Structured Design

Definition

Structured Design is a function-oriented design methodology that systematically transforms a DFD (Data Flow Diagram) into a Structure Chart, decomposing the system into a hierarchy of modules with well-defined interfaces, aiming for low coupling and high cohesion.

Process (Step-by-Step)

1. Draw the DFD for the system (from requirement analysis)
2. Identify the "center of the DFD" — classify the flow into:
   • Afferent (incoming) data flow — data moving toward the center, being transformed/refined
   • Efferent (outgoing) data flow — data moving away from the center toward output
   • Central transform — the core processing where input becomes output
3. Apply Transform Analysis or Transaction Analysis to convert the DFD into a Structure Chart
4. Refine the structure chart — apply design heuristics (fan-in/fan-out, coupling/cohesion rules) to improve module quality

Two Key Analysis Techniques

Transform Analysis

• Used when the DFD has a clear input → processing → output flow
• Maps to a structure chart with 3 main branches: Input, Process (Central Transform), Output

Transaction Analysis

• Used when the DFD has a transaction center — one input that branches into multiple different processing paths depending on the transaction type
• Maps to a structure chart where a dispatcher module calls different sub-modules per transaction type

Output: Structure Chart

A hierarchical diagram showing modules, their calling relationships, and data/control passed between them (covered in detail in the next topic).

Advantages

• Systematic, repeatable process from analysis (DFD) to design (Structure Chart)
• Encourages modularity, low coupling, high cohesion
• Well-established, widely taught methodology

Disadvantages

• Rigid separation of data and function (less natural for OOP-style systems)
• Can be tedious for very large systems

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Exam tip: Be ready to explain the DFD → Structure Chart transformation process and distinguish Transform Analysis (single input→output flow) from Transaction Analysis (branching by transaction type) — frequently tested.