11.2 Constructs 程序结构简答

Scope

本页重点是选择并 justify loop/construct。IF、CASE、FOR、WHILE、REPEAT 完整语法留到代码专项。

Selection

• IF selects between paths based on Boolean condition；nested IF handles dependent decisions。
• CASE selects one branch from multiple discrete values and is clearer than a long chain of equality tests。

Loop structures

Count-controlled loop

Use when the number of iterations is known before the loop starts，例如 processing every element in a fixed-size array or repeating exactly 50 times。

Pre-condition loop

Condition is tested before the body, so it may execute zero times。Use when continuation/termination depends on a condition that must be checked first。

Post-condition loop

Condition is tested after the body, so it executes at least once。Use when input/process must occur once before the stopping condition can be known。

Justification patterns

Use a count-controlled loop because the number of iterations is known: it repeats once for each character in the string.

Use a conditional loop because the number of iterations is not known and the loop ends when a sentinel value is input.

Use a conditional loop so the search can terminate as soon as the first match is found, avoiding unnecessary iterations.

第一分给 structure name，第二分给 scenario-specific reason。

Efficiency

An efficient loop avoids unnecessary work：

• Stop searching after a required item is found。
• Do not repeatedly call a function whose inputs have not changed。
• Use a known count rather than repeatedly checking an expensive condition when appropriate。
• Avoid processing data after the result is already determined。

Exam checklist

• Iteration count 是否已知？
• Loop body 是否必须至少运行一次？
• 是否需要 sentinel/EOF/match 控制停止？
• Justification 是否联系具体 data/condition？
• 是否可 early exit 减少重复处理？

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完整 selection 与 iteration pseudocode 留到专项练习。