3.2 dictionary

1. If there is no dictionary

1) Define a variable to save the information of a student

# Use list
stu1 = ['Xiao Ming',28,'male',170,63,80,89,99]
print(stu1[0],stu1[-1]) #Xiaoming 99

# Use dictionary
stu2 ={'name':'Xiao Ming','age':28,'Gender':'male','height':170,'weight':63,'language':80,'mathematics':89,'English':99}
print(stu2['name'],stu2['English']) #Xiaoming 99

2) Selection of dictionaries and lists

If multiple data with the same meaning (all names, ages, students, etc.) are saved at the same time, use the list;

If you save multiple data with different meanings at the same time, use a dictionary.

2. What is a dictionary (dict)

1) The dictionary is a container data type. It takes {} as the flag of the container, and multiple key value pairs are separated by commas: {key 1: value 1, key 2: value 2, key 3: value 3...}
2) The dictionary is changeable (it supports addition, deletion and modification); Dictionary is unnecessary (subscript operation is not supported)
3) Dictionary requirements: the elements of the dictionary are key value pairs
Key requirements: a The key must be data of immutable type (generally using string) b. the key is unique
Value requirements: no requirements

# Empty dictionary
dict1 = {}
print(len(dict1),type(dict1)) #0 <class 'dict'>

3. The dictionary is out of order

print({'a':10,'b':20} == {'b':20,'a':10}) #True

The key must be data of immutable type (string, Boolean, null value, number, tuple)

dict2 = {10:20,'ab':30,(10,20):40}
print(dict2) #{10: 20, 'ab': 30, (10, 20): 40}

# dict2 = {10:20,'ab':30,[10,20]:40}
# print(dict2) # unhashable type: 'list'

Key is unique

dict3 = {'a':100,'b':20,'a':30}
print(dict3) #{'a': 30, 'b': 20}

3.2.1 dictionary lookup

1. Look up single - get the value corresponding to a key in the dictionary

1) DICTIONARY [key] - get the value corresponding to the specified key in the dictionary. If the key does not exist, an error will be reported
2) Dictionary Get (key) - get the value corresponding to the specified key in the dictionary; If the key does not exist, no error will be reported, and None will be returned
Dictionaries. Get (key, default) - get the value corresponding to the specified key in the dictionary; If the key does not exist, no error will be reported, and the default value will be returned

dog = {'name': 'Wangcai', 'age': 3, 'breed': 'Earth dog', 'gender': 'Bitch'}
print(dog['name'], dog['gender']) #Wangcai bitch
print(dog.get('name'), dog.get('age')) #Wangcai 3
# print(dog['color']) # KeyError: 'color'
print(dog.get('color'))  # None
print(dog.get('color', 'white'))  # White (if it does not exist, the default value is returned)

3) Dictionary in real life

Define a variable to save the information of a class: class name, location, lecturer, head teacher, and all students

class1 = {
    'class_name': 'Python2204',
    'address': '15 teach',
    'lecturer': {'name': 'Yu Ting', 'age': 18, 'qq': '726550822', 'gender': 'female'},
    'class_teacher': {'name': 'Quiet', 'tel': '110'},
    'students': [
        {'name': 'stu1', 'age': 21, 'major': 'accounting', 'tel': '120', 'contacts': {'name': 'Zhang San', 'tel': '162723'}},
        {'name': 'stu2', 'age': 30, 'major': 'Electronics', 'tel': '219223', 'contacts': {'name': 'Xiao Ming', 'tel': '281912'}},
        {'name': 'stu3', 'age': 19, 'major': 'tourism management', 'tel': '123233', 'contacts': {'name': 'floret', 'tel': '886552'}},
        {'name': 'stu4', 'age': 25, 'major': 'signal communication', 'tel': '4444221', 'contacts': {'name': 'Li Si', 'tel': '22342345'}},
        {'name': 'stu5', 'age': 25, 'major': 'Mechanics', 'tel': '223111', 'contacts': {'name': 'Wang Wu', 'tel': '555632'}},
        {'name': 'stu6', 'age': 23, 'major': 'mathematics', 'tel': '234234', 'contacts': {'name': 'Zhao Liu', 'tel': '96533'}}
    ]
}

# 1) Print class name and classroom
print(class1['class_name'], class1['address']) #Python2204 15 teach

# 2) Print the name and age of the class instructor
lecturer = class1['lecturer']
print(lecturer['name'], lecturer['age']) #Yu Ting 18

# print(class1['lecturer']['name'])    # {'name': 'Yu Ting','age': 18,'qq':'726550822','gender': 'female'}['name']

# 3) Print the phone number of the head teacher
print(class1['class_teacher']['tel']) #110

# 4) Print the names of all students
# Method 1:
all_student = class1['students']
for stu in all_student:
    print(stu['name'])

# Method 2:
# ['stu1', 'stu2', ..., 'stu6']
result = [x['name'] for x in class1['students']]
print(result) #['stu1', 'stu2', 'stu3', 'stu4', 'stu5', 'stu6']

# 5) Calculate the average age of all students
# Method 1:
all_student = class1['students']
total_age = stu_count = 0
for stu in all_student:
    total_age += stu['age']
    stu_count += 1
print('average age:', total_age / stu_count) #Average age: 23.833333333333332

# Method 2
print('average age:', sum([x['age'] for x in class1['students']]) / len(class1['students']))

# 6) Print the names of all students' contacts
# Method 1:
all_student = class1['students']
for x in all_student:
    print(x['contacts']['name'])

# Method 2:
result = [x['contacts']['name'] for x in class1['students']]
print(result)

# 7) Print the names of students whose phone numbers of all contacts end with 2
# ['stu2', 'stu3', 'stu5']
# Method 1:
result = [x['name'] for x in class1['students'] if x['contacts']['tel'][-1] == '2']
print(result)

# Method 2:
for stu in class1['students']:
    if stu['contacts']['tel'][-1] == '2':
        print(stu['name'])

2. Traverse the dictionary

Method 1:

for key in Dictionary:
Loop body (to yes key obtained by variable)

Method 2:
for key, value in dictionary.items():
Circulatory body

dog = {'name': 'Wangcai', 'age': 3, 'breed': 'Earth dog', 'gender': 'Bitch'}
for x in dog:
    print('x:', x, dog[x])

for key, value in dog.items():
    print(key, value)

3.2.2 addition, deletion and modification of dictionary

1. Change - modify the value corresponding to a key

DICTIONARY [key] = new value - modify the value corresponding to the specified key in the dictionary to the specified new value

dog = {'name': 'Wangcai', 'age': 3, 'breed': 'Earth dog', 'gender': 'Bitch'}
print(dog)      # {'name': 'Wangcai', 'age': 3,'breed':' hyena ',' gender':' bitch '}

dog['name'] = 'Caicai'
print(dog)      # {'name': 'Caicai', 'age': 3,'breed':' hyena ',' gender':' bitch '}

1. Add - add key value pairs

1) DICTIONARY [key] = value - if the key does not exist, add a key value pair to the dictionary; If the key exists, the value of the key value pair will be modified
2) Dictionary SetDefault (key, value) - add the specified key value pair to the dictionary; If the key exists, the original value will be retained (it will not be modified)

dog = {'name': 'Wangcai', 'age': 3, 'breed': 'Earth dog', 'gender': 'Bitch'}
print(dog)      # {'name': 'Wangcai', 'age': 3,'breed':' hyena ',' gender':' bitch '}

dog['color'] = 'yellow'
print(dog)      # {'name': 'Wangcai', 'age': 3,'breed':' hyena ',' gender':' bitch ',' color':' yellow '}

dog.setdefault('weight', 15)
print(dog)      # {'name': 'Wangcai', 'age': 3,'breed':' hyena ',' gender':' bitch ',' color':' yellow ',' weight': 15}

# The existence of the key will modify the value of the key value pair
dog['age'] = 10
print(dog)      # {'name': 'Wangcai', 'age': 10,'breed':' hyena ',' gender':' bitch ',' color':' yellow ',' weight': 15}

# If the key exists, the original value will be retained (it will not be modified)
dog.setdefault('gender', 'male dog')
print(dog)

goods_list = [
    {'name': 'Instant noodles', 'price': 4, 'discount': 0.9},
    {'name': 'Ham sausage', 'price': 1.5},
    {'name': 'mineral water', 'price': 2, 'discount': 0.8},
    {'name': 'bread', 'price': 5.5},
    {'name': 'Yogurt', 'price': 7.5}
]
# Add a discount value of 1 to products without discount
for goods in goods_list:
    goods.setdefault('discount', 1)
print(goods_list)

1. Delete - delete key value pairs

1) del DICTIONARY [key] - delete the key value pair corresponding to the specified key in the dictionary

2) Dictionary Pop (key) - take out the value corresponding to the specified key in the dictionary

dog = {'name': 'Wangcai', 'age': 10, 'breed': 'Earth dog', 'gender': 'Bitch', 'color': 'yellow', 'weight': 15}
print(dog)      # {'name': 'Wangcai', 'age': 10,'breed':' hyena ',' gender':' bitch ',' color':' yellow ',' weight': 15}


del dog['gender']
print(dog)      # {'name': 'Wangcai', 'age': 10,'breed':' hyena ',' color':' yellow ',' weight': 15}

color = dog.pop('color')
print(dog, color)      # {'name': 'Wangcai', 'age': 10,'breed':' hyena ',' weight': 15}

3.2.3 relevant operations

Dictionary does not support +, *, >, <, > =<=

1. In and not in - determine whether the specified key exists or does not exist in the dictionary

Key in dictionary

Key not in dictionary

dict1 = {'a': 10, 'b': 20, 'c': 30}
print(10 in dict1)      # False
print('b' in dict1)     # True

2. Correlation function: len, dict

1) Len (Dictionary) - get the number of key value pairs in the dictionary

2) Dict (data) - converts the specified data into a dictionary

Requirements for data that can be converted into Dictionaries:
a. The data itself is a sequence
b. The element in the sequence must be a small sequence with only two elements
c. The first element of a small sequence must be data of immutable type

result = dict(['ab', 'cd', 'ef'])
print(result) # {'a': 'b', 'c': 'd', 'e': 'f'}

result = dict([('name', 'xiaoming'), ('age', 18)])
print(result) # {'name': 'xiaoming', 'age': 18}

Note: when the dictionary is converted into a list, all the keys of the dictionary are taken as the elements of the list

dog = {'name': 'Wangcai', 'age': 10, 'breed': 'Earth dog', 'gender': 'Bitch', 'color': 'yellow', 'weight': 15}
print(list(dog))        # ['name', 'age', 'breed', 'gender', 'color', 'weight']

3. Related methods: dictionary.xxx()

1) Dictionary clear() - clear the dictionary
2) Dictionary copy() - copy the dictionary to produce an identical new dictionary
3) Dictionary keys() - get all the keys in the dictionary and return a sequence
Dictionaries. values() - get all the values of the dictionary and return a sequence
Dictionaries. items() - convert the dictionary into a sequence, and convert each key value pair into a tuple

dog = {'name': 'Wangcai', 'age': 10, 'breed': 'Earth dog', 'gender': 'Bitch', 'color': 'yellow', 'weight': 15}
print(dog.keys()) #dict_keys(['name', 'age', 'breed', 'gender', 'color', 'weight'])
print(dog.values()) #dict_values(['Wangcai', 10, 'local dog', 'bitch', 'yellow', 15])
print(dog.items()) #dict_items([('name',' Wangcai '), ('age', 10), ('breed',' hyena '), ('gender', 'bitch'), ('color',' yellow '), ('weight', 15)])

4) Dictionary 1.update (dictionary 2) - add all key value pairs in dictionary 2 to dictionary 1

d1 = {'a': 10, 'b': 20}
d2 = {'x': 100, 'y': 200}
d1.update(d2)
print(d1)       # {'a': 10, 'b': 20, 'x': 100, 'y': 200}

Homework today

1. Define a variable to store a student's information. Student confidence includes: name, age, grade (single subject), phone number, and gender

   stu1 = {'name':'Zhang San','age':21,'score_Chinese':90,'score_math':100,'score_English':99,'tel':110,'gender':'male'}
   print(stu1)
   

2. Define a list, and save the information of 6 students in the list (student information includes: name, age, grade (single subject), phone number, gender (male, female, unknown))

   (1) Count the number of failed students

   (2) Print the names and corresponding grades of the underage students who failed

   # Method 1:
   for i in list1:
      if i['score'] < 60:
         print(i['name'],i['score'])
   # Method 2:
   list2=[(i['name'],i['score']) for i in list1 if i['score'] < 60]
   print(list2)
   

   (3) Average age of all boys

   count = 0
   sum = 0
   for i in list1:
      if i['gender'] == 'male':
         count +=1
         sum += i['age']
   print(sum/count)
   

   (4) Print the name of the student whose mobile phone tail number is 8

   name1=[i['name'] for i in list1 if i['tel'] % 10 == 8]
   print(name1)
   

   (5) Print the highest score and the name of the corresponding student

   list3=[]
   for i in list1:
      list3.append(i['score'])
      list3.sort()
      if i['score'] == list3[-1]:
         print(i['score'],i['name'])
   

   (6) Delete all students of Unknown Gender

   list4=[list1[i] for i in range(len(list1)) if list1[i]['gender'] != 'Unknown']
   print(list4)
   

   (7) Sort the list from big to small according to students' grades (struggle, give up if you can't)

   for i in range(1,len(list4)):
      for j in range(0,len(list4)-i):
         if list4[j]['score'] > list4[j+1]['score']:
            list4[j]['score'],list4[j+1]['score']=list4[j+1]['score'],list4[j]['score']
   print(list4)
   

3. Define a variable to save the information of a class. The class information includes: class name, classroom location, head teacher information, lecturer information, and all students in the class (determine the data type and specific information according to the actual situation)

   class1={
      'class_name':'python2204',
      'class_location':'15classroom',
      'class_teacher':{'name':'Sister Jing','gender':'female','position':'unknown','tel':234567890,'QQ':234523,'WX':1563214},
      'lecturer':[{'name': 'Sister Ting','gender':'female','position':'lecturer','tel':15632452,'QQ':563255,'WX':56321563},
                  {'name': 'Brother Fu','gender':'male','position':'lecturer','tel':14523422,'QQ':63252452,'WX':523563}],
      'students': [
           {'name': 'stu1', 'age': 21,'gender':'male', 'major': 'accounting', 'tel': '120', 'contacts': {'name': 'Zhang San', 'tel': '162723'}},
           {'name': 'stu2', 'age': 26,'gender':'female', 'major': 'Electronics', 'tel': '219223', 'contacts': {'name': 'Xiao Ming', 'tel': '281912'}},
           {'name': 'stu3', 'age': 19,'gender':'male', 'major': 'tourism management', 'tel': '123233', 'contacts': {'name': 'floret', 'tel': '886552'}},
           {'name': 'stu4', 'age': 22,'gender':'female', 'major': 'signal communication', 'tel': '4444221', 'contacts': {'name': 'Li Si', 'tel': '22342345'}},
           {'name': 'stu5', 'age': 21,'gender':'male', 'major': 'Mechanics', 'tel': '223111', 'contacts': {'name': 'Wang Wu', 'tel': '555632'}},
           {'name': 'stu6', 'age': 230,'gender':'female', 'major': 'mathematics', 'tel': '234234', 'contacts': {'name': 'Zhao Liu', 'tel': '96533'}}
       ]
   }
   

4. It is known that a list stores dictionaries corresponding to multiple dogs:

   dogs = [
     {'name': 'Beibei', 'color': 'white', 'breed': 'Silver Fox', 'age': 3, 'gender': 'mother'},
     {'name': 'tearful', 'color': 'grey', 'breed': 'Fadou', 'age': 2},
     {'name': 'Caicai', 'color': 'black', 'breed': 'Earth dog', 'age': 5, 'gender': 'common'},
     {'name': 'steamed stuffed bun', 'color': 'yellow', 'breed': 'Siberian Husky', 'age': 1},
     {'name': 'cola', 'color': 'white', 'breed': 'Silver Fox', 'age': 2},
     {'name': 'Wangcai', 'color': 'yellow', 'breed': 'Earth dog', 'age': 2, 'gender': 'mother'}
   ]
   

   (1) Use the list derivation to obtain the breeds of all dogs

   ['silver fox', 'fadou', 'local dog', 'husky', 'silver fox', 'local dog']

   dog_breed = [i['breed'] for i in dogs]
   print(dog_breed)
   

   (2) Use list derivation to get the names of all white dogs

   ['Beibei', 'Cola']

   white_dog_name = [i['name'] for i in dogs if i['color'] == 'white']
   print(white_dog_name)
   

   (3) Add gender 'male' to dogs without gender in dogs

   for i in dogs:
      i.setdefault('gender','common')
   print(dogs)
   

   (4) Count the number of 'silver foxes'

   count = 0
   for i in dogs:
      if i['breed'] == 'Silver Fox':
         count += 1
   print(count)