Programming Logic and Design Fourth Edition, Introductory

Programming Logic and DesignFourth Edition, Introductory Chapter 2 Understanding Structure

Objectives • Describe the features of unstructured spaghetti code • Describe the three basic structures – sequence, selection, and loop • Use a priming read • Appreciate the need for structure • Recognize structure Programming Logic and Design, Introductory, Fourth Edition

Objectives (continued) • Describe three special structures – case, do-while, and do-until Programming Logic and Design, Introductory, Fourth Edition

Understanding Unstructured Spaghetti Code • Spaghetticode – logically snarled program statements • Can be the result of poor program design Example: college admissions criteria Programming Logic and Design, Introductory, Fourth Edition

Understanding Unstructured Spaghetti Code (continued) Programming Logic and Design, Introductory, Fourth Edition

Understanding Unstructured Spaghetti Code (continued) • Spaghetti code programs often work, but are difficult to read and maintain • Convoluted logic usually requires more code Programming Logic and Design, Introductory, Fourth Edition

Understanding the Three Basic Structures • Structure: a basic unit of programming logic • Any program can be constructed from only three basic types of structures • Sequence • Selection • Loop Programming Logic and Design, Introductory, Fourth Edition

Understanding the Three Basic Structures (continued) • Sequence structure • A set of instructions, performed sequentially with no branching Programming Logic and Design, Introductory, Fourth Edition

Understanding the Three Basic Structures (continued) • Selection structure • Asks a question, then takes one of two possible courses of action based on the answer • Also called a decision structure or an if-then-else Programming Logic and Design, Introductory, Fourth Edition

Understanding the Three Basic Structures (continued) • Dual-alternative if: contains two alternatives Programming Logic and Design, Introductory, Fourth Edition

Understanding the Three Basic Structures (continued) • Single-alternative if: contains one alternative Programming Logic and Design, Introductory, Fourth Edition

Understanding the Three Basic Structures (continued) • Single-alternative if • Else clause is not required • Null case: situation where nothing is done Programming Logic and Design, Introductory, Fourth Edition

Understanding the Three Basic Structures (continued) • Loop structure • Repeats a set of actions based on the answer to a question • Also called repetition or iteration • Question is asked first in the most common form of loop Programming Logic and Design, Introductory, Fourth Edition

Understanding the Three Basic Structures (continued) • Loop structure Programming Logic and Design, Introductory, Fourth Edition

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 • End-structure statements • Indicate the end of a structure • endif: ends an if-then-else structure • endwhile: ends a loop structure Programming Logic and Design, Introductory, Fourth Edition

Understanding the Three Basic Structures (continued) Programming Logic and Design, Introductory, Fourth Edition

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: group of statements that execute as a single unit Programming Logic and Design, Introductory, Fourth Edition

Understanding the Three Basic Structures (continued) Programming Logic and Design, Introductory, Fourth Edition

Understanding the Three Basic Structures (continued) • Each structure has one entry and one exit point • Structures attach to others only at entry or exit points Programming Logic and Design, Introductory, Fourth Edition

Using the Priming Read • Priming read (or priming input): • Reads the first input data record • Outside the loop that reads the rest of the records • Helps keep the program structured • 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 Programming Logic and Design, Introductory, Fourth Edition

Using the Priming Read (continued) • Unstructured loop: Programming Logic and Design, Introductory, Fourth Edition

Using the Priming Read (continued) • Structured but nonfunctional loop Programming Logic and Design, Introductory, Fourth Edition

Using the Priming Read (continued) • Functional but non-structured loop Programming Logic and Design, Introductory, Fourth Edition

Using the Priming Read (continued) • Functional and structured loop Programming Logic and Design, Introductory, Fourth Edition

Using the Priming Read (continued) • Priming read sets up the process so the loop can be structured • To analyze a flowchart’s structure, try writing pseudocode for it Programming Logic and Design, Introductory, Fourth Edition

Using the Priming Read (continued) • What is wrong with this design? Programming Logic and Design, Introductory, Fourth Edition

Understanding the Reasons for Structure • Advantages of structure: • Provides clarity • Professionalism • Efficiency • Ease of maintenance • Supports modularity Programming Logic and Design, Introductory, Fourth Edition

Understanding the Reasons for Structure (continued) Programming Logic and Design, Introductory, Fourth Edition

Recognizing Structure • Any set of instructions can be expressed in structured format • Is this flowchart structured? Programming Logic and Design, Introductory, Fourth Edition

Recognizing Structure (continued) • Is this flowchart structured? Programming Logic and Design, Introductory, Fourth Edition

Recognizing Structure (continued) • To make it structured, pull each symbol out and rework • B begins a selection structure Programming Logic and Design, Introductory, Fourth Edition

Recognizing Structure (continued) • Pull up on the flowline from the left side of B Programming Logic and Design, Introductory, Fourth Edition

Recognizing Structure (continued) • Next, pull up the flowline on the right side of B Programming Logic and Design, Introductory, Fourth Edition

Recognizing Structure (continued) • Pull up the flowline on the left side of D and untangle it from the B selection by repeating C Programming Logic and Design, Introductory, Fourth Edition

Recognizing Structure (continued) • Now pull up the flowline on the right side of D Programming Logic and Design, Introductory, Fourth Edition

Recognizing Structure (continued) • Bring together the loose ends of D and of B Programming Logic and Design, Introductory, Fourth Edition

Three Special Structures – Case, DoWhile, and DoUntil • Many languages allow three additional structures: • case structure • do-while structure • do-until structure • Case Structure: • Decisions with more than two alternatives • Tests a variable against a series of values and takes action based on a match • Nested if-then-else statements will do what a case structure does Programming Logic and Design, Introductory, Fourth Edition

Three Special Structures – Case, DoWhile, and DoUntil (continued) • Using nested if-then-else for multiple alternatives Programming Logic and Design, Introductory, Fourth Edition

Three Special Structures – Case, DoWhile, and DoUntil (continued) • Using a case structure for multiple alternatives Programming Logic and Design, Introductory, Fourth Edition

Three Special Structures – Case, DoWhile, and DoUntil (continued) • do-while and do-until loops • Question is asked at the end of the loop structure • Ensures that the loop statements are always used at least once Programming Logic and Design, Introductory, Fourth Edition

Three Special Structures – Case, DoWhile, and DoUntil (continued) • do-while loop executes as long as the question’s answer is Yes or True • do-until loop executes as long as the question’s answer is No or False (until it becomes Yes or True) Programming Logic and Design, Introductory, Fourth Edition

Three Special Structures – Case, DoWhile, and DoUntil (continued) • while loop with question at beginning is called a pretest loop • do-while and do-until with question at end are called posttest loops • A posttest loop can be replaced with a sequence followed by a pretest while loop Programming Logic and Design, Introductory, Fourth Edition

Three Special Structures – Case, DoWhile, and DoUntil (continued) Programming Logic and Design, Introductory, Fourth Edition

Three Special Structures – Case, DoWhile, and DoUntil (continued) • How can this design be made structured? Programming Logic and Design, Introductory, Fourth Edition

Three Special Structures – Case, DoWhile, and DoUntil (continued) • Repeat the needed step to enforce structure Programming Logic and Design, Introductory, Fourth Edition

Summary • Spaghetti code: snarled program logic • Three basic structures: sequence, selection, loop • These three can be combined by stacking and nesting • Priming read: statement that reads the first input data record • Structured techniques promote clarity, professionalism, efficiency, and modularity Programming Logic and Design, Introductory, Fourth Edition

Programming Logic and Design Fourth Edition, Introductory