From entry to prison-deduction

Getting started N day

Comprehension: expressions used to quickly and easily generate lists or dictionaries

• List comprehension

• basic structure

  List=[expression for variable in sequence]

  Generate a new list, the elements in the list are the result of the expression corresponding to each loop

  Is equivalent to:

  List=[]

  for variable in sequence:

  ​ List.append(expression)

  list1 = [10 for i in range(3)]
  print(list1)  # [10, 10, 10]
  
  list2 = [2 * i for i in range(1, 4)]
  print(list2)  # [2, 4, 6]
  

  Exercise: Write a list comprehension to produce a list of elements that satisfy the following rules: str0,str3,str6,...,str99

  list3=[f'str{i}'for i in range(0,100,3)]
  print(list3)
  

• Conditional structure

  List=[expression for variable in sequence if conditional statement]

  Is equivalent to:

  List=[]

  for variable in sequence:

  ​ if conditional statement:

  ​ List.append (expression)

  Exercise: Write a list comprehension to produce a list of elements that satisfy the following rules: str1,str3,str6,...,str99

  method one

  list1=[f'str{i}' for i in range(1,100) if i % 3 == 0 or i == 1 ]
  print(list1)
  

  Method 2: ternary operator

  list2=['str1' if i == 0 else f'str{i}' for i in range(0,100,3)]
  print(list2)
  

• Multiple loop structure 1:

  List=[expression for variable 1 in sequence 1 for variable 2 in sequence 2]

  Is equivalent to:

  List=[]

  for variable 1 in sequence 1:

  ​ for variable 2 in sequence 2:

  ​ List.append(expression)

• Multiple loop structure 2:

  List=[expression for variable 1 in sequence 1 if conditional statement for variable 2 in sequence 2 If conditional statement]

  Is equivalent to:

  List=[]

  for variable 1 in sequence 1:

  ​ if conditional statement:

  ​ for variable 2 in sequence 2:

  ​ if conditional statement:

  ​ List.append(expression)

• # 集合推导式
  a1 = {
        
        i for i in 'hello'}
  print(a1)  # {'h', 'e', 'l', 'o'}
  
  # 元组和集合推导式
  a2 = tuple(x * 2 for x in 'hello')
  print(a2)  # ('hh', 'ee', 'll', 'll', 'oo')
  
  # 字典推导式
  a3 = {
        
        x: x * 2 for x in 'hello'}
  print(a3)  # {'h': 'hh', 'e': 'ee', 'l': 'll', 'o': 'oo'}
  
  a4 = dict((x, x * 2) for x in 'hello')
  print(a4)  # {'h': 'hh', 'e': 'ee', 'l': 'll', 'o': 'oo'}
  
  # 练习3 通过字典推导式交换一个字典的键和值 dic={'a':10,'b':20,'c':30}
  dic1 = {
        
        'a': 10, 'b': 20, 'c': 30}
  # dic12 = dict((dic1[i], i) for i in dic1)
  dic2 = {
        
        dic1[i]: i for i in dic1}
  print(dic2)
  
  # 通过字典推导式交换一个字典的键和值 如果值是可变的在新的字典中不交换
  dic3 = {
        
        'a': 10, 'b': 20, 'c': 30, 'd': [10, 20]}
  dic4 = dict((i,dic3[i]) if type(dic3[i]) in (list,set,dict) else (dic3[i],i) for i in dic3)
  print(dic4)