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📜  Python Set

📅  最后修改于: 2020-10-30 06:01:10             🧑  作者: Mango

Python Set

Python集是无序项目的集合。集合中的每个元素必须是唯一的，不可变的，并且集合会删除重复的元素。集是可变的，这意味着我们可以在创建后对其进行修改。

与Python的其他集合不同，集合的元素没有附加索引，即，我们无法通过索引直接访问集合的任何元素。但是，我们可以将它们全部print在一起，也可以通过遍历集合来获取元素列表。

创建一个集合

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

示例1：使用花括号

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'}

looping through the set elements ... 
Friday
Tuesday
Monday
Saturday
Thursday
Sunday
Wednesday

示例2：使用set()方法

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'}

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。

# 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：



Traceback (most recent call last)
 in 
      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具有不可变元素，而set2具有一个可变元素作为列表。在检查set2的类型时，它引发了一个错误，这意味着set只能包含不可变元素。

创建一个空集有点不同，因为空的花括号{}也也用于创建字典。因此， Python提供了不带参数的set()方法来创建空集。

# Empty curly braces will create dictionary
set3 = {}
print(type(set3))

# Empty set using set() function
set4 = set()
print(type(set4))

输出：

让我们看看如果我们向集合提供重复元素会发生什么。

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由多个重复元素组成，当我们打印它时，从集合中删除了重复项。

将项目添加到集合中

Python提供了add()方法和update()方法，可用于将某些特定项目添加到集合中。 add()方法用于添加单个元素，而update()方法用于向集合添加多个元素。考虑以下示例。

示例：1-使用add()方法

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

要在集合中添加多个项目， Python提供了update()方法。它接受iterable作为参数。

考虑以下示例。

示例-2使用update()函数

Months = set(["January","February", "March", "April", "May", "June"])  
print("\nprinting the original set ... ")  
print(Months)  
print("\nupdating the original set ... ")  
Months.update(["July","August","September","October"]);  
print("\nprinting the modified set ... ")   
print(Months);

输出：

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

updating the original set ... 
printing the modified set ... 
{'January', 'February', 'April', 'August', 'October', 'May', 'June', 'July', 'September', 'March'}

从集合中删除项目

Python提供了throw()方法和remove()方法，可用于从集合中删除项目。这些函数之间的区别是，如果该项在集合中不存在，则使用discard()函数，则该集合保持不变，而remove()方法将出现错误。

考虑以下示例。

示例1：使用discard()方法

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

Python还提供了remove()方法来从集合中删除项目。考虑以下示例，使用remove()方法删除项目。

示例2-使用remove()函数

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：

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

Removing some months from the set...

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

我们还可以使用pop()方法删除该项目。通常，pop()方法将始终删除最后一项，但是集合是无序的，我们无法确定将从集合中弹出哪个元素。

考虑以下示例，使用pop()方法从集合中删除该项目。

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'}

在上面的代码中，Month集合的最后一个元素是March，但是pop()方法删除了June和January，因为该集合是无序的，并且pop()方法无法确定该集合的最后一个元素。

Python提供了clear()方法来删除集合中的所有项目。

考虑以下示例。

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

输出：

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

Removing all the items from the set...

Printing the modified set...
set()

Dispose()和remove()之间的区别

尽管discard()和remove()方法都执行相同的任务，但是discard()和remove()之间有一个主要区别。

如果使用set()中不存在要使用set()删除的键，则Python将不会给出错误。该程序将保持其控制流。

另一方面，如果集合中不存在使用remove()从集合中删除的项目，则Python会引发错误。

考虑以下示例。

例-

Months = set(["January","February", "March", "April", "May", "June"])  
print("\nprinting the original set ... ")  
print(Months)  
print("\nRemoving items through discard() method...");  
Months.discard("Feb"); #will not give an error although the key feb is not available in the set  
print("\nprinting the modified set...")  
print(Months)  
print("\nRemoving items through remove() method...");  
Months.remove("Jan") #will give an error as the key jan is not available in the set.   
print("\nPrinting the modified set...")  
print(Months)

输出：

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

Removing items through discard() method...

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

Removing items through remove() method...
Traceback (most recent call last):
  File "set.py", line 9, in 
    Months.remove("Jan")
KeyError: 'Jan'

Python设置操作

可以执行数学运算，如并集，相交，差和对称差。 Python提供了使用运算符或方法执行这些操作的工具。我们将这些操作描述如下。

两套并集

使用管道(|)运算符可计算出两组的并集。这两个集合的并集包含两个集合中都存在的所有项目。

考虑以下示例来计算两个集合的并集。

示例1：使用union |运算符

Days1 = {"Monday","Tuesday","Wednesday","Thursday", "Sunday"}  
Days2 = {"Friday","Saturday","Sunday"}  
print(Days1|Days2) #printing the union of the sets

输出：

{'Friday', 'Sunday', 'Saturday', 'Tuesday', 'Wednesday', 'Monday', 'Thursday'}

Python还提供了union()方法，该方法还可用于计算两个集合的并集。考虑以下示例。

示例2：使用union()方法

Days1 = {"Monday","Tuesday","Wednesday","Thursday"}  
Days2 = {"Friday","Saturday","Sunday"}  
print(Days1.union(Days2)) #printing the union of the sets

输出：

{'Friday', 'Monday', 'Tuesday', 'Thursday', 'Wednesday', 'Sunday', 'Saturday'}

两套相交

两组的交集可以通过＆和运算符或交集()函数。这两个集合的交集作为两个集合中共有的元素的集合给出。

考虑以下示例。

示例1：使用＆运算符

Days1 = {"Monday","Tuesday", "Wednesday", "Thursday"}  
Days2 = {"Monday","Tuesday","Sunday", "Friday"}  
print(Days1&Days2) #prints the intersection of the two sets

输出：

{'Monday', 'Tuesday'}

示例2：使用intersection()方法

set1 = {"Devansh","John", "David", "Martin"}  
set2 = {"Steve", "Milan", "David", "Martin"}  
print(set1.intersection(set2)) #prints the intersection of the two sets

输出：

{'Martin', 'David'}

范例3：

set1 = {1,2,3,4,5,6,7}
set2 = {1,2,20,32,5,9}
set3 = set1.intersection(set2)
print(set3)

输出：

{1,2,5}

section_update()方法

section_update()方法从原始集中删除两个集中都不存在的项目(如果指定了多个，则为所有集中)。

交集更新()方法与交集()方法的不同之处在于，它通过删除不需要的项来修改原始集，而交集()方法返回一个新集。

考虑以下示例。

a = {"Devansh", "bob", "castle"}  
b = {"castle", "dude", "emyway"}  
c = {"fuson", "gaurav", "castle"}  
  
a.intersection_update(b, c)  
  
print(a)

输出：

{'castle'}

两组之间的差异

可以通过使用减法(-)运算符或交集()方法来计算两组的差。假设有两个集合A和B，并且差为AB，则表示将获得集合B中不存在的A元素，从而得到结果集合。

考虑以下示例。

示例1：使用减法(-)运算符

Days1 = {"Monday",  "Tuesday", "Wednesday", "Thursday"}  
Days2 = {"Monday", "Tuesday", "Sunday"}  
print(Days1-Days2) #{"Wednesday", "Thursday" will be printed}

输出：

{'Thursday', 'Wednesday'}

示例2：使用difference()方法

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'}

两组的对称差

两组的对称差通过^运算符或symmetric_difference()方法计算。集的对称差异，它删除了两个集中都存在的元素。考虑以下示例：

示例-1：使用^运算符

a = {1,2,3,4,5,6}
b = {1,2,9,8,10}
c = a^b
print(c)

输出：

{3, 4, 5, 6, 8, 9, 10}

示例-2：使用symmetric_difference()方法

a = {1,2,3,4,5,6}
b = {1,2,9,8,10}
c = a.symmetric_difference(b)
print(c)

输出：

{3, 4, 5, 6, 8, 9, 10}

设置比较

Python允许我们通过使用我们可以检查一组是否是一个子集，超集，或等同的其他组使用比较运算符，即，<，>，<=，> =，==与集。根据集合中存在的项目返回布尔值true或false。

考虑以下示例。

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

输出：

True
False
False

冷冻套

冻结集是普通集的不变形式，即，冻结集的项目无法更改，因此可以用作字典中的键。

创建后，冻结集合的元素无法更改。我们不能通过使用诸如add()或remove()之类的方法来更改或追加冻结集的内容。

Frozenset()方法用于创建Frozenset对象。可迭代的序列将传递给此方法，该方法将转换为冻结集，作为该方法的返回类型。

考虑以下示例以创建冻结集。

Frozenset = frozenset([1,2,3,4,5])   
print(type(Frozenset))  
print("\nprinting the content of frozen set...")  
for i in Frozenset:  
    print(i);  
Frozenset.add(6) #gives an error since we cannot change the content of Frozenset after creation

输出：



printing the content of frozen set...
1
2
3
4
5
Traceback (most recent call last):
  File "set.py", line 6, in 
    Frozenset.add(6) #gives an error since we can change the content of Frozenset after creation 
AttributeError: 'frozenset' object has no attribute 'add'

字典的Frozenset

如果将字典作为序列传递给Frozenset()方法内的序列，它将仅接收字典中的键，并返回一个包含字典键作为其元素的Frozenset。

考虑以下示例。

Dictionary = {"Name":"John", "Country":"USA", "ID":101}   
print(type(Dictionary))  
Frozenset = frozenset(Dictionary); #Frozenset will contain the keys of the dictionary  
print(type(Frozenset))  
for i in Frozenset:   
    print(i)

输出：



Name
Country
ID

设置编程示例

示例-1：编写程序以从集中删除给定的数字。

my_set = {1,2,3,4,5,6,12,24}
n = int(input("Enter the number you want to remove"))
my_set.discard(n)
print("After Removing:",my_set)

输出：

Enter the number you want to remove:12
After Removing: {1, 2, 3, 4, 5, 6, 24}

示例-2：编写一个程序将多个元素添加到集合中。

set1 = set([1,2,4,"John","CS"])
set1.update(["Apple","Mango","Grapes"])
print(set1)

输出：

{1, 2, 4, 'Apple', 'John', 'CS', 'Mango', 'Grapes'}

示例-3：编写程序以查找两个集合之间的联合。

set1 = set(["Peter","Joseph", 65,59,96])
set2  = set(["Peter",1,2,"Joseph"])
set3 = set1.union(set2)
print(set3)

输出：

{96, 65, 2, 'Joseph', 1, 'Peter', 59}

例4：编写程序以查找两个集合之间的交集。

set1 = {23,44,56,67,90,45,"Javatpoint"}
set2 = {13,23,56,76,"Sachin"}
set3 = set1.intersection(set2)
print(set3)

输出：

{56, 23}

示例-5：编写程序以将元素添加到Frozenset。

set1 = {23,44,56,67,90,45,"Javatpoint"}
set2 = {13,23,56,76,"Sachin"}
set3 = set1.intersection(set2)
print(set3)

输出：

TypeError: 'frozenset' object does not support item assignment

上面的代码提出了一个错误，因为冻结集是不可变的，创建后无法更改。

示例-6：编写程序以查找issuperset，issubset和superset。

set1 = set(["Peter","James","Camroon","Ricky","Donald"])
set2 = set(["Camroon","Washington","Peter"])
set3 = set(["Peter"])

issubset = set1 >= set2
print(issubset)
issuperset = set1 <= set2
print(issuperset)
issubset = set3 <= set2
print(issubset)
issuperset = set2 >= set3
print(issuperset)

输出：

False
False
True
True

Python内置设置方法

Python包含以下与这些集合一起使用的方法。

SN Method Description
1 add(item) It adds an item to the set. It has no effect if the item is already present in the set.
2 clear() It deletes all the items from the set.
3 copy() It returns a shallow copy of the set.
4 difference_update(….) It modifies this set by removing all the items that are also present in the specified sets.
5 discard(item) It removes the specified item from the set.
6 intersection() It returns a new set that contains only the common elements of both the sets. (all the sets if more than two are specified).
7 intersection_update(….) It removes the items from the original set that are not present in both the sets (all the sets if more than one are specified).
8 Isdisjoint(….) Return True if two sets have a null intersection.
9 Issubset(….) Report whether another set contains this set.
10 Issuperset(….) Report whether this set contains another set.
11 pop() Remove and return an arbitrary set element that is the last element of the set. Raises KeyError if the set is empty.
12 remove(item) Remove an element from a set; it must be a member. If the element is not a member, raise a KeyError.
13 symmetric_difference(….) Remove an element from a set; it must be a member. If the element is not a member, raise a KeyError.
14 symmetric_difference_update(….) Update a set with the symmetric difference of itself and another.
15 union(….) Return the union of sets as a new set.
(i.e. all elements that are in either set.)
16 update() Update a set with the union of itself and others.