集合set
1.由不同元素组成的集合，集合是一组无序排列的，集合中的元素必须是不可变的
-定义集合
第一种：
jh = {1,2,3,4}
print(type(jh),jh)
运行结果：
<class 'set'> {1, 2, 3, 4} Process finished with exit code 0
第二种：
jh = set('hello') print(type(jh),jh)
运行结果：
<class 'set'> {'e', 'o', 'h', 'l'} Process finished with exit code 0
-添加元素
jh = {1,2,3,4}
jh.add("nihao")
print(jh)
{1, 2, 3, 4, 'nihao'}
Process finished with exit code 0
-删除
*.clear 清除集合
*.pop 随机删除
jh = {1,2,3,4}
jh.pop()
print(jh)
运行结果：
{2, 3, 4}
Process finished with exit code 0
*.remove 删除指定元素（如果指定的元素不在，会报错）
jh = {1,2,3,4}
jh.remove(4)
print(jh)
运行结果：
{1, 2, 3}
Process finished with exit code 0
*.discard 删除指定元素（如果指定元素不在，不会报错）
-交集 &
math = {'xm','xh','xg'}
english ={'xm','xh'}
print(math.intersection(english))
运行结果：
{'xh', 'xm'}
Process finished with exit code 0
-并集 |
math = {'xm','xh','xg','xx'}
english ={'xm','xh','dm','john'}
print(math.union(english))
运行结果：
{'xg', 'dm', 'john', 'xm', 'xx', 'xh'}
Process finished with exit code 0
-差集（也可以两个集合做减法）
math = {'xm','xh','xg','xx'}
english ={'xm','xh','dm','john'}
print(math.difference(english))
print(english.difference(math))
运行结果：
{'xg', 'xx'}
{'dm', 'john'}
Process finished with exit code 0