【Python】10、python内置数据结构之集合

一、集合
1、集合的定义

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

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}

集合是无序的，元素不能重复，元素要能被哈希（hash，不可变）

二、集合的操作
1、增

z## set.add()

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

In [87]: s.add("a")   # 原地增加单个元素，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()  # 原地增加可迭代对象的元素

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、删

## 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)   # 元素不存在时，不会报错

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

小结：
remove 删除给定的元素，元素不存在时，抛出KeyError
discard 删除给定的元素，元素不存在时，什么也不做
pop 随机删除一个元素并返回，集合为空返回KeyError，
clear 清空集合

3、改
set不能修改单个元素

4、查找
集合不能通过索引，集合不是线性结构，没有索引
集合没有访问单个元素的方法
集合没有查找的方法

做成员运算（in和not in）的时候，set的效率远高于list（O(1)和O(n)）;
O(n)不一定小于O(1)，还需要看数据规模

三、集合运算
1、交集

## set.intersection()

In [1]: s1 = {1, 2, 3}

In [2]: s2 = {2, 3, 4}

In [3]: s1.intersection()
Out[3]: {1, 2, 3}

In [4]: s1.intersection(s2)   # 返回交集；不会修改原set
Out[4]: {2, 3}

In [26]: s2.intersection(s1)
Out[26]: {2, 3}

In [5]: s1.intersection([2,3])
Out[5]: {2, 3}

In [6]: help(set.intersection)

In [7]: s1.intersection(2)
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-7-94b820092aa3> in <module>()
----> 1 s1.intersection(2)

TypeError: 'int' object is not iterable

In [17]: s1.intersection_update(s2)   # set.intersection的_update版本，修改原set，返回None

In [18]: s1
Out[18]: {2, 3}

In [19]: s2
Out[19]: {2, 3, 4}

In [20]: s1 = {1, 2, 3}

In [21]: s2 = {2, 3, 4}

In [22]: s1 & s2         #  set重载了按位与运算为求交集运算
Out[22]: {2, 3}

In [23]: s1
Out[23]: {1, 2, 3}

In [24]: s2
Out[24]: {2, 3, 4}

2、差集

In [27]: s1
Out[27]: {1, 2, 3}

In [28]: s2
Out[28]: {2, 3, 4}

In [29]: s1.difference(s2)
Out[29]: {1}

In [30]: s2.difference(s1)
Out[30]: {4}

In [31]: s1
Out[31]: {1, 2, 3}

In [32]: s2
Out[32]: {2, 3, 4}

In [33]: s1.difference_update(s2)

In [34]: s1
Out[34]: {1}

In [35]: s2
Out[35]: {2, 3, 4}

In [38]: s1
Out[38]: {1, 2, 3}

In [39]: s2
Out[39]: {2, 3, 4}

In [40]: s1 - s2    # set重载了运算符- 执行差集计算，相当于s1.difference(s2)
Out[40]: {1}

In [41]: s2 - s1
Out[41]: {4}

In [42]: s1 + s2
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-42-1659087814e1> in <module>()
----> 1 s1 + s2

TypeError: unsupported operand type(s) for +: 'set' and 'set'

In [50]: s1.symmetric_difference(s2)   # 对称差集
Out[50]: {1, 4}

In [51]: s1.symmetric_difference_update(s2)

In [52]: s1
Out[52]: {1, 4}

In [53]: s2
Out[53]: {2, 3, 4}

In [55]: s1           # set重载了异或运算符，执行求对称差集运算
Out[55]: {1, 2, 3}

In [56]: s2
Out[56]: {2, 3, 4}

In [57]: s1 ^ s2
Out[57]: {1, 4}

3、并集

In [58]: s1
Out[58]: {1, 2, 3}

In [59]: s2
Out[59]: {2, 3, 4}

In [60]: s1.union(s2)    # 那set的union有update版本吗？其实update就是union的update版本
Out[60]: {1, 2, 3, 4}

In [61]: s1 | s2          # set重载了|运算符，执行求对称并集运算
Out[61]: {1, 2, 3, 4}

4、集合相关的判断

In [68]: s1 = {2, 3}

In [69]: s2 = {1, 2, 3, 4}

In [70]: s1.isdisjoint(s2)   # 是否没有交集
Out[70]: False

In [71]: s1.issubset(s2)     # 是否是子集
Out[71]: True

In [72]: s1.issuperset(s2)   # 是否是父超集
Out[72]: False

In [73]: s2.issuperset(s1)
Out[73]: True

In [74]: s1 = {"a", "b"}

In [75]: s1.isdisjoint(s2)
Out[75]: True

四、集合的应用和限制
set常用于去重和大规模数据时成员运算时较快
str、bytes、bytearray对元素有要求，必须是8位的int；0-255

集合的元素不能重复，必须可hash（可变的类型都不能hash）