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71 lines
1.2 KiB
Markdown
71 lines
1.2 KiB
Markdown
# 不可变集合
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对应于元组（`tuple`）与列表（`list`）的关系，对于集合（`set`），**Python**提供了一种叫做不可变集合（`frozen set`）的数据结构。
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使用 `frozenset` 来进行创建：
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In [1]:
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```py
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s = frozenset([1, 2, 3, 'a', 1])
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s
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```
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Out[1]:
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```py
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frozenset({1, 2, 3, 'a'})
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```
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与集合不同的是，不可变集合一旦创建就不可以改变。
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不可变集合的一个主要应用是用来作为字典的键，例如用一个字典来记录两个城市之间的距离：
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In [2]:
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```py
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flight_distance = {}
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city_pair = frozenset(['Los Angeles', 'New York'])
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flight_distance[city_pair] = 2498
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flight_distance[frozenset(['Austin', 'Los Angeles'])] = 1233
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flight_distance[frozenset(['Austin', 'New York'])] = 1515
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flight_distance
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```
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Out[2]:
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```py
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{frozenset({'Austin', 'New York'}): 1515,
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frozenset({'Austin', 'Los Angeles'}): 1233,
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frozenset({'Los Angeles', 'New York'}): 2498}
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```
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由于集合不分顺序，所以不同顺序不会影响查阅结果：
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In [3]:
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```py
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flight_distance[frozenset(['New York','Austin'])]
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```
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Out[3]:
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```py
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1515
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```
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In [4]:
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```py
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flight_distance[frozenset(['Austin','New York'])]
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```
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Out[4]:
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```py
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1515
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``` |