python数据类型之集合
数据类型—集合set
特性：无序去重，元素必须是不可变类型，集合本身是可变的，可以存所有数据类型的任意多个数据，是可迭代的数据类型
官方帮助文档
class set(object)
| set() -> new empty set object
| set(iterable) -> new set object
|
| Build an unordered collection of unique elements.
|
| Methods defined here:
| add(...)
| Add an element to a set.
|
| This has no effect if the element is already present.
|
| clear(...)
| Remove all elements from this set.
|
| copy(...)
| Return a shallow copy of a set.
|
| difference(...)
| Return the difference of two or more sets as a new set.
|
| (i.e. all elements that are in this set but not the others.)
|
| difference_update(...)
| Remove all elements of another set from this set.
|
| discard(...)
| Remove an element from a set if it is a member.
|
| If the element is not a member, do nothing.
|
| intersection(...)
| Return the intersection of two sets as a new set.
|
| (i.e. all elements that are in both sets.)
|
| intersection_update(...)
| Update a set with the intersection of itself and another.
|
| isdisjoint(...)
| Return True if two sets have a null intersection.
|
| issubset(...)
| Report whether another set contains this set.
|
| issuperset(...)
| Report whether this set contains another set.
|
| pop(...)
| Remove and return an arbitrary set element.
| Raises KeyError if the set is empty.
|
| remove(...)
| Remove an element from a set; it must be a member.
|
| If the element is not a member, raise a KeyError.
|
| symmetric_difference(...)
| Return the symmetric difference of two sets as a new set.
|
| (i.e. all elements that are in exactly one of the sets.)
|
| symmetric_difference_update(...)
| Update a set with the symmetric difference of itself and another.
|
| union(...)
| Return the union of sets as a new set.
|
| (i.e. all elements that are in either set.)
|
| update(...)
| Update a set with the union of itself and others.
|
| ----------------------------------------------------------------------
| Data and other attributes defined here:
|
| __hash__ = None
创建集合
class set(object)
| set() -> new empty set object
| set(iterable) -> new set object
迭代创建
>>> set('123')
{'1', '3', '2'}
>>> set(['a','cd'])
{'cd', 'a'}
>>> a = set()
>>> a
set()
直接创建
>>> s = {1,2,3}
>>> s
{1, 2, 3}
集合的方法
['add', 'clear', 'copy', 'difference', 'difference_update', 'discard', 'intersection', 'intersection_update', 'isdisjoint', 'issubset', 'issuperset', 'pop', 'remove', 'symmetric_difference', 'symmetric_difference_update', 'union', 'update']
增加
set1.add(元素) # 把元素加入集合
>>> set1.add('b')
>>> set1
{'a', 'b'}
set1.update(元素)把元素加到集合中，可以多个值，迭代添加
>>> set1 = {1,2,3,4}
>>> set1.update("cd")
>>> set1
{1, 2, 3,'d', 4, 'c'}
删除
set1.pop() 随机删除，返回的是删除的值，集合为空的时候会报错
>>> set1 = {1,2,3,4}
>>> set1.pop()
1
set1.discard(元素) 没有返回值，没有指定的元素不会报错
>>> set1 = {1,2,3,4}
>>> set1.discard(1)
>>> set1
{2, 3, 4}
>>> set1 = {1,2,3,4} 没有指定元素时不会报错
>>> set1.discard(100)
set.remove(元素),没有元素会报错
>>>thisset = set(("Google", "Runoob", "Taobao"))
>>> thisset.remove("Taobao")
>>> print(thisset)
{'Google', 'Runoob'}
>>> thisset.remove("Facebook") # 不存在会发生错误
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
KeyError: 'Facebook'
set.clear() 清除集合内的所有元素
求交集（& 或者 intersection）
>>> set1 = {1,2,3,4}
>>> set2 = {3,4,5,6}
>>> set1.intersection(set2)
{3, 4}
>>> set1 & set2
{3, 4}
求并集（ | 或者 union）
>>> set1 = {1,2,3,4}
>>> set2 = {3,4,5,6}
>>> set1.union(set2)
{1, 2, 3, 4, 5, 6}
>>> set1 | set2
{1, 2, 3, 4, 5, 6}
求差集 （- 或者 difference）
>>> set1 = {1,2,3,4}
>>> set2 = {3,4,5,6}
>>> set1 - set2 set1独有的
{1, 2}
>>> set2 - set1 set2独有的
{5, 6}
>>> set1 = {1,2,3,4}
>>> set2 = {3,4,5,6}
>>> set1.difference(set2) 返回的时set1中除去去两个集合相同的元素
{1, 2}
>>> set2.difference(set1) 返回的时set2中除去去两个集合相同的元素
{5, 6}
对称差集 除去两个集合相同的元素（^ 或者 symmetric_difference）
>>> set1 = {1,2,3,4}
>>> set2 = {3,4,5,6}
>>> set1 ^ set2
{1, 2, 5, 6}
>>> set1.symmetric_difference(set2)
{1, 2, 5, 6}
判断一个集合是否是另一个集合的子集，子集与超集返回布尔值
set1 = {1,2,3}
set2 = {1,2,3,4,5,6}
print(set1 < set2) # True
print(set1.issubset(set2)) # 这两个相同，都是说明set1是set2子集。
print(set2 > set1) # True
print(set2.issuperset(set1)) # 这两个相同，都是说明set2是set1超集。
frozenset
把集合冻住，不能删除，就可以循环
>>> s = frozenset(set1)
>>> print(s,type(s))
frozenset({1, 2, 3}) <class 'frozenset'>
>>> set1.pop()
Traceback (most recent call last):
File "<pyshell#65>", line 1, in <module>
set1.pop()
AttributeError: 'frozenset' object has no attribute 'pop'
>>> for i in set1: # 变成不可变数据类型，可以遍历循环
print(i)
2
3
4