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 true statement block ENDDO WHILE condition p is true statement 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, Comprehensive9 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)

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 something endfor

Using a for Loop (continued)  Example: for count = 0 to 99 print 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