Python数据结构之四——set（集合）
Python版本：3.6.2 操作系统：Windows 作者：SmallWZQ
经过几天的回顾和学习，我终于把Python 3.x中的基础知识介绍好啦。下面将要继续什么呢？让我想想先~~~嗯，还是先整理一下近期有关Python基础知识的随笔吧。
Python编程软件的安装与使用——Windows、Linux和Mac
Python基础——输出[print()]与输入[input()]
上述六篇均为Python 3.x的基础知识。九尺高台，起于累土。学习Python就要从最基本开始，经过逐步的积累，才能有所成就。
Python基础知识再次回顾好了，接下来该干嘛呢？这不继续Python数据结构了吗？
上次我写了有关Python数据结构（列表、元组、字典）的3篇随笔：
本篇随笔将开始一段关于set（集合）之旅吧。
什么是集合呢？
说到集合，我首先想到了高中的数学。高中，人生学习中最繁忙的一段时光。直到现在，我能回忆起最多的就是学习、学习、还是读书……言归正传，高一时的数学，我们就接触到了集合。书中应该是这样定义的：
集合：由一个或多个确定的元素所构成的整体。若x是集合A的元素，则记作x∈A。
集合中的元素有三个特征：
1. 确定性：集合中的元素必须是确定的；
2. 互异性：集合中的元素互不相同，例如：集合A={1，a}，则a不能等于1）；
3. 无序性：集合中的元素没有先后之分，例如：集合{3,4,5}和{3,5,4}算作同一个集合。
Python 3.x中的set特征与数学中类似。我们之前学过list、tuple以及dict。其实，set与dict大致相同，但set没有Value，只有key。因此，set只是一组key的集合。由于key不能重复，所以，在set中，没有重复的key。
创建集合
1.1 创建空集合
在集合中，创建空集合（set）必须使用函数set()。
1 #创建空集合 2 >>>a = set() 3 >>>a 4 set() 5 >>>type(a) 6 <class 'set'>
注：不能使用{}，{}用于创建空字典。
1.2 创建非空集合
非空集合可以用大括号{}或 set() 函数来创建。
1 #创建集合 2 >>>a={'a','b','c','d'} 3 >>>b=set('abcdefabcd') 4 >>>c=set({'a':1,'b':2,'c':3}) 5 >>>d=set(['a','b','c','a']) 6 #运行结果 7 >>>print(a,type(a)) 8 {'c', 'd', 'b', 'a'} <class 'set'> 9 >>>print(b,type(b)) 10 {'f', 'e', 'b', 'c', 'd', 'a'} <class 'set'> 11 >>>print(c,type(c)) 12 {'b', 'a','c'} <class 'set'> 13 >>>print(d,type(d)) 14 {'c', 'b', 'a'} <class 'set'>
特别地，set中的元素是无序的，并且重复元素在set中自动被过滤。
1 #set中重复元素被自动过滤 2 >>>s = {1,2,,1,2,4,4,3,3} 3 >>>s 4 {1,2,3,4}
功能属性
set有很多很多的功能属性。你们不信？不信的话，继续往下看呗~~~
set功能属性如下：
1 class set(object): 2 """ 3 set() -> new empty set object 4 set(iterable) -> new set object 5 6 Build an unordered collection of unique elements. 7 """ 8 def add(self, *args, **kwargs): # real signature unknown 9 """ 10 Add an element to a set. 11 12 This has no effect if the element is already present. 13 """ 14 pass 15 16 def clear(self, *args, **kwargs): # real signature unknown 17 """ Remove all elements from this set. """ 18 pass 19 20 def copy(self, *args, **kwargs): # real signature unknown 21 """ Return a shallow copy of a set. """ 22 pass 23 24 def difference(self, *args, **kwargs): # real signature unknown 25 """ 26 Return the difference of two or more sets as a new set. 27 28 (i.e. all elements that are in this set but not the others.) 29 """ 30 pass 31 32 def difference_update(self, *args, **kwargs): # real signature unknown 33 """ Remove all elements of another set from this set. """ 34 pass 35 36 def discard(self, *args, **kwargs): # real signature unknown 37 """ 38 Remove an element from a set if it is a member. 39 40 If the element is not a member, do nothing. 41 """ 42 pass 43 44 def intersection(self, *args, **kwargs): # real signature unknown 45 """ 46 Return the intersection of two sets as a new set. 47 48 (i.e. all elements that are in both sets.) 49 """ 50 pass 51 52 def intersection_update(self, *args, **kwargs): # real signature unknown 53 """ Update a set with the intersection of itself and another. """ 54 pass 55 56 def isdisjoint(self, *args, **kwargs): # real signature unknown 57 """ Return True if two sets have a null intersection. """ 58 pass 59 60 def issubset(self, *args, **kwargs): # real signature unknown 61 """ Report whether another set contains this set. """ 62 pass 63 64 def issuperset(self, *args, **kwargs): # real signature unknown 65 """ Report whether this set contains another set. """ 66 pass 67 68 def pop(self, *args, **kwargs): # real signature unknown 69 """ 70 Remove and return an arbitrary set element. 71 Raises KeyError if the set is empty. 72 """ 73 pass 74 75 def remove(self, *args, **kwargs): # real signature unknown 76 """ 77 Remove an element from a set; it must be a member. 78 79 If the element is not a member, raise a KeyError. 80 """ 81 pass 82 83 def symmetric_difference(self, *args, **kwargs): # real signature unknown 84 """ 85 Return the symmetric difference of two sets as a new set. 86 87 (i.e. all elements that are in exactly one of the sets.) 88 """ 89 pass 90 91 def symmetric_difference_update(self, *args, **kwargs): # real signature unknown 92 """ Update a set with the symmetric difference of itself and another. """ 93 pass 94 95 def union(self, *args, **kwargs): # real signature unknown 96 """ 97 Return the union of sets as a new set. 98 99 (i.e. all elements that are in either set.) 100 """ 101 pass 102 103 def update(self, *args, **kwargs): # real signature unknown 104 """ Update a set with the union of itself and others. """ 105 pass 106 107 def __and__(self, *args, **kwargs): # real signature unknown 108 """ Return self&value. """ 109 pass 110 111 def __contains__(self, y): # real signature unknown; restored from __doc__ 112 """ x.__contains__(y) <==> y in x. """ 113 pass 114 115 def __eq__(self, *args, **kwargs): # real signature unknown 116 """ Return self==value. """ 117 pass 118 119 def __getattribute__(self, *args, **kwargs): # real signature unknown 120 """ Return getattr(self, name). """ 121 pass 122 123 def __ge__(self, *args, **kwargs): # real signature unknown 124 """ Return self>=value. """ 125 pass 126 127 def __gt__(self, *args, **kwargs): # real signature unknown 128 """ Return self>value. """ 129 pass 130 131 def __iand__(self, *args, **kwargs): # real signature unknown 132 """ Return self&=value. """ 133 pass 134 135 def __init__(self, seq=()): # known special case of set.__init__ 136 """ 137 set() -> new empty set object 138 set(iterable) -> new set object 139 140 Build an unordered collection of unique elements. 141 # (copied from class doc) 142 """ 143 pass 144 145 def __ior__(self, *args, **kwargs): # real signature unknown 146 """ Return self|=value. """ 147 pass 148 149 def __isub__(self, *args, **kwargs): # real signature unknown 150 """ Return self-=value. """ 151 pass 152 153 def __iter__(self, *args, **kwargs): # real signature unknown 154 """ Implement iter(self). """ 155 pass 156 157 def __ixor__(self, *args, **kwargs): # real signature unknown 158 """ Return self^=value. """ 159 pass 160 161 def __len__(self, *args, **kwargs): # real signature unknown 162 """ Return len(self). """ 163 pass 164 165 def __le__(self, *args, **kwargs): # real signature unknown 166 """ Return self<=value. """ 167 pass 168 169 def __lt__(self, *args, **kwargs): # real signature unknown 170 """ Return self<value. """ 171 pass 172 173 @staticmethod # known case of __new__ 174 def __new__(*args, **kwargs): # real signature unknown 175 """ Create and return a new object. See help(type) for accurate signature. """ 176 pass 177 178 def __ne__(self, *args, **kwargs): # real signature unknown 179 """ Return self!=value. """ 180 pass 181 182 def __or__(self, *args, **kwargs): # real signature unknown 183 """ Return self|value. """ 184 pass 185 186 def __rand__(self, *args, **kwargs): # real signature unknown 187 """ Return value&self. """ 188 pass 189 190 def __reduce__(self, *args, **kwargs): # real signature unknown 191 """ Return state information for pickling. """ 192 pass 193 194 def __repr__(self, *args, **kwargs): # real signature unknown 195 """ Return repr(self). """ 196 pass 197 198 def __ror__(self, *args, **kwargs): # real signature unknown 199 """ Return value|self. """ 200 pass 201 202 def __rsub__(self, *args, **kwargs): # real signature unknown 203 """ Return value-self. """ 204 pass 205 206 def __rxor__(self, *args, **kwargs): # real signature unknown 207 """ Return value^self. """ 208 pass 209 210 def __sizeof__(self): # real signature unknown; restored from __doc__ 211 """ S.__sizeof__() -> size of S in memory, in bytes """ 212 pass 213 214 def __sub__(self, *args, **kwargs): # real signature unknown 215 """ Return self-value. """ 216 pass 217 218 def __xor__(self, *args, **kwargs): # real signature unknown 219 """ Return self^value. """ 220 pass 221 222 __hash__ = None
set功能属性虽多，但平时常用的也就那么几个。
常用属性
1. 添加元素
在集合中添加元素，可以使用add()方法，并且不生成一个新的集合。
1 #添加元素：add() 2 >>>s = {1,2,3} 3 >>>s.add(4) 4 >>>s 5 {1,2,3,4} 6 >>>s.add('g') 7 >>>s 8 {1,2,3,4,'g'} 9 >>>s.add(4) 10 >>>s 11 {1,2,3,4,'g'}
add()方法可以向set中添加元素，可以重复添加，但不会有效果。
2. 删除元素
set中利用remove()方法可以删除集合中的元素。
1 #删除元素 2 >>>s 3 {1,2,3,4,'g'} 4 >>>s.remove('g') 5 >>>s 6 {1,2,3,4}
3. 清空元素
clear()方法可以清空set中的元素。
1 #清空元素 2 >>>a = {1,2,3,4} 3 >>>b = a.clear() 4 >>>print(a,type(a)) 5 set() <class 'set'> 6 >>>print(b,type(b)) 7 None <class 'NoneType'>
4. 复制元素
copy()方法只能浅拷贝set中的元素，并生成一个新的集合。
1 #浅拷贝：copy() 2 >>>a = {1,(9,2),3} 3 >>>b = a.copy() 4 >>>print(a,id(a)) 5 {(9, 2), 1, 3} 2097937619880 6 >>>print(b,id(b)) 7 {(9, 2), 1, 3} 2097937620776 8 9 #赋值 10 >>>s = {1,2,3,4} 11 >>>d = s 12 >>>print(s,id(s)) 13 {1, 2, 3, 4} 2097937785128 14 >>>print(d,id(d)) 15 {1, 2, 3, 4} 2097937785128
5. pop()
pop()方法用于从set中随机取一个元素。记住，是随机的~~~
1 #pop()方法 2 >>>s = {1,2,3,4,5,'g','s'} 3 >>>s.pop() 4 'g' 5 >>>s.pop() 6 3
6. set集合操作
set与数学中的集合类似，是无序的和无重复元素的集合。因此，在Python中，set可以进行交集、并集、补集等操作。
| 数学符号 | Python符号 | 含义 |
| - 或\ | - | 差集，相对补集 |
| ∩ | & | 交集 |
| ∪ | | | 并集 |
| ≠ | != | 不等于 |
| ＝ | == | 等于 |
| ∈ | in | 是成员关系 |
| ∉ | not in | 非成员关系 |
1 #set集合操作 2 >>>s = {1,2,3,4} 3 >>>d = {2.3.5.6} 4 >>>s & d 5 {2.3} 6 >>>s | d 7 {1,2,3,4,5,6} 8 >>>s - d 9 {1,4} 10 >>>d - s 11 {5,6}
set和dict的唯一区别仅在于没有存储对应的value，但是，set的原理和dict一样，所以，同样不可以放入可变对象，因为无法判断两个可变对象是否相等，也就无法保证set内部“不会有重复元素”。因此，最常用的key是字符串。
“思想者”
set中存储着key，集合中不能放入可变的对象。之前的文章也说过：tuple是不可变的，而list是可变的。因此，set中是可以存储tuple的。这是真的吗？
时间是检验真理的唯一标准。下面请看示例代码：
1 #tuple可以作为集合中的元素 2 >>>s = {(1,),(1,2,3),1,2,'g'} 3 >>>s 4 {(1,),(1,2,3),1,2,'g'} 5 6 #tuple也有失灵的时候 7 >>>t = (1,2,[1,2,3],4) 8 >>>type(t) 9 <class 'tuple'> 10 >>>d = {1,2,(1,2,[1,2,3],4)} 11 Traceback (most recent call last): 12 File "<stdin>", line 1, in <module> 13 TypeError: unhashable type: 'list'
为什么会有错误呢？我也不清楚哎~~~这里面的道道很深，请读者细细体会。
set是一种数据结构。如果要详细的介绍set，我应该可以去出书了。这篇随笔只是起到入门的效果。
正所谓“师傅”领进门，修行靠大家嘛！
posted on 2018-03-02 09:49 SmallWZQ 阅读(11638) 评论(1) 编辑 收藏 举报
