2.1 列表的创建与操作 / Creating and Manipulating Lists

中文：列表是有序可变序列，用方括号创建。索引从 0 开始，负数索引从末尾倒数。
English: Lists are ordered, mutable sequences created with square brackets. Indexing starts at 0; negative indices count from the end.

python

fruits = ["apple", "banana", "cherry"] print(fruits[0]) # apple 第一个 / first print(fruits[-1]) # cherry 最后一个 / last print(fruits[1:3]) # ['banana', 'cherry'] 切片 / slicing fruits.append("date") # 末尾添加 / append at end fruits.insert(1, "blueberry") # 指定位置插入 / insert at index fruits.remove("banana") # 按值删除 / remove by value last = fruits.pop() # 弹出末尾 / pop last print(fruits) # ['apple', 'blueberry', 'cherry'] print(last) # date

2.2 列表推导式 / List Comprehensions

中文：列表推导式用一行代码生成列表，语法为[表达式 for 元素 in 可迭代对象 if 条件]。
English: A list comprehension builds a list in one line:[expression for item in iterable if condition].

python

numbers = [1, 2, 3, 4, 5, 6] # 每个元素平方 / square each element squares = [n ** 2 for n in numbers] print(squares) # [1, 4, 9, 16, 25, 36] # 只保留偶数 / keep only even numbers evens = [n for n in numbers if n % 2 == 0] print(evens) # [2, 4, 6] # 带条件表达式 / with conditional expression labels = ["偶 / even" if n % 2 == 0 else "奇 / odd" for n in numbers] print(labels)

2.3 字典的创建与遍历 / Creating and Iterating Dictionaries

中文：字典存储键值对，键必须可哈希且唯一。用get()访问不存在的键可返回默认值，避免KeyError。
English: Dictionaries store key-value pairs. Keys must be hashable and unique. Useget()to avoidKeyErrorwhen a key may be missing.

python

student = { "name": "Alice", "age": 20, "major": "Computer Science" } print(student["name"]) # Alice print(student.get("gpa", "N/A")) # N/A 默认值 / default student["gpa"] = 3.8 # 新增键 / add key student["age"] = 21 # 修改值 / update value # 遍历 / iteration for key, value in student.items(): print(f"{key}: {value}") # 只遍历键 / iterate keys only print(list(student.keys()))

2.4 条件判断 / Conditional Statements

中文：elif是 else if 的缩写。Python 用缩进表示代码块，通常为 4 个空格。
English:elifis short for else-if. Python uses indentation (typically 4 spaces) to define blocks.

python

score = 85 if score >= 90: grade = "A" elif score >= 80: grade = "B" elif score >= 70: grade = "C" else: grade = "D" print(f"分数 / Score: {score}, 等级 / Grade: {grade}")

2.5 循环 / Loops

中文：for用于遍历可迭代对象，while在条件为真时重复执行。break跳出循环，continue跳过本次迭代。
English:foriterates over an iterable;whilerepeats while a condition is true.breakexits the loop;continueskips the current iteration.

python

# for 循环 / for loop for i in range(1, 6): # 1 到 5 / 1 to 5 if i == 3: continue # 跳过 3 / skip 3 print(i, end=" ") print() # 1 2 4 5 # while 循环 / while loop count = 0 while count < 3: print(f"count = {count}") count += 1 # 带 else 的循环 / loop with else for n in [2, 4, 6]: if n % 2 != 0: print("发现奇数 / found odd") break else: print("全部是偶数 / all even")

2.6 练习 / Exercise

中文：统计一段文本中每个单词出现的次数，按次数降序输出前 3 个。
English: Count word frequencies in a text and print the top 3 in descending order.

python

text = "the quick brown fox jumps over the lazy dog the fox" words = text.split() counter = {} for word in words: counter[word] = counter.get(word, 0) + 1 # 按次数降序排序 / sort by count descending top3 = sorted(counter.items(), key=lambda x: x[1], reverse=True)[:3] for word, count in top3: print(f"{word}: {count}")

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