Strings and Text Processing字符串与文本处理

How to use this guide如何使用本指南

Strings are present in every curriculum, but the depth of coverage varies. AP CSP Topic 3.4 directly assesses string operations via the exam reference sheet. Ontario ICS3C A1.2 is the most explicit string-manipulation expectation, naming character swapping, capitalisation, extracting substrings, and counting occurrences — all covered here. Ontario ICS3U A1.1 covers strings as a variable type. BC has no dedicated string standard but anchors to "manipulate numbers and text" in Computer Studies 10. Alberta CSE1110/CSE1120 name strings as a data type and concatenation as an operator. The table below shows which rows are core for you; each section cites the curriculum document it was checked against.字符串出现在所有课程大纲中，但覆盖深度各有不同。AP CSP 主题 3.4 通过考试参考手册直接评估字符串操作。安大略 ICS3C A1.2 是最明确的字符串操作期望，点名了字符交换、首字母大写、提取子字符串和计算出现次数——本指南均有涵盖。安大略 ICS3U A1.1 将字符串作为变量类型涵盖。BC 没有专门的字符串标准，但依附于 Computer Studies 10 中的"操作数字和文本"。阿尔伯塔 CSE1110/CSE1120 将字符串列为数据类型，将拼接列为运算符。下表显示哪些行对你是核心内容；每节均注明所依据的课纲文件。

String Basics and Indexing字符串基础与索引

name = "Dingrui"
print(name[0])    # D
print(name[3])    # g
print(name[-1])   # i
print(name[-3])   # u
print(len(name))  # 7

String Slicing字符串切片

date = "2024-06-15"
year  = date[0:4]   # "2024"
month = date[5:7]   # "06"
day   = date[8:10]  # "15"
print(year, month, day)  # 2024 06 15

String Methods字符串方法

row = "  Alice , Biology , 92 "
fields = row.split(",")          # ["  Alice ", " Biology ", " 92 "]
clean  = [f.strip().lower() for f in fields]
print(clean)                     # ["alice", "biology", "92"]

Concatenation and String Formatting字符串拼接与格式化

name    = "Alice"
subject = "Biology"
score   = 92

# Method 1: + concatenation
line1 = name + " | " + subject + " | " + str(score)

# Method 2: f-string
line2 = f"{name} | {subject} | {score}"

# Method 3: .format()
line3 = "{} | {} | {}".format(name, subject, score)

print(line1)  # Alice | Biology | 92
print(line2)  # Alice | Biology | 92
print(line3)  # Alice | Biology | 92

Searching Within Strings在字符串中搜索

sentence = "the quick brown fox"

# in — membership test
print("fox" in sentence)        # True
print("cat" in sentence)        # False

# find — first index (-1 if absent)
print(sentence.find("quick"))   # 4
print(sentence.find("cat"))     # -1

# count — occurrences
print(sentence.count("o"))      # 2  (brown, fox)
print(sentence.count("the"))    # 1

Looping Over Characters遍历字符

word   = "education"
vowels = "aeiou"

# Style 1: for-each (cleaner)
count1 = 0
for ch in word:
    if ch in vowels:
        count1 += 1
print(count1)   # 5

# Style 2: for-range (index-based)
count2 = 0
for i in range(len(word)):
    if word[i] in vowels:
        count2 += 1
print(count2)   # 5

Text-Processing Worked Example: Word Count文本处理综合例题：词频统计

sentence = "the cat sat on the mat and the cat"

# Step 1: Normalise
text  = sentence.lower().strip()

# Step 2: Split into words
words = text.split()           # ["the","cat","sat","on","the","mat","and","the","cat"]

# Step 3: Count total words
total = len(words)
print(f"Total words: {total}") # Total words: 9

# Step 4: Frequency of each unique word
freq = {}
for w in words:
    if w in freq:
        freq[w] += 1
    else:
        freq[w] = 1
print(freq)
# {'the': 3, 'cat': 2, 'sat': 1, 'on': 1, 'mat': 1, 'and': 1}

# Step 5: Most frequent word
top = max(freq, key=freq.get)
print(f"Most frequent: '{top}' ({freq[top]} times)")
# Most frequent: 'the' (3 times)

# Step 6: Target search
target = "cat"
print(f"'{target}' appears: {'yes' if target in words else 'no'}")

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

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。

• ✓Given any string, state the character at a specific positive index and negative index without running code. Explain why indices start at 0. 🇺🇸 AP CSP 3.4 / 🇨🇦 AB CSE1110 2.4.3给定任意字符串，不运行代码说出特定正索引和负索引处的字符。解释为何索引从 0 开始。🇺🇸 AP CSP 3.4 / 🇨🇦 AB CSE1110 2.4.3
• ✓Predict the output of any slice expression s[start:stop] or s[start:stop:step] by applying the "stop is excluded" rule. 🇺🇸 AP CSP Skill 4.B / 🇨🇦 ON ICS3C A1.2通过应用"stop 不含"规则，预测任意切片表达式 s[start:stop] 或 s[start:stop:step] 的输出。🇺🇸 AP CSP 技能 4.B / 🇨🇦 ON ICS3C A1.2
• ✓Use upper(), lower(), split(), join(), and replace() correctly and explain why assigning the return value is required. 🇨🇦 ON ICS3C A1.2 / AB CSE1120 2.6正确使用 upper()、lower()、split()、join() 和 replace()，并解释为何需要赋值返回值。🇨🇦 ON ICS3C A1.2 / AB CSE1120 2.6
• ✓Write a formatted output line using an f-string that embeds at least one variable and one computed expression. 🇨🇦 AB CSE1110 2.4.6 / AB CSE1120 2.6用 f-string 写出包含至少一个变量和一个计算表达式的格式化输出行。🇨🇦 AB CSE1110 2.4.6 / AB CSE1120 2.6
• ✓Distinguish the return values of in (bool), find() (int / -1), and count() (int); predict each for a given string and target without running the code. 🇺🇸 AP CSP 3.4 / 🇨🇦 ON ICS3C A1.2区分 in（布尔值）、find()（整数 / -1）和 count()（整数）的返回值；对给定字符串和目标，不运行代码预测各自结果。🇺🇸 AP CSP 3.4 / 🇨🇦 ON ICS3C A1.2
• ✓Write a for-each loop that counts vowels in a string from scratch, without using built-in methods. 🇨🇦 ON ICS3C A1.2 / BC CS10从头编写一个 for-each 循环，不使用内置方法统计字符串中的元音。🇨🇦 ON ICS3C A1.2 / BC CS10
• ✓Explain when to use for ch in s vs for i in range(len(s)), and give a task that requires each form. 🇨🇦 ON ICS3C A1.2解释何时使用 for ch in s，何时使用 for i in range(len(s))，并各举一个需要该形式的任务。🇨🇦 ON ICS3C A1.2
• ✓Write the three-step word-count pipeline (lower(), split(), dict loop) for a given sentence and produce the correct frequency dictionary. 🇨🇦 ON ICS3C A1.2 "count occurrences of a word"为给定句子编写三步词频统计流水线（lower()、split()、字典循环）并生成正确的频率字典。🇨🇦 ON ICS3C A1.2"计算单词出现次数"
• ✓Trace a string-expression chain (e.g., " Hello ".strip().lower().replace("h","j")) step by step, writing the intermediate string after each method call. 🇺🇸 AP CSP Skill 4.B逐步追踪字符串表达式链（如 " Hello ".strip().lower().replace("h","j")），在每次方法调用后写出中间字符串。🇺🇸 AP CSP 技能 4.B
• ✓Extract the year, month, and day from a date string of the form "YYYY-MM-DD" using slicing, without using split(). 🇨🇦 ON ICS3C A1.2 "extract a portion"仅用切片（不用 split()）从 "YYYY-MM-DD" 格式的日期字符串中提取年、月、日。🇨🇦 ON ICS3C A1.2"提取一部分"
• ✓Honors — ICS3C / CSE1120 Write a program that removes all punctuation from a sentence before doing a word-count, producing clean frequency data. 🇨🇦 ON ICS3C A1.2 / AB CSE1120 2.6荣誉 — ICS3C / CSE1120 编写一个程序，在词频统计前去除句子中的所有标点，生成干净的频率数据。🇨🇦 ON ICS3C A1.2 / AB CSE1120 2.6

What This Feeds Into本单元的去向

String manipulation is a prerequisite skill for almost every real program. Within the HS Computer Science series, the data-structures and searching units depend on your ability to tokenise text, compare strings, and loop character by character. Both downstream AP courses test string operations directly. The link below points to the AP CSA guide confirmed to exist in this repo; AP CSA Unit 1 introduces Java String methods (which closely parallel the Python methods in this guide).字符串操作是几乎所有真实程序的先决技能。在 HS Computer Science 系列中，数据结构和搜索单元依赖于你分词、比较字符串和逐字符循环的能力。两门下游 AP 课程都直接测试字符串操作。以下链接指向本仓库中已确认存在的 AP CSA 指南；AP CSA Unit 1 介绍 Java String 方法（与本指南中的 Python 方法高度对应）。