Programming Logic and Design Seventh Edition

1. Programming Logic and DesignSeventh Edition Chapter 3 Understanding Structure

2. Objectives In this chapter, you will learn about: • The three basic control structures: • Sequence • Selection • Repetition ( aka looping and iteration ) • The need for structure • The features of unstructured spaghetti code • Recognizing structure • Structuring and modularizing unstructured logic • Using a priming input to structure a program

3. Three Basic Control StructuresSequence, Selection, Repetition/Iteration/Looping

4. RAPTOR Symbols SequenceControl Structures Selection and Repetition Control Structures

5. Understanding Unstructured Spaghetti Code • Spaghetti code • Logically snarled program statements • Can be the result of poor program design • Programs often work but are difficult to read and maintain • Convoluted logic usually requires more code • Unstructured programs • Do notfollow the rules of structured logic • Structured programs • Do follow rules of structured logic • RAPTOR does not permit you to draw an unstructured flowchart

6. Figure 3-1Spaghetti code logic for washing a dog

7. Understanding the Three Basic Structures • Structure • A Control Structure is a basic unit of programming logic • Sequence • Perform a single action ( input, output, assign, … ) • Selection ( aka decision ) • Ask a question, take one of two actions • Dual-alternative or single-alternativeif's • Loop ( aka repetition/iteration ) • Repeat actions based on answer to a question

8. Understanding the Three Basic Structures (continued) There is a distinction between sequence and a sequence structure. The book doesn't make this distinction which is unfortunate. On the right, you see 3 sequence structures being executed in sequence ( or sequentially ). To the right of that, you see 3 selection structures in a sequence. You enter a control structure through an entry point and exit from an exit point. Control structures can be stacked and nested. Stacked control structures are always executed in sequence. Figure 3-2 Sequence

9. Understanding the Three Basic Structures (continued) question usually referred to as the condition no yes statement statement Figure 3-3Selection structure

10. Understanding the Three Basic Structures (continued) • Dual-alternative if • Contains two alternatives • If-then-else structure Pseudocode keywords: if, then, do, else, endif ifsomeCondition [is true] then do statement_1 else do statement_2 endif statements

11. Understanding the Three Basic Structures (continued) • Single-alternative if • Else clause is not required • If the condition does not evaluate to true, the statement(s) are not executed and control passes to the next structure • null case • Situation where nothing is done ifemployee belongs to dentalPlan then do deduct $40 from employeeGrossPay endif

12. Understanding the Three Basic Structures (continued) Figure 3-4Single-alternative selection structure

13. Understanding the Three Basic Structures (continued) • Loop structure • Repeats a set of actions based on the answer to a question • Loop body • Also called repetition or iteration • Question is asked first in the most common form of a loop • while … do (pre-test) or do … while (post-test) • RAPTOR lets you put the test anywhere you like!

14. Understanding the Three Basic Structures (continued) Connector When using drawing software such as Visio Figure 3-5Loop structure (pre-test form – while … do)

15. Understanding the Three Basic Structures (continued) • Loop structure [ pre-test loop ] whiletestCondition continues to be true do someStatement endwhile while count less than 11 do printCount() //module call do add 1 to count endwhile

16. Understanding the Three Basic Structures (continued) • All logic problems can be solved using only these three structures • Structures can be combined in an infinite number of ways • Stacking • Attaching structures end-to-end • Nesting • discussed a few slides later… • End-structure statements • Indicate the end of a structure • endif ends an if-then-else structure • endwhile ends a loop structure

17. Understanding the Three Basic Structures (continued) Connector Figure 3-6 Structured flowchart and pseudocode with three stacked structures

18. Understanding the Three Basic Structures (continued) • Any individual task or step in a structure can be replaced by a structure • Nesting • Placing one structure within another • Indent the nested structure’s statements • Block [ aka compound statement ] • Group of statements that execute as a single unit • Java uses { } to enlcose these

19. Understanding the Three Basic Structures (continued) block or compound statement Figure 3-7 Flowchart and pseudocode showing nested structures [ sequence nested within a selection ]

20. Understanding the Three Basic Structures (continued) entry point and exit point for a structure structures have ONE of each Figure 3-8 Flowchart and pseudocode showing nested structures a loop nested within a sequence, nested within a selection

21. Understanding the Three Basic Structures (continued) Figure 3-9 Flowchart and pseudocode for selection structure with nested sequence and loop structures

22. Understanding the Three Basic Structures (continued) • Structured programs have the following characteristics: • Include only combinations of the three basic structures • Each of the structures has a single entry point and a single exit point • Structures can be stacked or connected to one another only at their entry or exit points • Selection and Loop structures can have nestedstructures

23. Using a Priming Input to Structurea Program • Priming read (or priming input) • Reads the first input data record outside of the loop that reads the rest of the records • Helps keep the program structured • Note: it is not always necessary to do this… • Analyze a flowchart for structure one step at a time • Watch for unstructured loops that do not follow this order: • First ask a question • Take action based on the answer • Return to ask the question again

24. Using a Priming Input to Structure a Program (continued) Figure 3-15Structured, but nonfunctional, flowchart of number-doubling problem

25. Using a Priming Input to Structure a Program (continued) RAPTOR would let you do this because there are languages that support a “mid-test” condition. Our book only considers pre-test and post-test loops as structured. Figure 3-16Functional but unstructuredflowchart Programming Logic & Design, Sixth Edition

26. Using a Priming Input to Structure a Program (continued) • Priming read sets up the process so the loop can be structured • To analyze a flowchart’s structure, try writing pseudocode for it start getinputNumber //priming read while not eof calculatedAnswer = inputNumber * 2 print calculatedAnswer get inputNumber endwhile stop

27. Figure 3-17Functional, structured flowchart for the number-doubling problem

28. Figure 3-18Structured but incorrect solution to the number-doubling problem Programming Logic & Design, Sixth Edition

29. Understanding the Reasons for Structure • Clarity • Professionalism • Efficiency • Ease of maintenance • Supports modularity

30. Recognizing Structure • Any set of instructions can be expressed in structured format • Any task to which you can apply rules can be expressed logically using sequence, selection, loop • It can be difficult to detect whether a flowchart is structured

31. Flowchart Status • Flowcharts fall in to one of the 3 following categories: • structured AND functional • structured but NOT functional • functional but NOT structured • These categories will be presented over the next several slides.

32. Recognizing Structure (continued) What are the structures? Are they: stacked? nested? Are they: structured? functional? Figure 3-20 Example 2 Programming Logic & Design, Sixth Edition

33. Recognizing Structure (continued) This is a no no! Functional but not Structured Figure 3-21 Example 3

34. Recognizing Structure (continued) • Single process like G is part of an acceptable structure • At least the beginning of a sequence structure Figure 3-22Untangling Example 3, first step

35. Recognizing Structure (continued) • H begins a selection structure • Sequences never have decisions in them • Logic never returns to G Figure 3-23Untangling Example 3, second step

36. Recognizing Structure (continued) • Pull up on the flowline from the left side of H Figure 3-24 Untangling Example 3, third step

37. Recognizing Structure (continued) • Next, pull up the flowline on the right side of H Figure 3-25Untangling Example 3, fourth step

38. Recognizing Structure (continued) • Pull up the flowline on the left side of I and untangle it from the B selection by repeating J Figure 3-26Untangling Example 3, fifth step

39. Recognizing Structure (continued) • Now pull up the flowline on the right side of I Figure 3-27Untangling Example 3, sixth step

40. Recognizing Structure (continued) • Bring together the loose ends of I and of H Figure 3-28 Finished flowchart and pseudocode for untangling Example 3

41. Structuring and ModularizingUnstructured Logic • Dog-washing process • Unstructured • Can be reconfigured to be structured • First step simple sequence Figure 3-29 Washing the dog, part 1

42. Structuring and ModularizingUnstructured Logic (continued) • After the dog runs away • Catch the dog and determine whether he runs away again • A loop begins Figure 3-30 Washing the dog, part 2

43. Figure 3-31 Washing the dog, part 3 Programming Logic & Design, Sixth Edition

44. Figure 3-33 Structured dog-washing flowchart and pseudocode Programming Logic & Design, Sixth Edition

45. Figure 3-34 Modularized version of the dog-washing program Programming Logic & Design, Sixth Edition

46. Summary • Spaghetti code • Snarled program logic • Three basic structures • Sequence, selection, and loop • Combined by stacking and nesting • Priming read • Statement that reads the first input data record

47. Summary (continued) • Structured techniques promote: • Clarity • Professionalism • Efficiency • Modularity • Flowchart can be made structured by untangling