12.2 Program Design 程序设计简答

Scope

本页整理 structure chart 与 state-transition diagram 的 purpose、symbols 和 reading。完整绘图及由图写 pseudocode 留到专项。

Structure chart purpose

A structure chart：

• Shows how a problem/program is decomposed into modules/sub-tasks。
• Shows module hierarchy and which module calls another。
• Shows selection and repetition of module calls。
• Shows parameters/data passed between modules and return values。
• Can distinguish procedure/function relationships through interface information。

它不显示每一条 algorithm statement 的执行顺序；那是 flowchart/pseudocode 的任务。

Structure chart symbols

• Rectangle：module/procedure/function。
• Connecting line：calling relationship/hierarchy。
• Diamond：selection/decision；one of alternative modules is called at run time。
• Curved/circular arrow：iteration/repetition；the connected calls are repeated, usually in shown order。
• Data arrow：parameter passed to a module。
• Return arrow：function return value；BYREF may be represented bidirectionally according to notation。

Explain 题必须用图中的 module names，例如：

The curved arrow represents iteration: Module-A repeatedly calls Sub-Y1, then Sub-Y2, then Sub-9.

Benefits of modules in design

• A specific task is isolated as one module。
• Repeated tasks can be reused/called from several places。
• Complexity is reduced。
• Independent testing/debugging and maintenance are easier。
• Parameters reduce reliance on global variables and make data flow clear。

State-transition diagrams

Document a system whose behaviour depends on its current state and an event/input。

• State：current mode/condition of the system。
• Transition：directed change from current state to next state。
• Event/input：causes transition。
• Output/action：may occur during transition。
• Self-loop：event occurs but system remains in the same state。
• Start/initial state and final/halting state may be indicated。

Reading method

• Begin at stated initial/current state。
• Follow only the arrow whose event matches the next input。
• Record any output on that transition。
• Update current state to the arrow destination。
• Repeat for each input in order。

For heater example, Temp < 10 changes Off to On；Temp > 20 changes On to Off。Intermediate temperatures use self-loops, avoiding rapid contradictory switching。

Exam checklist

• Structure chart purpose 是否写 module hierarchy + data/parameters？
• Diamond 与 curved arrow 是否分别识别 selection/iteration？
• Explain symbol 是否引用具体 module names？
• State transition 是否区分 current state、input、output、next state？
• Arrow direction 与 event label 是否准确？

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Structure chart/state-transition diagram 绘制及转 pseudocode 留到专项。