Python Data Types - Collections

0. What is a collection?

A set is an unordered, non-repeating combination of data , and its main functions are as follows:

Deduplication, turning a list into a collection, it will automatically deduplicate

Relationship test, test the intersection, difference, union and other relationships before two sets of data

Set (set): Grouping different elements together to form a set is the basic data type of python.

set elements: the members that make up the set ( non-repeatable )

A collection object is an unordered set of hashable (immutable, dictionary-like keys) values : members of the collection can act as dictionary keys

Collection classification: mutable collection, immutable collection

Mutable set (set): elements can be added and removed, non-hashable, cannot be used as a dictionary key, and cannot be used as elements of other sets

Immutable collections (frozenset): the exact opposite of the above

li = [[1,2], ' a ' , ' b ' ]
s =set(li) # TypeError: unhashable type: 'list'
print(s)

a=set( ' alex li a ' )   #There are multiple a 
print (a)
{ '  ' , ' l ' , ' i ' , ' a ' , ' x ' , ' e ' }     #The set will remove duplicates and print them out of order 

b =[ ' aaa ' , ' bbb ' , ' aaa ' ]
s=set(b)
print(s)
{ ' aaa ' , ' bbb ' }       #If there are duplicates in the list, the duplicates will be removed

1. Create a collection

Since the collection does not have its own syntax format, it can only be created through the methods set() and frozenset() of the collection , or {} to create

s1 = set('alvin')
 
s2 = frozenset( ' yuan ' )   #immutable set 

s3 ={1,2,3 }


print(s1,type(s1))  #{'l', 'v', 'i', 'a', 'n'} <class 'set'>
print(s2,type(s2))  #frozenset({'n', 'y', 'a', 'u'}) <class 'frozenset'>

2. Access the collection

Since the collection itself is unordered, indexing or slicing operations cannot be created for the collection, only loop traversal or use in, not in to access or judge the elements of the collection.

s1 = set('alvin')
print('a' in s1)
print('b' in s1)
#s1[1]  #TypeError: 'set' object does not support indexing
 
for i in s1:
    print(i)
#    
# True
# False
# v
# n
# l
# i
# a

3. Update the collection

Updates can be made using the following built-in methods:

set.add(): add

set.remove()：删

set.update()：改

Note: Only mutable collections can be updated:

# s1 = frozenset('alvin')
# s1.add(0)  #AttributeError: 'frozenset' object has no attribute 'add'
 
s2=set('alvin')
s2.add( ' mm ' )    # 'mm' will be stored as a whole 
print (s2)   # { 'mm' , 'l', 'n', 'a', 'i', 'v'} 
 
s2. update( ' HO ' ) #Add multiple elements, split the HO, and add it as a single element. If there are duplicates, it will be de-duplicated 
print (s2)   # {'mm', 'l', 'n', ' a', 'i', 'H', 'O' , 'v'}

s2.update([12, ' alex ' , ' eee ' , ' alvin ' ])   #In the list, the element is a whole, print (s2) # {'l', 'n', ' alvin' , 'a' , 12 , 'i', 'alex' , 'v', 'eee' } 
s2.remove( ' l ' )    #Specify delete, when the value does not exist, an error will be reported print (s2)   # {'mm', ' n', 'a', 'i', 'H', 'O', 'v'} 
s2.discard( 'vvvv ' ) #Specify delete, when the value does not exist, no error will be reported 
s2.pop()   



#Delete an element randomly s2.clear 
() #Empty the   collection del 
s2 : delete the collection itself

4. Collection type operators

1 in , not in: determine whether an element is in the set

2 Set equivalence and inequality (==, !=)

3 Subsets, Supersets

4 print( a.isdisjoint (b)), returns true if the two sets are not related in any way