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Programming Logic and DesignFourth Edition, Introductory

Chapter 2

Understanding Structure

• 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

• Describe three special structures – case, do-while, and do-until

Programming Logic and Design, Introductory, Fourth Edition

• Spaghetticode – logically snarled program statements
• Can be the result of poor program design

Example: college admissions criteria

Programming Logic and Design, Introductory, Fourth Edition

Programming Logic and Design, Introductory, Fourth Edition

• 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

• 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

• Sequence structure
• A set of instructions, performed sequentially with no branching

Programming Logic and Design, Introductory, Fourth Edition

• 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

• Dual-alternative if: contains two alternatives

Programming Logic and Design, Introductory, Fourth Edition

• Single-alternative if: contains one alternative

Programming Logic and Design, Introductory, Fourth Edition

• Single-alternative if
• Else clause is not required
• Null case: situation where nothing is done

Programming Logic and Design, Introductory, Fourth Edition

• 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

• Loop structure

Programming Logic and Design, Introductory, Fourth Edition

• 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

Programming Logic and Design, Introductory, Fourth Edition

• 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

Programming Logic and Design, Introductory, Fourth Edition

Programming Logic and Design, Introductory, Fourth Edition

Programming Logic and Design, Introductory, Fourth Edition

• 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

• 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

• Unstructured loop:

Programming Logic and Design, Introductory, Fourth Edition

• Structured but nonfunctional loop

Programming Logic and Design, Introductory, Fourth Edition

• Functional but non-structured loop

Programming Logic and Design, Introductory, Fourth Edition

• Functional and structured loop

Programming Logic and Design, Introductory, Fourth Edition

• 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

• What is wrong with this design?

Programming Logic and Design, Introductory, Fourth Edition

• 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

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

• Is this flowchart structured?

Programming Logic and Design, Introductory, Fourth Edition

• To make it structured, pull each symbol out and rework
• B begins a selection structure

Programming Logic and Design, Introductory, Fourth Edition

• Pull up on the flowline from the left side of B

Programming Logic and Design, Introductory, Fourth Edition

• Next, pull up the flowline on the right side of B

Programming Logic and Design, Introductory, Fourth Edition

• 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

• Now pull up the flowline on the right side of D

Programming Logic and Design, Introductory, Fourth Edition

• Bring together the loose ends of D and of B

Programming Logic and Design, Introductory, Fourth Edition

• 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

• Using nested if-then-else for multiple alternatives

Programming Logic and Design, Introductory, Fourth Edition

• Using a case structure for multiple alternatives

Programming Logic and Design, Introductory, Fourth Edition

• 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

• 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

• 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

Programming Logic and Design, Introductory, Fourth Edition

Programming Logic and Design, Introductory, Fourth Edition

• How can this design be made structured?

Programming Logic and Design, Introductory, Fourth Edition

• Repeat the needed step to enforce structure

Programming Logic and Design, Introductory, Fourth Edition

• 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

• Flowchart can be made structured by untangling
• case structure: handles questions with multiple alternatives
• while loop: a pretest loop that asks the question first
• while loop statements may never be executed if the answer is No
• do-while and do-until loops: posttest loops that ask the question last

Programming Logic and Design, Introductory, Fourth Edition

• do-while and do-until loop statements are always executed at least once
• Posttest loop can be replaced by a sequence followed by a while loop

Programming Logic and Design, Introductory, Fourth Edition