集合

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

# 包含元素的集合
fruits = {"apple", "banana", "cherry"}
numbers = {1, 2, 3, 4, 5}

# 从其他序列创建
letters = set("hello")  # {'h', 'e', 'l', 'o'}
numbers = set(range(5))  # {0, 1, 2, 3, 4}

# 集合推导式
squares = {x**2 for x in range(10)}
print(squares)  # {0, 1, 4, 9, 16, 25, 36, 49, 64, 81}

# 添加元素
fruits = {"apple", "banana"}
fruits.add("orange")
fruits.add("apple")  # 重复元素不会添加
print(fruits)  # {'apple', 'banana', 'orange'}

# 更新集合
fruits.update(["cherry", "mango"])
fruits.update({"grape", "pear"})
print(fruits)  # 添加多个元素

# 删除元素
fruits.remove("banana")  # 如果元素不存在会引发 KeyError
fruits.discard("kiwi")   # 即使元素不存在也不会报错

popped = fruits.pop()    # 随机删除并返回一个元素
print(f"Removed: {popped}")

# 清空集合
fruits.clear()
print(fruits)  # set()

A = {1, 2, 3}
B = {3, 4, 5}
print(A | B)       # {1, 2, 3, 4, 5}
print(A.union(B))  # {1, 2, 3, 4, 5}

print(A & B)              # {3}
print(A.intersection(B))  # {3}

print(A - B)              # {1, 2}
print(A.difference(B))    # {1, 2}

print(A ^ B)                      # {1, 2, 4, 5}
print(A.symmetric_difference(B))  # {1, 2, 4, 5}

fs = frozenset([1, 2, 3])
d = {fs: "value"}  # 合法

# fs.add(4)  # 报错: frozenset 不可变

A = {1, 2, 3, 4, 5}
B = {4, 5, 6, 7, 8}

# 并集
print(A | B)   # {1, 2, 3, 4, 5, 6, 7, 8}
print(A.union(B))  # 同上

# 交集
print(A & B)   # {4, 5}
print(A.intersection(B))  # 同上

# 差集
print(A - B)   # {1, 2, 3}
print(A.difference(B))  # 同上

# 对称差集 (仅在其中一个集合中的元素)
print(A ^ B)   # {1, 2, 3, 6, 7, 8}
print(A.symmetric_difference(B))  # 同上

# 子集和超集
print({1, 2} <= A)  # True (子集)
print(A >= {1, 2})  # True (超集)
print({1, 6} <= A)  # False

# 不相交测试
print(A.isdisjoint({6, 7, 8}))  # False (有交集)
print(A.isdisjoint({9, 10}))    # True

# 长度
print(len({1, 2, 3}))  # 3

# 成员测试
print(2 in {1, 2, 3})  # True
print(5 not in {1, 2, 3})  # True

# 复制
numbers = {1, 2, 3}
numbers_copy = numbers.copy()

# 创建不可变集合
immutable = frozenset([1, 2, 3])

# 不可变集合可以作为字典的键
set_dict = {frozenset({1, 2}): "value"}
print(set_dict[frozenset({2, 1})])  # 'value'

# 不可变集合不支持修改操作
# immutable.add(4)  # 会引发 AttributeError

# 作为函数参数
def unique_count(elements):
    return len(set(elements))

# print(unique_count([1, 2, 2, 3, 3, 3]))  # 3

# 返回集合
def get_vowels(text):
    vowels = {"a", "e", "i", "o", "u"}
    return vowels.intersection(text.lower())

# print(get_vowels("Hello World"))  # {'e', 'o'}

def common_friends(friends1, friends2):
    """返回两个朋友列表的共同好友"""
    set1 = set(friends1)
    set2 = set(friends2)
    return set1 & set2

# alice_friends = ["Bob", "Charlie", "Diana"]
# bob_friends = ["Alice", "Charlie", "Eve"]
# print(common_friends(alice_friends, bob_friends))  # {'Charlie'}

def unique_words(filename):
    """统计文件中不同单词的数量"""
    with open(filename, "r") as f:
        words = set()
        for line in f:
            words.update(line.strip().split())
        return len(words)

# print(unique_words("sample.txt"))

def set_cover(universe, subsets):
    """近似解决集合覆盖问题"""
    elements = set(universe)
    selected = []

    while elements:
        # 选择覆盖最多剩余元素的子集
        subset = max(subsets, key=lambda s: len(elements & s))
        selected.append(subset)
        elements -= subset

    return selected

# universe = {1, 2, 3, 4, 5}
# subsets = [{1, 2, 3}, {2, 4}, {3, 4, 5}, {1, 5}]
# print(set_cover(universe, subsets))  # 可能输出 [{1, 2, 3}, {3, 4, 5}]