Python教程-Python 集合（Set）

Python 集合是一种无序项目的集合。集合中的每个元素必须是唯一的、不可变的，并且集合会移除重复的元素。集合是可变的，这意味着我们可以在创建后修改它。

与Python中的其他集合不同，集合的元素没有附加索引，即我们不能通过索引直接访问集合的任何元素。然而，我们可以一起打印它们，或者可以通过循环遍历集合获取元素的列表。

创建集合

集合可以通过用大括号 {} 括起逗号分隔的不可变项来创建。Python 还提供了 set() 方法，该方法可以用于通过传递序列来创建集合。

使用花括号创建集合

Days = {"Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday", "Sunday"}    
print(Days)    
print(type(Days))    
print("looping through the set elements ... ")    
for i in Days:    
    print(i)  

输出：

{'Friday', 'Tuesday', 'Monday', 'Saturday', 'Thursday', 'Sunday', 'Wednesday'}
<class 'set'>
looping through the set elements ... 
Friday
Tuesday
Monday
Saturday
Thursday
Sunday
Wednesday

使用 set() 方法创建集合

Days = set(["Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday", "Sunday"])    
print(Days)    
print(type(Days))    
print("looping through the set elements ... ")    
for i in Days:    
    print(i)    

输出：

{'Friday', 'Wednesday', 'Thursday', 'Saturday', 'Monday', 'Tuesday', 'Sunday'}
<class 'set'>
looping through the set elements ... 
Friday
Wednesday
Thursday
Saturday
Monday
Tuesday
Sunday

它可以包含任何类型的元素，例如整数、浮点数、元组等。但是可变元素（列表、字典、集合）不能是集合的成员。考虑以下示例。

创建包含不可变元素的集合

# Creating a set which have immutable elements  
set1 = {1,2,3, "JavaTpoint", 20.5, 14}  
print(type(set1))  
#Creating a set which have mutable element  
set2 = {1,2,3,["Javatpoint",4]}  
print(type(set2))  

输出：

<class 'set'>

Traceback (most recent call last)
<ipython-input-5-9605bb6fbc68> in <module>
      4 
      5 #Creating a set which holds mutable elements
----> 6 set2 = {1,2,3,["Javatpoint",4]}
      7 print(type(set2))

TypeError: unhashable type: 'list'

在上面的代码中，我们创建了两个集合，集合 set1 包含不可变元素，而 set2 包含一个可变元素（列表）。在检查 set2 的类型时，引发了一个错误，这意味着集合只能包含不可变元素。

创建空集合有点不同，因为空的花括号 {} 也用于创建字典。因此，Python 提供了使用无参数的 set() 方法来创建空集合。

创建空集合

# Empty curly braces will create dictionary  
set3 = {}  
print(type(set3))  
  
# Empty set using set() function  
set4 = set()  
print(type(set4)) 

输出：

<class 'dict'>
<class 'set'>

如果我们向集合提供重复的元素，看看会发生什么。

集合中的重复元素

set5 = {1,2,4,4,5,8,9,9,10}  
print("Return set with unique elements:",set5)  

输出：

Return set with unique elements: {1, 2, 4, 5, 8, 9, 10}

在上面的代码中，我们可以看到 set5 包含多个重复的元素，但在打印时会去除集合中的重复项。

向集合添加元素

Python 提供了 add() 方法和 update() 方法，用于向集合添加特定的项。add() 方法用于添加单个元素，而 update() 方法用于添加多个元素。

使用 add() 方法添加元素

Months = set(["January","February", "March", "April", "May", "June"])    
print("\nprinting the original set ... ")    
print(months)    
print("\nAdding other months to the set...");    
Months.add("July");    
Months.add ("August");    
print("\nPrinting the modified set...");    
print(Months)    
print("\nlooping through the set elements ... ")    
for i in Months:    
    print(i)    

输出：

printing the original set ... 
{'February', 'May', 'April', 'March', 'June', 'January'}

Adding other months to the set...

Printing the modified set...
{'February', 'July', 'May', 'April', 'March', 'August', 'June', 'January'}

looping through the set elements ... 
February
July
May
April
March
August
June
January 

使用 update() 方法添加多个元素

months = set(["January","February", "March", "April", "May", "June"])    
print("\nprinting the original set ... ")    
print(months)    
print("\nRemoving some months from the set...");    
months.discard("January");    
months.discard("May");    
print("\nPrinting the modified set...");    
print(months)    
print("\nlooping through the set elements ... ")    
for i in months:    
    print(i)    

输出：

printing the original set ... 
{'February', 'January', 'March', 'April', 'June', 'May'}

Removing some months from the set...

Printing the modified set...
{'February', 'March', 'April', 'June'}

looping through the set elements ... 
February
March
April
June

从集合中删除元素

Python 提供了几种方法来从集合中删除元素。可以使用 remove()、discard() 或 pop() 方法来执行此操作。

使用 remove() 方法删除元素

months = set(["January","February", "March", "April", "May", "June"])    
print("\nprinting the original set ... ")    
print(months)    
print("\nRemoving some months from the set...");    
months.remove("January");    
months.remove("May");    
print("\nPrinting the modified set...");    
print(months)    

输出：

months = set(["January","February", "March", "April", "May", "June"])    
print("\nprinting the original set ... ")    
print(months)    
print("\nRemoving some months from the set...");    
months.remove("January");    
months.remove("May");    
print("\nPrinting the modified set...");    
print(months)    

请注意，如果我们尝试删除不存在的元素，将引发错误。

使用 discard() 方法删除元素

months = set(["January","February", "March", "April", "May", "June"])    
print("\nprinting the original set ... ")    
print(months)    
print("\nRemoving some months from the set...");    
months.discard("January");    
months.discard("May");    
print("\nPrinting the modified set...");    
print(months)    
print("\nlooping through the set elements ... ")    
for i in months:    
    print(i)    

输出：

printing the original set ... 
{'February', 'January', 'March', 'April', 'June', 'May'}

Removing some months from the set...

Printing the modified set...
{'February', 'March', 'April', 'June'}

looping through the set elements ... 
February
March
April
June

使用 pop() 方法删除元素

Months = set(["January","February", "March", "April", "May", "June"])    
print("\nprinting the original set ... ")    
print(Months)    
print("\nRemoving some months from the set...");    
Months.pop();    
Months.pop();    
print("\nPrinting the modified set...");    
print(Months)    

输出：

printing the original set ... 
{'June', 'January', 'May', 'April', 'February', 'March'}

Removing some months from the set...

Printing the modified set...
{'May', 'April', 'February', 'March'}

请注意，pop() 方法删除集合中的一个元素，但由于集合是无序的，因此无法预测删除的是哪个元素。

集合操作

Python 还提供了多种用于执行集合操作的内置方法，如并集、交集、差集等。

并集

可以使用 union() 方法或 | 运算符找到两个集合的并集。

# Create three sets  
set1 = {1, 2, 3}  
set2 = {2, 3, 4}  
set3 = {3, 4, 5}  
  
# Find the common elements between the three sets  
common_elements = set1.union(set2, set3)  
  
# Print the common elements  
print(common_elements)  

输出：

{1, 2, 3, 4, 5}

交集

可以使用 intersection() 方法或 & 运算符找到两个集合的交集。

# Create three sets  
set1 = {1, 2, 3}  
set2 = {2, 3, 4}  
set3 = {3, 4, 5}  
  
# Find the common elements between the three sets  
common_elements = set1.intersection(set2, set3)  
  
# Print the common elements  
print(common_elements)  

输出：

{3}

差集

可以使用 difference() 方法或 - 运算符找到两个集合的差集。

Days1 = {"Monday",  "Tuesday", "Wednesday", "Thursday"}    
Days2 = {"Monday", "Tuesday", "Sunday"}    
print(Days1.difference(Days2)) # prints the difference of the two sets Days1 and Days2    

输出：

{'Thursday', 'Wednesday'}

对称差集

可以使用 symmetric_difference() 方法或 ^ 运算符找到两个集合的对称差集（即只在一个集合中出现的元素）。

Days1 = {"Monday",  "Tuesday", "Wednesday", "Thursday"}    
Days2 = {"Monday", "Tuesday"}    
Days3 = {"Monday", "Tuesday", "Friday"}    
    
#Days1 is the superset of Days2 hence it will print true.     
print (Days1>Days2)     
    
#prints false since Days1 is not the subset of Days2     
print (Days1<Days2)    
    
#prints false since Days2 and Days3 are not equivalent     
print (Days2 == Days3)    

输出：

True
False
False