python入门之python开发笔记基本数据类型集合

谁借莪１个温暖的怀抱￠ 2023-10-03 18:18 31阅读 0赞

### 本节重点 ###

1.让学员了解什么是集合以及集合的作用

2.让学员掌握集合的定义和基本特征

3.让学员掌握集合的关系运算和常用操作

### 引子 ###

现在有一个linux班一个python班，我们创建两个列表，把班里的学生表示出来：

l = \[‘张三’,‘李四’,‘老男孩’\]

p = \[‘张三’,‘李四’,‘alex’\]

现在要找出既在linux班上课也在python班上课的学生，应该怎么找？

l= ['张三','李四','老男孩']
    p = ['张三','李四','alex']
    l_p = []
    for i in l:
        if i in p:
            l_p.append(i)
    print(l_p)

上面这种方法可以实现我们的需求，但是比较麻烦，而且当列表变得非常长，执行代码的效率也是问题。

接下来，我们要学习一种新的数据类型，能够非常轻松的帮我们完成这个任务。

### 认识集合 ###

集合是一个数学概念：由一个或多个确定的元素所构成的整体叫做集合。

**集合中的元素有三个特征：**

1.确定性（元素必须可hash）

2.互异性（去重）

3.无序性（集合中的元素没有先后之分），如集合\{3,4,5\}和\{3,5,4\}算作同一个集合。

> 注意：集合存在的意义就在于**去重和关系运算**

### 用集合解决问题 ###

l= {'张三','李四','老男孩'}  #集合定义
    p = {'张三','李四','alex'}
    l_p = l&p    #集合求交集
    print(l_p)

![image][]

### 集合的定义 ###

l= {1,2,3,1}  #此处应说明集合“去重”的效果
    #定义可变集合
    >>> set_test=set('hello') #此处应说明集合的“无序性”
    >>> set_test
    {'l', 'o', 'e', 'h'}
    #改为不可变集合frozenset
    >>> f_set_test=frozenset(set_test)
    >>> f_set_test
    frozenset({'l', 'e', 'h', 'o'})

### 集合的**关系运算** ###

除了刚刚我们学过的交集之外，集合还可以做其他的关系运算。

继续以引子为例，现在已知两个集合分别是学习linux班的同学和学习python班的同学

&.&=:交集——既学习linux课程也学习python课程的同学

![image][image 1]

l= {'张三','李四','老男孩'}
    p = {'张三','李四','alex'}
    print(l.intersection(p))
    print(l&p)

|,|=:合集，也叫并集——linux班和python班的所有同学

> 这里同学们不可以单纯的认为python班的人数+linux班的人数就是结果，如果两班人数相加应该是6人。但学习Linux班的张三和李四还同时在python班学习，因此我们实际上总共只有四个学员。**求并集除了合并效果之外还有去重功能**

![image][image 2]

l= {'张三','李四','老男孩'}
    p = {'张三','李四','alex'}
    print(l.union(p))
    print(l|p)

－,－=:差集——只在linux而不python班的同学

![image][image 3]

l= {'张三','李四','老男孩'}
    p = {'张三','李四','alex'}
    print(l.difference(p))
    print(l-p)

,=:对称差集——只在linux班或只在python班的同学

![image][image 4]

a = {1,2,3}
    b = {2,3,4,5}
    print(a.symmetric_difference(b))
    print(a^b)

包含关系

in,not in：判断某元素是否在集合内  
＝＝,！＝:判断两个集合是否相等

两个集合之间一般有三种关系，相交、包含、不相交。在Python中分别用下面的方法判断：

*  set.isdisjoint(s)：判断两个集合是不是不相交
 *  set.issuperset(s)：判断集合是不是包含其他集合，等同于a>=b
 *  set.issubset(s)：判断集合是不是被其他集合包含，等同于a<=b

### **集合的常用操作** ###

**元素的增加**

单个元素的增加 : add()，add的作用类似列表中的append

对序列的增加 : update()，而update类似extend方法，update方法可以支持同时传入多个参数：

>>> a={1,2}
    >>> a.update([3,4],[1,2,7])
    >>> a
    {1, 2, 3, 4, 7}
    >>> a.update("hello")
    >>> a
    {1, 2, 3, 4, 7, 'h', 'e', 'l', 'o'}
    >>> a.add("hello")
    >>> a
    {1, 2, 3, 4, 'hello', 7, 'h', 'e', 'l', 'o'}

**元素的删除**

集合删除单个元素有两种方法：

元素不在原集合中时：

set.discard(x)不会抛出异常

set.remove(x)会抛出KeyError错误

>>> a={1,2,3,4}
    >>> a.discard(1)
    >>> a
    {2, 3, 4}
    >>> a.discard(1)
    >>> a
    {2, 3, 4}
    >>> a.remove(1)
    Traceback (most recent call last):
      File "<input>", line 1, in <module>
    KeyError: 1

pop()：由于集合是无序的，pop返回的结果不能确定，且当集合为空时调用pop会抛出KeyError错误，

clear():清空集合

>>> a={3,"a",2.1,1}
    >>> a.pop()
    >>> a.pop()
    >>> a.clear()
    >>> a
    set()
    >>> a.pop()
    Traceback (most recent call last):
      File "<input>", line 1, in <module>
    KeyError: 'pop from an empty set'

### 集合的工厂函数 ###

class set(object):
        """
        set() -> new empty set object
        set(iterable) -> new set object
    
        Build an unordered collection of unique elements.
        """
        def add(self, *args, **kwargs): # real signature unknown
            """
            Add an element to a set.
    
            This has no effect if the element is already present.
            """
            pass
    
        def clear(self, *args, **kwargs): # real signature unknown
            """ Remove all elements from this set. """
            pass
    
        def copy(self, *args, **kwargs): # real signature unknown
            """ Return a shallow copy of a set. """
            pass
    
        def difference(self, *args, **kwargs): # real signature unknown
            """
            相当于s1-s2
    
            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.)
            """
            pass
    
        def difference_update(self, *args, **kwargs): # real signature unknown
            """ Remove all elements of another set from this set. """
            pass
    
        def discard(self, *args, **kwargs): # real signature unknown
            """
            与remove功能相同，删除元素不存在时不会抛出异常
    
            Remove an element from a set if it is a member.
    
            If the element is not a member, do nothing.
            """
            pass
    
        def intersection(self, *args, **kwargs): # real signature unknown
            """
            相当于s1&s2
    
            Return the intersection of two sets as a new set.
    
            (i.e. all elements that are in both sets.)
            """
            pass
    
        def intersection_update(self, *args, **kwargs): # real signature unknown
            """ Update a set with the intersection of itself and another. """
            pass
    
        def isdisjoint(self, *args, **kwargs): # real signature unknown
            """ Return True if two sets have a null intersection. """
            pass
    
        def issubset(self, *args, **kwargs): # real signature unknown
            """ 
            相当于s1<=s2
    
            Report whether another set contains this set. """
            pass
    
        def issuperset(self, *args, **kwargs): # real signature unknown
            """
            相当于s1>=s2
    
             Report whether this set contains another set. """
            pass
    
        def pop(self, *args, **kwargs): # real signature unknown
            """
            Remove and return an arbitrary set element.
            Raises KeyError if the set is empty.
            """
            pass
    
        def remove(self, *args, **kwargs): # real signature unknown
            """
            Remove an element from a set; it must be a member.
    
            If the element is not a member, raise a KeyError.
            """
            pass
    
        def symmetric_difference(self, *args, **kwargs): # real signature unknown
            """
            相当于s1^s2
    
            Return the symmetric difference of two sets as a new set.
    
            (i.e. all elements that are in exactly one of the sets.)
            """
            pass
    
        def symmetric_difference_update(self, *args, **kwargs): # real signature unknown
            """ Update a set with the symmetric difference of itself and another. """
            pass
    
        def union(self, *args, **kwargs): # real signature unknown
            """
            相当于s1|s2
    
            Return the union of sets as a new set.
    
            (i.e. all elements that are in either set.)
            """
            pass
    
        def update(self, *args, **kwargs): # real signature unknown
            """ Update a set with the union of itself and others. """
            pass
    
    ...略...

[image]: https://img-blog.csdnimg.cn/img_convert/5a86b28370253be925a3bab9d567637a.png
[image 1]: https://img-blog.csdnimg.cn/img_convert/a625d17c9f0b547dda722df7317199bb.png
[image 2]: https://img-blog.csdnimg.cn/img_convert/1de3238223442d32de566832de713c73.png
[image 3]: https://img-blog.csdnimg.cn/img_convert/1b2282afda9658ce57ba6f42530c5954.png
[image 4]: https://img-blog.csdnimg.cn/img_convert/16fff984f16dd3db67ff56a94119214d.png