Programming Logic and Design Seventh Edition

Programming Logic and DesignSeventh 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

Three Basic Control StructuresSequence, Selection, Repetition/Iteration/Looping Connector Use where flowlines come together in a flowchart

RAPTOR Flowchart Symbols SequenceControl Structures Selection and Repetition Control Structures

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

Understanding the Three Basic Control Structures (continued) expression usually referred to as the condition null case No structure to be executed Figure 3-4Single-alternative if selection structure

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 ifemployee belongs to dentalPlanthen deduct $40 from employeeGrossPay endif

Understanding the Three Basic Control Structures (continued) no yes structure structure Figure 3-3Dual-alternative ifselection structure

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

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 elifkeyword • 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

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!

Understanding the Three Basic Control Structures (continued) Entry Point Connector When using drawing software such as Visio Figure 3-5Loop structure (pre-test form – while loop)

Understanding the Three Basic Control Structures (continued) • Loop structure [ pre-test loop ] whiletestConditionis true dosomeStatement endwhile while count < 11 printCount() //module call count += 1 endwhile

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 • enddoends a post-test loop structure

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

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)

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 ]

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

Understanding the Three Basic Structures (continued) selection Note: The diamond flowchart symbol is used for both selection and loop structures. 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 A loop structure will always have flowlines merging above the test expression and the body of the loop loop Figure 3-9 Flowchart and pseudocode for selection structure with nested sequence and loop structures

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 nestedstructures

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

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

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 getinputNumber//priming input while not eof calculatedAnswer = inputNumber * 2 printcalculatedAnswer getinputNumber endwhile stop

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 3-17number-doubling problem

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

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

Flowchart Status • Flowcharts fall into one of the 2 following categories: • structured • unstructured

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

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

Recognizing Structure (continued) Nested selection structures in RAPTOR Programming Logic and Design, Seventh Edition

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

Programming Logic and Design Seventh Edition