3. Dictionaries

3.1 Characteristics

• Dictionary is the most flexible built-in data type used in python outside the list
• A dictionary is a variable data type
• The difference between list and list
  • List order, dictionary disorder
  • Lists access elements by index, and dictionaries access elements by key
• Basic Dictionary Format

dic={key1:value1,key2:value2,......keyn:valuen}

• Values can be of any data type, but keys can't be, they must be immutable data types, that is, keys must be hashed.
• supplement
  • Kohashi: immutable
  • Not Hash: Variable
• Characteristic
  • Key: value, key-value pairs connected by colons
  • key: value, an element of a dictionary
  • Keyvalue pairs are separated by commas
  • The whole dictionary is wrapped in {}

3.2 Creation

Empty Dictionary

>>> dic={}
>>> type(dic)
<class 'dict'>

Multivariate Dictionary

info = {'id':31901001,'name':'Sheep','sex':'boy'}

>>> dic=dict(id=1001,name='sheep',age=16)
>>> dic
{'id': 1001, 'name': 'sheep', 'age': 16}

Strong turn (use of zip)

zip()
key=('id','name','age')
value=(1001,'sheep',16)
tp=dict(zip(key,value))
print(tp,type(zip),type(tp))
#{'id': 1001, 'name': 'sheep', 'age': 16} 
#<class 'type'> <class 'dict'>

Create a dictionary through the fromkeys method

• format

dict.fromkeys(seq,val=None)

• Create and return a dictionary
  • Use the elements in sep as keys in a dictionary
  • val is the initial value of all keys in the dictionary and does not provide a default of None.

dic=dict.fromkeys('Tom')
print(dic)
#{'T': None, 'o': None, 'm': None}

dic=dic.fromkeys(['name','sex','age'],33)
print(dic)
#{'name': 33, 'sex': 33, 'age': 33}

3.3 Common Operations

3.3.1 Increase

• Add key-value pairs directly (dic[key]=value)
  • By using the variable name ['key']='value'
    • If key does not exist, create a new key-value pair [key:value]
    • Update the value if it exists

dic={}
dic['id'] = 1001
dic['name'] = 'sheep'
dic['age'] = 16
print(dic)
#{'id': 1001, 'name': 'sheep', 'age': 16}

dic['name'] = 'Tom'
print(dic)
#{'id': 1001, 'name': 'Tom', 'age': 16}

• Example: Simple add queries

lis=[{'id':1001,'name':'Tom'},{'id':1002,'name':'Sheep'}]
def sel_name(id):
    id=int(id)
    for i in lis:
        if i['id']==id:
            return i['name']
    return 1;
def add_lis(id,name):
    id=int(id)
    lis.append({'id':id,'name':name})

while True:
    a = input('-----------\n1.Adding members\n2.adopt id Search for personnel\n0.Exit and view\n-----------\n')
    a = int(a)
    if a == 1:
        id = input('Please enter Add id:')
        while True:
            if sel_name(id) == 1:
                break
            else:
                print('id Has been registered!!!')
                id = input('Please re-enter id: ')
        name = input('Please enter the added name:')
        add_lis(id, name)
        print('Added Successfully')
    elif a == 2:
        id = input('Please enter id: ')
        if sel_name(id) == 1:
            print("No one can find it.")
        else:
            print('name:', sel_name(id))
    elif a == 0:
        print(lis)
        exit()
    else:
        print('Input error!')
    st=input('Press any key to continue!')

• dic.update(dic)

  Update key: value pairs in dictionary dic to dict

  • If the keys of the two dictionaries are completely different, add dic's key values to dict altogether.
  • If there are two identical keys, update

  dic={'id':1001,'name':'sheep'}
  dic2={'sex':'boy','name':'tom'}
  dic2.update(dic)
  print(dic2)
  #{'sex': 'boy', 'name': 'sheep', 'id': 1001}
  

3.3.2 Delete

• dict.pop

  • Format: dict.pop(key[,default])
    • If key exists in dict, delete and return dict[key]
    • If it does not exist, the default value is returned, the default value is not given, and the error is reported.

  dic2={'sex': 'boy', 'name': 'sheep', 'id': 1001}
  dic2.pop('sex')
  print(dic2)
  #{'name': 'sheep', 'id': 1001}
  dic2.pop('age')
  #Report errors
  a=dic2.pop('sex')
  print(a)
  #boy
  

• dict.popitem()

  • Then delete a pair of key-value pairs
  • Tuples with return values

  dic2={'sex': 'boy', 'id': 1001,'name': 'sheep'}
  a=dic2.popitem()
  print(dic2,a)
  #{'sex': 'boy', 'id': 1001} ('name', 'sheep')
  

• del dict[]

  Delete base

  dic2={'sex': 'boy', 'id': 1001,'name': 'sheep'}
  del dic2['sex']
  print(dic2)
  #{'id': 1001, 'name': 'sheep'}
  

• dict.clear()

  empty

  dic2={'sex': 'boy', 'id': 1001,'name': 'sheep'}
  dic2.clear()
  print(dic2)
  #{}
  

3.3.3 Change

• directly modify

dic={'sex': 'boy', 'id': 1001,'name': 'sheep'}
dic['id']=1002
print(dic)
#{'sex': 'boy', 'id': 1002, 'name': 'sheep'}

• dict.setdefault()
  • Build, exist, unchanged, return the value in the dictionary
  • No, add the corresponding key-value pair to the dictionary and return the corresponding value.

dic={'sex': 'boy', 'id': 1001,'name': 'sheep'}
print(dic.setdefault('id',999))
print(dic)
print(dic.setdefault('age',16))
print(dic)
###########
1001
{'sex': 'boy', 'id': 1001, 'name': 'sheep'}
16
{'sex': 'boy', 'id': 1001, 'name': 'sheep', 'age': 16}

3.3.4 check

• Access to dictionary values
  • Access value through the key of the dictionary

print(dic['id'])
#Error reporting if return does not exist

• dict.get() accesses value
  • The grammatical format dict.get(key[,default])
  • Uncertainty about whether a key exists in a dictionary and want to manipulate it, such as getting a value --> get method
  • When key does not exist, no exception is thrown and None is returned.

dic={'sex': 'boy', 'id': 1001,'name': 'sheep'}
print(dic.get('age'))
#None
print(dic.get('age',12))
#12

3.4 traversal

• dict.keys() returns all keys
• dict.values() returns all values
• dict.items() returns key and values

dic={'sex': 'boy', 'id': 1001,'name': 'sheep'}
print(dic.keys())
print(dic.values())
print(dic.items())
#dict_keys(['sex', 'id', 'name'])
#dict_values(['boy', 1001, 'sheep'])
#dict_items([('sex', 'boy'), ('id', 1001), ('name', 'sheep')])

for i in dic.items():
    print(i[0],':',i[1])
#sex : boy
#id : 1001
#name : sheep
for i,j in dic.items():
    print(i,':',j)
#Ibid.

#Give an example
li=[22,33,55,66,88,45,95,99,79]
dic={'>=60':[],'<60':[]}
for i in li:
    if i>=60:
        dic['>=60'].append(i)
    else:
        dic['<60'].append(i)
print(dic)

4. Sets

set()

4.1 Characteristics

• disorder
• Not repeatable
• Immutable
  • Internal elements are hashable
  • The set itself is not hashing
• Unit data sets enclosed in {}
• purpose
  • Duplicate removal
  • Relational testing

4.2 Creation

4.2.1 Creation of Empty Sets

>>> st=set()
>>> st
set()
>>> st={}
>>> type(st)
<class 'dict'>

4.2.2 Multivariate Set Creation

>>> st={1,2,5,6,9}
>>> type(st)
<class 'set'>

4.3 Strong Turn

>>> li= ['city','cc','aa']
>>> st=set(li)
>>> type(st)
<class 'set'>
>>> st
{'cc', 'city', 'aa'}

>>> s='city'
>>> st=set(s)
>>> st
{'c', 't', 'i', 'y'}

>>> dic={'id':101,'name':'tom'}
>>> st=set(dic)
>>> st
{'id', 'name'}
>>> lu=list(dic)
>>> lu
['id', 'name']

4.4 Basic Operations

4.4.1 Increase

• set.add()
• set.update()

4.4.2 Delete

• set.pop() Deletes one of the smallest sorted elements
• set.discard() removes elements that do not exist and will report errors
• set.remove()
• del set

>>> li=[1,2,3,4,5,6,'a','b']
>>> s=set(li)
>>> s.pop()
1
>>> s.discard('a')
{2, 3, 4, 5, 6, 'b'}
>>> s.remove(2)
{3, 4, 5, 6, 'b'}

Irreversible and unverifiable

4.5 traversal

li=['a','b','c',1,2,3]
s=set(li)
for a,b in enumerate(s,0):
    print('(',a,b,')',end=' ')
#( 0 1 ) ( 1 2 ) ( 2 3 ) ( 3 b ) ( 4 a ) ( 5 c ) 

4.6 Set Basic Operations

4.6.1 subset

>>> a=set('asd')
>>> b=set('dsa')
>>> a,b
({'d', 'a', 's'}, {'d', 'a', 's'})
>>> a==b
True
>>> a>b
False

4.6.2 intersection

• &
• set.intersection

a=set('accqa')
b=set('gqebgeb')
print(a&b)
#{'q'}
print(a.intersection(b))
#{'q'}

4.6.3 Union

• |
• set.union()

print(a|b)
#{'q', 'b', 'e', 'g', 'c', 'a'}
print(a.union(b))
#{'q', 'b', 'e', 'g', 'c', 'a'}

4.6.4 difference set

• set.diference()

print(a-b)
print(a.difference(b))
#{'c', 'a'}