Programming Logic and Design Seventh Edition

1. Programming Logic and DesignSeventh Edition Chapter 5 Looping

2. Objectives • In this chapter, you will learn about: • The advantages of looping • Using a loop control variable (lcv) • Nested loops • Avoiding common loop mistakes • Using a for loop • Common loop applications • Loop control structure in RAPTOR • RAPTOR Intermediate mode: user-defined procedure

3. Understanding the Advantages of Looping • Looping makes computer programming efficient and worthwhile • Write one set of instructions to operate on multiple, separate sets of data • Loop: • control structure that repeats a set of actions (loop body) while some condition remains true or until some condition becomes true.

4. Loop Control Structure in RAPTOR In RAPTOR: you repeat the body of the loop UNTIL the condition evaluates to true. RAPTOR lets you structure your loop as one of three types: Pre-Test loop ( while / for ) Mid-Test loopillegal in our book Post-Test loop ( do … while ) Our book only shows us pre-test loops. RAPTOR Loop Symbol Note: In most languages (including Java) you repeat the loop bodywhile the condition remains true

5. Understanding the Advantages of Looping (continued) Figure 5-1 The loop structure (pre-test loop) Our book calls this a while loop

6. LoopingDefinite and Indefinite Loops Looping may be achieved using either a definite loop or an indefinite loop • A definite loop is also referred to as a counter-controlled loop • An indefinite loop is also referred to as a sentinel-controlled loop • For a definite loop, you know when you are creating the loop how many times the loop body should be executed • For an indefinite loop, the number of times the body of the loop should be executed can be different for each run of a program

7. Using a Counter-Controlled while loop • As long as a boolean expression (the condition) remains true, the while loop’s body executes • Essential Steps: • Create and initialize a loop control variable (lcv) • Determine the upper or lower limit for the lcv • Determine the step (increment or decrement) for the lcv • Determine the boolean expression (condition) that will control the loop • Each iteration of the loop, update the lcv • Note: for a counter-controlled loop, a loop control variable is also referred to as a counter

8. Using a Definite (Counter-Controlled) Loop Must the condition be true or false for the loop body to be executed? In RAPTOR, what would the condition be? lcv is: count Figure 5-3 A counted while loop that outputs “Hello” four times

9. Using an Indefinite Loop (Sentinel-Controlled) with a Sentinel Value • Indefinite loop • May be performed a different number of times each time the program executes • Essential steps: • Identify a sentinel value (value outside the range of valid input data) or sentinel module expression (such as RAPTOR's End_of_Input procedure) • Determine the boolean expression (condition) to be used to exit the loop • name = QUIT //where QUIT is a named constant sentinel value • End_of_input //RAPTOR sentinel module expression • Java  hasNext( ) //Java method to evaluate end of input condition • Each iteration of the loop get a new input value to compare to the sentinel value or evaluate the sentinel module expression

10. Figure 5-4 An indefinite while loop that displays “Hello” as long as the user wants to continue

11. Understanding the Loop in a Program’s Mainline Logic • Three steps that should occur in every properly functioning loop • Initialize the variable that will control the loop ( sentinel or counter value ) • Test the loop control variable ( lcv ) to determine whether the loop body executes • Update (aka alter) the loop control variable

12. Nested Loops • Nested loops: • loop within another loop • Outer loop: • loop that contains the other loop • Inner loop: • loop that is contained • Needed when values of two (or more) variables repeat to produce every combination of values

13. Figure 5-8 Flowchart and pseudocode for AnswerSheet program

14. 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

15. Avoiding Common Loop Mistakes (continued) • Mistake: • neglecting to initialize the loop control variable • Example: get name statement removed • Value of name unknown or garbage • Program may end before any labels printed • 100 labels printed with an invalid name

16. Figure 5-10Incorrect logic for greeting program because the loop control variable initialization is missing

17. Avoiding Common Loop Mistakes (continued) • Mistake: • neglecting to alter the loop control variable • Example: • Remove get name instruction from outer loop • User never enters a name after the first one • Inner loop executes infinitely • [ Always … ] incorrect to create a loop that cannot terminate

18. Figure 5-10 Incorrect logic for greeting program because the loop control variable is not altered Programming Logic & Design, Sixth Edition

19. Avoiding Common Loop Mistakes (continued) • Mistake: • using the wrong comparison with the loop control variable • Programmers must use correct comparison • Seriousness depends on actions performed within a loop • Overcharge insurance customer by one month • Overbook a flight on airline application • Dispense extra medication to patients in pharmacy

20. Programming Logic & Design, Sixth Edition Figure 5-12 Incorrect logic for greeting program because the wrong test is made with the loop control variable

21. Avoiding Common Loop Mistakes (continued) • Mistake: • including statements inside the loop that belong outside the loop • Example: • discount every item by 30 percent • Inefficient because the same value is calculated 100 separate times for each price that is entered • Move outside loop for efficiency

22. Figure 5-13Inefficient way to produce 100 discount price stickers for differently priced items

23. Figure 5-14Improved discount sticker-making program

24. Using a forLoop • for statement or for loop is a definite loop • Puts all of the loop control expressions in the for loop header • Initialize • Test • Update • Takes the form: [initial, final] for loopControlVariable = initialValue to finalValue [step stepValue] do something endfor

25. Using a forLoop (continued) • Example pseudocode for count = 0 to 3 step 1 [0,3] output “Hello” end for • Initializescount to 0 • Checks count against the limit value 3 (test) • If evaluation is true, for statement body prints the label • Increases count by 1 (update)

26. Using a for Loop (continued) • while statement could be used in place of for statement • Step value: • number used to increase (decrease) a loop control variable on each pass through a loop • Programming languages can: • Require a statement that indicates the step value • Have a step value default of 1 • Specify step value when each pass through the loop changes the loop control variable by a value other than 1

27. Common Loop Applications • Using a loop to count and accumulate totals • Examples • Business reports often include totals • List of real estate sold and total value • Count of number of records processed • Counters and Accumulators: • Counter usually is incremented by one each iteration • Accumulator has some value added to it each iteration • Running sum

28. Common Loop Applications (continued) • Accumulate total real estate prices • Declare numeric variable at beginning • Initialize the accumulator to 0 • Read each transaction’s data record • Add its value to accumulator variable • Keep reading records until eof is encountered • Variables exist only for the life of the application • Run the application a second time; variables may occupy different memory location

29. Common Loop Applications (continued) 5 records processed Figure 5-16 Month-end real estate sales report

30. Figure 5-17 Flowchart and pseudocode for real estate sales report program

31. Common Loop Applications (continued) • Use a loop to validate data • When prompting a user for data, no guarantee that data is valid • Validate data: • make sure data falls in acceptable ranges • Example: user enters birth month • If number is less than 1 or greater than 12 • Display error message and stop the program • Assign default value for the month • Reprompt the user for valid input

32. Figure 5-18 Reprompting a user once after an invalid month is entered Programming Logic & Design, Sixth Edition

33. Figure 5-19 Reprompting a user using a loop after an invalid month is entered Programming Logic & Design, Sixth Edition

34. Common Loop Applications (continued) • Limiting a reprompting loop • Reprompting can be frustrating to a user if it continues indefinitely • Maintain count of the number of reprompts • Forcing a data item means: • Override incorrect data by setting the variable to a specific value

35. Common Loop Applications (continued) • Validating a data type • Validating data requires a variety of methods • isNumeric() or similar method • Provided with the language translator you use to write your programs • Black box • isChar() or isWhitespace() • Accept user data as a string • Use a method to convert to correct to data type

36. Common Loop Applications (continued) Figure 5-21 Checking data for correct type

37. Common Loop Applications (continued) • Validating reasonableness and consistency of data • Many data items can be checked for reasonableness • Good defensive programs try to foresee all possible inconsistencies and errors

38. Summary • Use a loop to: • write one set of instructions that operates on multiple, separate sets of data • Three steps must occur in every loop • Initialize loop control variable • Compare variable to some value • Alter the variable that controls the loop • Nested loops: • loops within loops • Nested loops maintain two individual loop control variables • Alter each at the appropriate time

39. 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 loop • number of iterations is known • while loop used with indefinite loop • number of iterations is not known

40. Summary (continued) • for loop expressions: • Initialize • Test • Update • Accumulator • variable that accumulates values creating a running sum • Counter • Variable used to keep track of the number of occurrences of something (records, customers processed, etc.) • Loops are often used to ensure user data is valid