Program Logic and Design: Lecture on Basic Structures

Introduction

• Goal: Discuss three basic programming structures.
• Avoid "spaghetti code" (unstructured programs).
• Focus on structured programming.

Three Basic Structures

Sequence Structure

• Definition: Performs actions in a linear order.
• Characteristics: No branching or skipping tasks.
• Example: Driving directions - actions executed step-by-step (e.g., turn right, turn left).
• Diagram Representation: Stacking boxes on top of each other.

Selection Structures

• Definition: Also known as decision structures; they involve making decisions.
• Symbol: Diamond shape in flowcharts.
• Types:
  • Dual Alternative Ifs (If-Then-Else):
    • Contains two alternatives (yes or no)
    • Example: If traffic is backed up, follow an alternate path.
  • Single Alternative Ifs:
    • Contains only one alternative followed by an end (e.g., If it’s raining, then take an umbrella).
    • Null case: If condition is false, nothing happens.
• Flowchart: Begins with a decision symbol and branches join at the bottom.

Loop Structures

• Definition: Repeats a set of actions while a condition remains true (called repetition or iteration).
• Common Form: While loop, where condition is tested first.
• Example: While hungry, continue eating.
• Flowchart Representation: Loop body symbols return to the decision point until the condition is false.

Stacking Structures

• Concept: Structures can be stacked end to end (sequence, selection, loop).
• Mistakes to Avoid: Do not interweave structures.
• End Statements:
  • End If: Ends an if-then-else structure.
  • End While: Ends a loop structure.
• Variability: Structures can be stacked in infinite ways.
• Import Rule: Yes/No for flowcharts, True/False for pseudocode.

Nesting Structures

• Definition: Placing one structure within another and indenting accordingly.
• Block: Group of statements executed as a single unit (e.g., curly braces {} in Java/C++).
• Examples:
  • Selection structure nested within a sequence.
  • Loop within a selection.
• Alignment: End statements (End If, End While) should be vertically aligned.

Characteristics of Structured Programs

• Single Entry/Exit Points:
  • Each structure has a single entry and single exit point.
  • These points are where structures can be connected.
• Combining Structures:
  • Structures can be connected or nested within each other only at entry/exit points.

Summary

• Foundational Structures:
  • Sequences, Selections, and Loops form the basis of structured programs.
• Connections: Entry and exit points are crucial for stacking/nesting structures.
• Example: Page 94 & Figure 3.11 in the textbook for detailed examples and diagrams.

Next Topic

• Priming input to structure a program.