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