The creation and use of sets in python

description:

A set is an unordered sequence of non-repeating elements. Sets and lists are very similar

The difference between sets and lists:

• Only immutable objects can be stored in the collection
• The objects stored in the collection are unordered (not stored in the order in which the elements are inserted)
• Duplicate elements cannot and will not appear in the collection

---

Create a collection:

You can use braces {} or set() to create a collection. Note: To create an empty collection, you must use set() instead of {}, because {} is used to create an empty dictionary.

Method 1: Use {} to create a collection

s = {
    
    10,3,5,1,2,1,2,3,1,1,1,1}
print(s) # {1, 2, 3, 5, 10}
print(type(s)) # <class 'set'>

As can be seen from the code above, the objects stored in the collection are unordered, and there will be no duplicate elements (can be used for deduplication)

集合中只能存储不可变对象
a = {
    
    [1,2,3],[4,6,7]}
print(a) # 报错 TypeError: unhashable type: 'list'

Method 2: Use the set() function to create a collection

创建一个空集合
s = set() 
print(s) # set()
print(type(s)) # <class 'set'>

Use set() to convert sequences and dictionaries to sets. When using set() to convert dictionaries to sets, only the keys in the dictionary are included

s = set([1,3,4,4,5,1,1,2,3,4,5])
print(s) # {1, 2, 3, 4, 5}

s = set('hello')
print(s) # {'h', 'o', 'l', 'e'}

s = set({
    
    'a':1,'b':2,'c':3})
print(s) # {'a', 'c', 'b'}

---

The use of sets:

1. Use in and not in to check the elements in the collection

s = {
    
    'a','b',1,2,3,1}
print('c' in s)   # False
print(1 in s)     # True
print(2 not in s) # False

2. Use len() to get the number of elements in the collection

s = {
    
    'a','b',1,2,3,1}
print(s)      # {1, 2, 3, 'a', 'b'}
print(len(s)) # 5

3. add() adds an element to the collection. If the element already exists, no operation is performed.

s = {
    
    'a','b',1,2,3,1}
s.add(3)
s.add(10)
s.add('hello')
print(s) # {1, 2, 3, 10, 'hello', 'b', 'a'}

4. update() adds the elements in a collection to the current collection, update() can pass a sequence or a dictionary as a parameter, and the dictionary only uses keys

s1 = {
    
    1,2,3}
s2 = set("hello")
print(s2)     # {'l', 'o', 'e', 'h'}

s1.update(s2) # 将一个集合中的元素添加到当前集合中
print(s1)     # {'h', 1, 2, 3, 'e', 'o', 'l'}

s1 = {
    
    1,2,3}
s1.update((10,20,30,40)) # 传递一个序列作为参数
print(s1)     # {1, 2, 3, 40, 10, 20, 30}

s1 = {
    
    1,2,3}
s1.update({
    
    100:'aa',200:'bb',300:'cc',400:'dd'}) # 传递一个字典作为参数
print(s1)  # {400, 1, 2, 3, 100, 200, 300}

5. pop() randomly delete an element in the set and return

s = {
    
    4, 2, 3, 100, 40, 'o', 'a', 'h',}
result = s.pop()
print(result) # 2

6. remove(x) delete the specified element x in the collection

s = {
    
    4, 2, 3, 100, 40, 'o', 'a', 'h',}
s.remove(100)
print(s) # {2, 3, 4, 'h', 40, 'o', 'a'}

7. clear() clear the collection

s = {
    
    4, 2, 3, 100, 40, 'o', 'a', 'h',}
s.clear()
print(s) # set() 空集合