What is a Set in Python?

In Python, a set is an unordered (无序的) collection of unique (唯一的) elements. Unlike lists or tuples, sets do not allow duplicate (重复的) values, and their elements have no fixed position. Sets are defined using curly braces {} or the set() function. They are useful for tasks like removing duplicates from a list or checking membership (成员关系) efficiently.

Create a Set

You can create a set in two ways:

1. Using {} with elements separated by commas:

# A set of fruits (no duplicates)
fruits = {"apple", "banana", "cherry", "apple"}  # "apple" appears only once
print(fruits)  # Output: {'apple', 'banana', 'cherry'}

2. Using the set() function (useful for converting other data types like lists to sets):

# Convert a list to a set to remove duplicates
numbers = [1, 2, 2, 3, 4, 4, 4]
unique_numbers = set(numbers)
print(unique_numbers)  # Output: {1, 2, 3, 4}

# Create an empty set (note: {} creates a dictionary, not an empty set)
empty_set = set()

Important Features of Sets

• Unordered: Elements do not have a specific order, so you cannot access them by index.
• Unique: Each element appears only once; duplicate values are automatically removed.
• Mutable (可变的): You can add or remove elements after creating the set (but the elements themselves must be immutable, like numbers or strings).

Check if an Element Exists

Use the in keyword to check if an element is present in a set:

fruits = {"apple", "banana", "cherry"}
print("banana" in fruits)  # Output: True
print("grape" in fruits)  # Output: False

Add Elements to a Set

• add(): Adds a single element to the set.

fruits = {"apple", "banana"}
fruits.add("cherry")
print(fruits)  # Output: {'apple', 'banana', 'cherry'}

• update(): Adds multiple elements (from another set, list, or tuple).

vegetables = {"carrot", "potato"}
fruits.update(vegetables)  # Add all elements from vegetables set
fruits.update(["orange", "grape"])  # Add elements from a list
print(fruits)  # Output: {'apple', 'banana', 'cherry', 'carrot', 'potato', 'orange', 'grape'}

Remove Elements from a Set

• remove(value): Removes a specific element; raises an error if the element does not exist.

fruits = {"apple", "banana", "cherry"}
fruits.remove("banana")
print(fruits)  # Output: {'apple', 'cherry'}
# fruits.remove("grape")  # This will cause a KeyError

• discard(value): Removes an element if it exists; does nothing if the element is not found.

fruits.discard("grape")  # No error even if "grape" is not present

• pop(): Removes and returns a random element (since sets are unordered, the removed element is unpredictable).

random_fruit = fruits.pop()
print(random_fruit)  # Output: e.g., 'apple' (varies each time)
print(fruits)  # Output: {'cherry'} (if 'apple' was removed)

• clear(): Removes all elements from the set.

fruits.clear()
print(fruits)  # Output: set()

Set Operations

Sets support mathematical operations like union (并集), intersection (交集), difference (差集), and symmetric difference (对称差集).

1. Union (∪): Combines elements from two sets (duplicates removed).

set1 = {1, 2, 3}
set2 = {3, 4, 5}
union_set = set1.union(set2)  # or set1 | set2
print(union_set)  # Output: {1, 2, 3, 4, 5}

2. Intersection (∩): Returns elements common to both sets.

intersection_set = set1.intersection(set2)  # or set1 & set2
print(intersection_set)  # Output: {3}

3. Difference (-): Returns elements in the first set but not in the second.

difference_set = set1.difference(set2)  # or set1 - set2
print(difference_set)  # Output: {1, 2} (elements in set1 but not set2)

4. Symmetric Difference (⊕): Returns elements in either set but not in both.

symmetric_diff_set = set1.symmetric_difference(set2)  # or set1 ^ set2
print(symmetric_diff_set)  # Output: {1, 2, 4, 5}

Loop Through a Set

You can use a for loop to iterate over the elements in a set (order is not guaranteed):

fruits = {"apple", "banana", "cherry"}
for fruit in fruits:
    print(fruit)

Possible output (order may vary):

banana
apple
cherry

Set vs. List vs. Tuple: Key Differences

Example: Practical Use of Sets

Use Case 1: Remove Duplicates from a List
Suppose you have a list of scores with duplicates and want unique values:

scores = [85, 90, 85, 95, 90, 85]
unique_scores = set(scores)
print(unique_scores)  # Output: {85, 90, 95}

Use Case 2: Find Common Students in Two Classes

class1 = {"Alice", "Bob", "Charlie"}
class2 = {"Bob", "David", "Eve"}
common_students = class1.intersection(class2)
print(common_students)  # Output: {"Bob"}

Python集合介绍

什么是Python中的集合？

在Python中，**集合（set）**是一种无序的（unordered）唯一元素（unique elements）集合。与列表或元组不同，集合不允许重复（duplicate）值，且元素没有固定顺序。集合用花括号{}或set()函数定义。它们适用于从列表中删除重复项或高效检查成员关系（membership）等任务。

创建集合

创建集合有两种方式：

1. 使用{}并以逗号分隔元素：

# 水果集合（无重复项）
fruits = {"apple", "banana", "cherry", "apple"}  # "apple"仅出现一次
print(fruits)  # 输出：{'apple', 'banana', 'cherry'}

2. 使用set()函数（适用于将列表等其他数据类型转换为集合）：

# 将列表转换为集合以删除重复项
numbers = [1, 2, 2, 3, 4, 4, 4]
unique_numbers = set(numbers)
print(unique_numbers)  # 输出：{1, 2, 3, 4}

# 创建空集合（注意：{}创建的是字典，不是空集合）
empty_set = set()

集合的重要特性

• 无序性：元素没有特定顺序，因此不能通过索引访问。
• 唯一性：每个元素仅出现一次，重复值会被自动删除。
• 可变性（Mutable）：可以在创建后添加或删除元素（但元素本身必须是不可变的，如数字或字符串）。

检查元素是否存在

使用in关键字检查元素是否在集合中：

fruits = {"apple", "banana", "cherry"}
print("banana" in fruits)  # 输出：True
print("grape" in fruits)  # 输出：False

向集合中添加元素

• add()：向集合中添加单个元素。

fruits = {"apple", "banana"}
fruits.add("cherry")
print(fruits)  # 输出：{'apple', 'banana', 'cherry'}

• update()：添加多个元素（来自另一个集合、列表或元组）。

vegetables = {"carrot", "potato"}
fruits.update(vegetables)  # 添加vegetables集合中的所有元素
fruits.update(["orange", "grape"])  # 添加列表中的元素
print(fruits)  # 输出：{'apple', 'banana', 'cherry', 'carrot', 'potato', 'orange', 'grape'}

从集合中删除元素

• remove(value)：删除指定元素；若元素不存在则抛出错误。

fruits = {"apple", "banana", "cherry"}
fruits.remove("banana")
print(fruits)  # 输出：{'apple', 'cherry'}
# fruits.remove("grape")  # 这会导致KeyError错误

• discard(value)：若元素存在则删除；若不存在则不执行任何操作。

fruits.discard("grape")  # 即使"grape"不存在也不会报错

• pop()：删除并返回一个随机元素（由于集合无序，删除的元素不可预测）。

random_fruit = fruits.pop()
print(random_fruit)  # 输出：例如'apple'（每次运行结果可能不同）
print(fruits)  # 输出：{'cherry'}（假设删除了'apple'）

• clear()：清空集合中的所有元素。

fruits.clear()
print(fruits)  # 输出：set()

集合运算

集合支持并集（union）、交集（intersection）、差集（difference）和对称差集（symmetric difference）等数学运算。

1. 并集（∪）：合并两个集合的元素（去除重复项）。

set1 = {1, 2, 3}
set2 = {3, 4, 5}
union_set = set1.union(set2)  # 或 set1 | set2
print(union_set)  # 输出：{1, 2, 3, 4, 5}

2. 交集（∩）：返回两个集合的共同元素。

intersection_set = set1.intersection(set2)  # 或 set1 & set2
print(intersection_set)  # 输出：{3}

3. 差集（-）：返回第一个集合中存在但第二个集合中不存在的元素。

difference_set = set1.difference(set2)  # 或 set1 - set2
print(difference_set)  # 输出：{1, 2}（set1中有但set2中没有的元素）

4. 对称差集（⊕）：返回在任一集合中存在但不同时存在的元素。

symmetric_diff_set = set1.symmetric_difference(set2)  # 或 set1 ^ set2
print(symmetric_diff_set)  # 输出：{1, 2, 4, 5}

遍历集合

可以使用for循环遍历集合中的元素（顺序不固定）：

fruits = {"apple", "banana", "cherry"}
for fruit in fruits:
    print(fruit)

可能的输出（顺序可能不同）：

banana
apple
cherry

集合 vs. 列表 vs. 元组：主要区别

示例：集合的实际应用

场景1：从列表中删除重复项
假设你有一个包含重复分数的列表，需要获取唯一值：

scores = [85, 90, 85, 95, 90, 85]
unique_scores = set(scores)
print(unique_scores)  # 输出：{85, 90, 95}

场景2：查找两个班级的共同学生

class1 = {"Alice", "Bob", "Charlie"}
class2 = {"Bob", "David", "Eve"}
common_students = class1.intersection(class2)
print(common_students)  # 输出：{"Bob"}

专业词汇和不常用词汇表

set, /set/, 集合
unordered, /n'rdrd/, 无序的
unique, /ju'nik/, 唯一的
duplicate, /'duplket/, 重复的
membership, /'membrp/, 成员关系
mutable, /'mjutbl/, 可变的
union, /'junin/, 并集
intersection, /ntr'sekn/, 交集
difference, /'dfrns/, 差集
symmetric difference, /s'metrk 'dfrns/, 对称差集
iterate, /'tret/, 遍历