Neglecting to alter the loop control

variable Using the wrong comparison with the

loop control variable Including statements inside the loop

that belong outside the loop

Using a for Loop

• for statement or for loop is a definite loop

Provides three actions in one structure Initializes Evaluates Increments

Takes the form:for initialValue to finalValue

do somethingendfor

Using a for Loop (continued) Example:

for count = 0 to 99print LABEL_TEXT, name

endfor

Initializes count to 0 Checks count against the limit value 99 If evaluation is true, for statement

body prints the label Increases count by 1

Summary When using a loop, write one set of

instructions that operates on multiple, separate data

Three steps must occur in every loop: Initialize loop control variable Compare variable to some value Alter the variable that controls the loop

Summary (continued)

Common mistakes made by programmers: Neglecting to initialize loop control variable Neglecting to alter loop control variable Using wrong comparison with loop control

variable Including statements inside the loop that belong

outside the loop Most computer languages support a for

statement for loop used with definite loops

When number of iterations is known