Program design tools

Planning before you code

• Before coding, designers draw the program's structure.
• Two key tools: the structure chart 结构图 and the state-transition diagram 状态转换图.
• Both make the design clear and reveal gaps before any code is written.

Structure charts

• A structure chart shows the hierarchical decomposition 分解 of a program into modules (subroutines).
• Each module is a rectangle; lines link a caller (above) to its callees (below).
• Small arrows show parameters passed down and results returned up — so you can read the subroutine signatures off it.
• It's a design-stage tool that captures how the program is broken into parts.

A flowchart plans a program's logic during the design stage.

State-transition diagrams

• A state-transition diagram shows the states a system can be in and the events that move it between them.
• Each state is a circle; each transition is an arrow labelled with its event.
• Great for vending machines, traffic lights, locks and user interfaces.
• It makes missing transitions easy to spot ("what if a second coin is inserted while awaiting selection?").

A structure chart: modules with the parameters passed between them

Top-down design 自顶向下设计

• Top-down (stepwise) design breaks a big problem into smaller sub-problems.
• Each sub-problem becomes a module, making the program easier to write and test.
• A structure chart or state-transition diagram can be turned into equivalent pseudocode.

Black-box tests the specification; white-box tests the code paths

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