Programming Logic and Design Seventh Edition Chapter 3 Understanding Structure Objectives In this chapter, you will learn about: • The three basic control structures: – Sequence – Selection – Repetition ( aka looping and iteration ) • The need for structure • Using a priming input 2 Three Basic Control Structures Sequence, Selection, Repetition/Iteration/Looping Connector Use where flowlines come together in a flowchart 3 RAPTOR Flowchart Symbols Sequence Control Structures Selection and Repetition Control Structures 4 Understanding the Three Basic Control 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 that will execute in sequence (or sequentially). To the right of that, you see 3 selection structures that will execute in a sequence (sequentiall). 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 control structure Sequential execution of control structures 5 Understanding the Three Basic Control Structures (continued) expression usually referred to as the condition null case No structure to be executed Figure 3-4 Single-alternative if selection structure 6 Understanding the Three Basic Control Structures (continued) • Single-alternative if – Else clause is not required – If the condition does not evaluate to true, the structure(s) are not executed and control passes to the next structure • null case – Situation where nothing is done – This would be the "empty" else branch of a selection structure if employee belongs to dentalPlan then deduct $40 from employeeGrossPay endif 7 Understanding the Three Basic Control Structures (continued) no structure yes structure Figure 3-3 Dual-alternative if selection structure 8 Understanding the Three Basic Control Structures (continued) • Dual-alternative if – Contains two alternatives – If-then-else structure if someCondition [is true] then Pseudocode keywords: if, then, else, endif statement_1 structures else statement_2 endif 9 Understanding the Three Basic Control Structures (continued) • • • • • Some languages have an "else if" keyword The spelling of the keyword in languages varies (elseif / elsif / elif) Python uses the elif keyword Java and C++ do not have an "else-if" keyword Usage: if x == 1 then print 1 elif x == 2 then print 2 ... (additional elif statements) else print "no match for x" endif 10 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 (pre-test) or do … while (post-test) – RAPTOR lets you put the test anywhere you like! 11 Understanding the Three Basic Control Structures (continued) Entry Point Connector When using drawing software such as Visio Figure 3-5 12 Loop structure (pre-test form – while loop) Understanding the Three Basic Control Structures (continued) • Loop structure [ pre-test loop ] while testCondition is true do someStatement endwhile while count < 11 printCount() count += 1 endwhile 13 //module call Understanding the Three Basic Control Structures (continued) • All logic problems can be solved using only these three control structures • Structures can be combined in an infinite number of ways • Stacking – Attaching structures end-to-end at their entry/exit points • 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 pre-test loop structure – enddo ends a post-test loop structure 14 Understanding the Three Basic Control Structures (continued) Figure 3-6 Structured flowchart and pseudocode with three stacked structures 15 Understanding the Three Basic Control Structures (continued) • Any individual task or step can be inserted by creating a new structure or an existing structure can be replaced by a different 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 and C++ use { } to enlcose these – Python relies on indentation of the block (suite) 16 Understanding the Three Basic Control Structures (continued) block or compound statement Figure 3-7 Flowchart and pseudocode showing nested structures [ sequence nested within a selection ] 17 Understanding the Three Basic Control 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 18 Understanding the Three Basic Structures (continued) selection Note: The diamond flowchart symbol is used for both selection and loop structures. loop How do you know if you are looking at a selection or loop structure? A selection structure only has flowlines merging at the exit point for the structure Figure 3-9 Flowchart and pseudocode for selection structure with nested sequence and loop structures 19 A loop structure will always have flowlines merging above the test expression and the body of the loop Understanding the Three Basic Control Structures (continued) • Rules for creating a "structured" flowchart – Include only combinations of the three basic control structures (sequence, selection, loop) – Each of the structures has a single entry point and a single exit point – Structures can be stacked (connected) to one another only at their entry and exit points – Ideally whenever flowchart lines come together a connector is used (if drawn manually) – Selection and Loop structures can have nested structures 20 Using a Priming Input • Priming input (or priming read) – Does the first input operation outside of the loop that inputs the rest of the data – Inside the body of the loop the input operation is usually the last operation performed in the body of the loop – Note: it is not always necessary to do this… Some languages can “look ahead” to see if there is any data that can be input 21 Using a Priming Input (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 3-16 Programming Logic & Design, Sixth Edition 22 Using a Priming Input (continued) • Priming input sets up the process so the loop can be structured • To analyze a flowchart’s structure, try writing pseudocode for it start get inputNumber //priming input while not eof calculatedAnswer = inputNumber * 2 print calculatedAnswer get inputNumber endwhile stop 23 A priming input is OUTSIDE the loop structure. It precedes the structure. The input structure INSIDE the loop is usually the last structure nested in the loop. Figure 3-17 number-doubling problem 24 Figure 3-18 Structured but incorrect solution to the number-doubling problem Programming Logic & Design, Sixth Edition 25 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, and loop control structures 26 Flowchart Status • Flowcharts fall into one of the 2 following categories: 1. structured 2. unstructured 27 Recognizing Structure (continued) This is a no no! Understandable but not Structured Figure 3-21 Example 3 28 Recognizing Structure (continued) What are the structures? Are they: stacked? nested? Are they: structured? Figure 3-20 Example 2 Programming Logic & Design, Sixth Edition 29 Recognizing Structure (continued) Nested selection structures in RAPTOR Programming Logic and Design, Seventh Edition 30 Summary • Spaghetti code – Snarled program logic • Three basic control structures – Sequence, selection, and looping – Combined by stacking and/or nesting • Priming input – Statement that reads the first input data record 31