Control Flow控制流程

How to use this guide如何使用本指南

Control flow is assessed in every curriculum we map to. All four agree on the core constructs: boolean conditions, selection (if/elif/else), and iteration (while, for). Where they diverge is on nesting depth and loop-control keywords: Alberta CSE1120 explicitly names nested conditional and nested iterative blocks; AP CSP limits to what can be expressed in its reference-sheet pseudocode. The table below tells you which sections are core for you. Every section uses pseudocode first, then Python.控制流程在我们对照的所有大纲中均为考核内容。四套大纲对核心构造高度一致：布尔条件（boolean，布尔）、选择（selection，选择）和迭代（iteration，迭代）。它们的分歧在于嵌套深度和循环控制关键字：阿尔伯塔 CSE1120 明确点名嵌套条件块和嵌套迭代块；AP CSP 限于其参考手册伪代码所能表达的内容。下表告诉你哪些节属于你的核心。所有节均先用伪代码，再用 Python。

Boolean Conditions & Comparison布尔条件与比较

SET age TO 17
SET has_ticket TO True
IF age >= 18 AND has_ticket THEN
    OUTPUT "Admitted"
ELSE
    OUTPUT "Not admitted"
END IF

age = 17
has_ticket = True
if age >= 18 and has_ticket:
    print("Admitted")
else:
    print("Not admitted")   # outputs: Not admitted

if / elif / elseif / elif / else

IF condition THEN
    statements
ELIF other_condition THEN
    statements
ELSE
    statements
END IF

INPUT score
IF score >= 90 THEN
    OUTPUT "A"
ELIF score >= 80 THEN
    OUTPUT "B"
ELIF score >= 70 THEN
    OUTPUT "C"
ELIF score >= 60 THEN
    OUTPUT "D"
ELSE
    OUTPUT "F"
END IF

score = int(input())
if score >= 90:
    print("A")
elif score >= 80:
    print("B")
elif score >= 70:
    print("C")
elif score >= 60:
    print("D")
else:
    print("F")

Nested Conditionals嵌套条件

INPUT age, height
IF age >= 12 THEN
    IF height >= 140 THEN
        OUTPUT "Enjoy the ride!"
    ELSE
        OUTPUT "Too short."
    END IF
ELSE
    OUTPUT "Too young."
END IF

age = int(input())
height = int(input())
if age >= 12:
    if height >= 140:
        print("Enjoy the ride!")
    else:
        print("Too short.")
else:
    print("Too young.")

while Loopswhile 循环

WHILE condition:
    body statements
END WHILE

SET count TO 5
WHILE count >= 1:
    OUTPUT count
    SET count TO count - 1
OUTPUT "Blast off!"

count = 5
while count >= 1:
    print(count)
    count -= 1
print("Blast off!")

for Loopsfor 循环

FOR item IN collection:
    body statements
END FOR

INPUT n
SET total TO 0
FOR i FROM 1 TO n:
    SET total TO total + i
OUTPUT total

n = int(input())
total = 0
for i in range(1, n + 1):   # range(1, n+1) gives 1, 2, ..., n
    total += i
print(total)

Loop Control: break & continue循环控制：break 与 continue

numbers = [3, 7, 12, 5, 8]
for n in numbers:
    if n % 2 == 0:
        print("First even:", n)
        break   # stop as soon as found

scores = [85, -1, 92, -5, 78]
for s in scores:
    if s < 0:
        continue   # skip invalid (negative) scores
    print(s)

Nested Loops嵌套循环

FOR i FROM 1 TO 3:          -- outer loop: rows
    FOR j FROM 1 TO 3:      -- inner loop: columns
        OUTPUT i * j
    END FOR
END FOR

for i in range(1, 4):       # outer: rows 1, 2, 3
    for j in range(1, 4):   # inner: columns 1, 2, 3
        print(i * j, end=" ")
    print()                 # newline after each row

1 2 3
2 4 6
3 6 9

Exam Strategy and Common Pitfalls考试策略与常见陷阱

Control flow questions appear in every CS exam format: multiple-choice trace, written pseudocode, short-answer justify. The tactics below cover the three most common failure modes.控制流程问题出现在每种 CS 考试形式中：多项选择追踪、书面伪代码、简答论证。以下策略涵盖三种最常见的失分模式。

Flashcards闪卡

Practice Quiz综合测验

Readiness Checklist准备就绪清单

Tick each item when you can do it cold, without notes, on a first attempt.能在无笔记、首次尝试下完成，再勾选每一项。

• ✓Evaluate any boolean expression with comparison and logical operators (==, !=, <, >, <=, >=, and, or, not) and state whether it is True or False. Distinguish x = 5 (assignment) from x == 5 (comparison). 🇺🇸 AP CSP 3.5 / 🇨🇦 AB CSE1120用比较和逻辑运算符（==、!=、<、>、<=、>=、and、or、not）求值任意布尔表达式，并说明其为 True 还是 False。区分 x = 5（赋值）和 x == 5（比较）。🇺🇸 AP CSP 3.5 / 🇨🇦 AB CSE1120
• ✓Write a complete if/elif/else selection in pseudocode and Python for a problem with at least three branches (e.g., grade classifier). Trace it with a value that hits each branch. 🇨🇦 ON ICS3U A2.2 / AP CSP 3.6为至少三个分支的问题（如等级分类器）用伪代码和 Python 写出完整的 if/elif/else 选择结构。用触及每个分支的值追踪它。🇨🇦 ON ICS3U A2.2 / AP CSP 3.6
• ✓Write and trace a nested conditional (if inside if). Explain why the inner block does not run when the outer condition is False. 🇨🇦 AB CSE1120 2.8.1 / AP CSP 3.7写出并追踪嵌套条件（if 内嵌 if）。解释为何外层条件为 False 时内层块不运行。🇨🇦 AB CSE1120 2.8.1 / AP CSP 3.7
• ✓Write a while loop that counts, accumulates, or searches. Identify what variable must be updated inside the body to prevent an infinite loop. Trace the loop with a variable table. 🇨🇦 ON ICS3U A2.2 / AB CSE1120 outcome 1写一个用于计数、累加或搜索的 while 循环。找出必须在循环体内更新哪个变量以防止无限循环。用变量表追踪循环。🇨🇦 ON ICS3U A2.2 / AB CSE1120 结果 1
• ✓Write a for loop using range(). Compute range(1, n+1) and explain why n+1 is needed to include n. State the number of iterations for any given range. 🇨🇦 BC CP11 / AP CSP 3.8用 range() 写 for 循环。计算 range(1, n+1) 并解释为何需要 n+1 才能包含 n。说明任意给定范围的迭代次数。🇨🇦 BC CP11 / AP CSP 3.8
• ✓Explain the difference between break (exits loop) and continue (skips to next iteration). Write one example of each and trace through one full iteration showing the effect. 🇨🇦 AB CSE1120 outcome 2.8解释 break（退出循环）和 continue（跳到下一次迭代）的区别。各写一个示例，并追踪一次完整迭代以显示效果。🇨🇦 AB CSE1120 结果 2.8
• ✓Write a nested loop (e.g., multiplication table). Compute the total iterations (outer × inner). Trace one full outer iteration manually showing all inner iterations. 🇨🇦 AB CSE1120 2.8.2 / ON ICS3U A2.3写一个嵌套循环（如乘法表）。计算总迭代次数（外层 × 内层）。手工追踪一次完整的外层迭代，展示所有内层迭代。🇨🇦 AB CSE1120 2.8.2 / ON ICS3U A2.3
• ✓Justify choosing for vs while for a given problem scenario in one or two sentences, citing readability and/or the known-vs-unknown iteration count. 🇺🇸 CSTA 3A-AP-15用一两句话为给定问题场景论证选择 for 还是 while，引用可读性和/或已知与未知迭代次数。🇺🇸 CSTA 3A-AP-15
• ✓Identify an off-by-one error in a given loop: spot whether < should be <= (or vice versa), state the incorrect output vs the expected output, and write the fix. 🇨🇦 BC CP11 / AB CSE1120识别给定循环中的差一错误：判断 < 是否应改为 <=（或反之），说明错误输出与预期输出，并写出修复。🇨🇦 BC CP11 / AB CSE1120
• ✓State verbatim the CSTA standard 3A-AP-15 and the Ontario ICS3U expectation A2.2, and identify which topic of this guide each one maps to. 🇺🇸 CSTA / 🇨🇦 ON逐字陈述 CSTA 标准 3A-AP-15 和安大略 ICS3U 期望 A2.2，并找出本指南中各自对应的主题。🇺🇸 CSTA / 🇨🇦 ON
• ✓Write a complete program combining if/elif/else inside a for or while loop (e.g., iterate a list, classify each item, accumulate counts). Trace it with two input values. 🇨🇦 ON ICS3U A2.3 / AB CSE1120写一个完整程序，将 if/elif/else 嵌套在 for 或 while 循环内（如遍历列表、对每项分类、累计计数）。用两个输入值追踪它。🇨🇦 ON ICS3U A2.3 / AB CSE1120

What This Feeds Into本单元的去向

Control flow is the engine that makes programs useful. Every subsequent HS CS unit builds on the selection and iteration constructs practiced here: Functions (Unit 4) use conditionals inside function bodies; Data Structures (Unit 5) use for loops to traverse lists; Searching and Sorting (Unit 7) is nested loops and conditionals throughout. Both AP courses also assume control-flow fluency from day one.控制流程是让程序有用的引擎。后续每个 HS CS 单元都建立在这里练习的选择和迭代构造之上：函数（第 4 单元）在函数体内使用条件语句；数据结构（第 5 单元）使用 for 循环遍历列表；搜索与排序（第 7 单元）通篇是嵌套循环（nested loops，嵌套循环）和条件语句。两门 AP 课程也从第一天起就假定控制流程流利度。