一、集合

1、集合的定義

In [74]: s = {}
In [74]: s = {}    # 空大括號是空的字典

In [75]: type(s)
Out[75]: dict

In [77]: type(s)
Out[77]: set

In [78]: help(set)

Help on class set in module builtins:

class set(object)
 |  set() -> new empty set object
 |  set(iterable) -> new set object
 |  
 |  Build an unordered collection of unique elements.
 |  
 |  Methods defined here:
 
 
In [80]: s = set([1, 2])

In [81]: s
Out[81]: {1, 2}

In [82]: s = set("xxj")

In [83]: s
Out[83]: {‘j‘, ‘x‘}

In [84]: s = {1, 2, 1, 3}

In [85]: s
Out[85]: {1, 2, 3}

集合是無序的，元素不能重復，元素要能被哈希（hash，不可變）

二、集合的操作

1、增

z## set.add()

In [86]: s
Out[86]: {1, 2, 3}

In [87]: s.add("a")   # 原地增加單個元素，元素要可哈希

In [88]: s
Out[88]: {1, 2, 3, ‘a‘}

In [89]: s.add(3)

In [90]: s
Out[90]: {1, 2, 3, ‘a‘}

In [93]: s.add([1, 2])
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-93-2beaf0c16593> in <module>()
----> 1 s.add([1, 2])

TypeError: unhashable type: ‘list‘

In [94]: help(s.add)


In [95]: s.add((1, 2))

In [96]: s
Out[96]: {(1, 2), 1, 2, 3, ‘a‘}


## set.update()  # 原地增加可叠代對象的元素

In [99]: help(s.update)

Help on built-in function update:

update(...) method of builtins.set instance
    Update a set with the union of itself and others.

    
In [127]: s = set()

In [128]: s
Out[128]: set()

In [129]: type(s)
Out[129]: set  
    
In [101]: s.update(10)
-----------------------------------------------------------------------
TypeError                                 Traceback (most recent call l
<ipython-input-101-c184888ad9c5> in <module>()
----> 1 s.update(10)

TypeError: ‘int‘ object is not iterable


In [131]: s.update(["a"])

In [132]: s
Out[132]: {‘a‘}

In [133]: s.update(["a"], ["b"])

In [134]: s
Out[134]: {‘a‘, ‘b‘}

In [135]: s.update(["a"], ["b"], 1)
-----------------------------------------------------------------------
TypeError                             Traceback (most recent call last)
<ipython-input-135-fc556b8d9726> in <module>()
----> 1 s.update(["a"], ["b"], 1)

TypeError: ‘int‘ object is not iterable

In [136]: s.update(["a"], ["b"], "xj")

In [137]: s
Out[137]: {‘a‘, ‘b‘, ‘j‘, ‘x‘}

In [139]: s.update([["S", "B"]])
-----------------------------------------------------------------------
TypeError                             Traceback (most recent call last)
<ipython-input-139-da563f39a191> in <module>()
----> 1 s.update([["S", "B"]])

TypeError: unhashable type: ‘list‘

2、刪

## set.remove()

In [142]: s
Out[142]: {‘a‘, ‘b‘, ‘j‘, ‘x‘}

In [143]: s.remove("a")

In [144]: s
Out[144]: {‘b‘, ‘j‘, ‘x‘}

In [151]: s.remove("S")
-----------------------------------------------------------------------
KeyError                              Traceback (most recent call last)
<ipython-input-151-332efdd48daa> in <module>()
----> 1 s.remove("S")

KeyError: ‘S‘


## set.pop()

In [153]: s = {1, 2, 3, 4}

In [154]: s.pop()    
Out[154]: 1

In [155]: s
Out[155]: {2, 3, 4}

In [156]: s.pop(5)
-----------------------------------------------------------------------
TypeError                             Traceback (most recent call last)
<ipython-input-156-23a1c03efc29> in <module>()
----> 1 s.pop(5)

TypeError: pop() takes no arguments (1 given)

In [157]: s.pop()
Out[157]: 2

In [158]: s.pop()
Out[158]: 3

In [159]: s.pop()
Out[159]: 4

In [160]: s.pop()
-----------------------------------------------------------------------
KeyError                              Traceback (most recent call last)
<ipython-input-160-e76f41daca5e> in <module>()
----> 1 s.pop()

KeyError: ‘pop from an empty set‘


## set.discard()

In [165]: help(set.discard)

Help on method_descriptor:

discard(...)
    Remove an element from a set if it is a member.
    
    If the element is not a member, do nothing.
    
In [166]: s = {1, 2, 3}

In [167]: s.discard(2)

In [168]: s.discard(1, 3)
-----------------------------------------------------------------------
TypeError                             Traceback (most recent call last)
<ipython-input-168-8702b734cbc4> in <module>()
----> 1 s.discard(1, 3)

TypeError: discard() takes exactly one argument (2 given)

In [169]: s.discard(2)   # 元素不存在時，不會報錯

In [170]: s
Out[170]: {1, 3}

In [32]: s.clear()

In [33]: s
Out[33]: set()


In [47]: del(s)

In [48]: s
-----------------------------------------------------------------------
NameError                             Traceback (most recent call last)
<ipython-input-48-f4d5d0c0671b> in <module>()
----> 1 s

NameError: name ‘s‘ is not defined

小結：

remove 刪除給定的元素，元素不存在時，拋出KeyError

discard 刪除給定的元素，元素不存在時，什麽也不做

pop 隨機刪除一個元素並返回，集合為空返回KeyError，

clear 清空集合

3、改

set不能修改單個元素

4、查找

集合不能通過索引，集合不是線性結構，沒有索引

集合沒有訪問單個元素的方法

集合沒有查找的方法

做成員運算（in和not in）的時候，set的效率遠高於list（O(1)和O(n)）;

O(n)不一定小於O(1)，還需要看數據規模

三、集合的集合操作

【Python】10、python內置數據結構之集合