day13 ternary expressions with, list formula, generator expression, anonymous function, built-in functions

A triplet of expressions:

• grammar:

  Condition holds return value if left to determine the conditions else condition does not hold the right of return value

  # 需求: 让用户输入用户名，输入的用户如果不是tank，为其后缀添加_DSB
  username = input('请输入用户名:').strip()
  new_username = username if username == 'tank' else username + '_DSB'
  print(new_username)

List of formula:

• You can generate a list of his party achieved.

• grammar:

  list = [a value taken for each, for each arbitrary value of an object may be taken out in an iterative iterables]

  It is right for the number of cycles, and may be taken out of each value iterables

  for the left can add value to the current list

  list = [a value for each iteration may be withdrawn in the object iterable]

  list = [value for each value of the iteration can be retrieved object if the object is determined in the iteration may be]

  # 列表生成式
  # 普通的
  new_list = []
  for i in range(1, 6):
      new_list.append(i)
  print(new_list)
  
  #列表
  new_list = [i for i in range(1, 6)]
  print(new_list)

Generator expressions (formula builder)

• 1. What is a generator?
  Generation tools, the generator is a custom iterator, essentially iterators

• If the function comprises the yield keyword, and then call the function, the code does not perform the function thereof, i.e., the return value obtained amount generator object.

• List formula: If the data amount h using

  advantage:

  You can rely on index values

  Disadvantages:

  Waste of resources

• Builder formula: If the large amount of data using

• (Line for line in range (1, 6)) ---> g ​​generators (1, 2, 3, 4, 5)

  advantage:

  Save resources

  Disadvantages:

  The value is not convenient

  # 生成一个有1000个值的生成器
  g = (line for line in range(1, 1000001))
  # <generator object <genexpr> at 0x00000203262318E0>
  print(g)
  
  # 列表生成式实现
  list1 = [line for line in range(1, 1000001)]
  print(list1)
  

Anonymous function

• No name function

  Parameter is left, the right is the return value

  lambda :

  PS: The reason, because there is no name, call the name of the function + ()

  Anonymous function requires a one-time use.

  Note: Anonymous functions using meaningless alone, it makes sense to be used in conjunction with "built-in function"

Built-in functions

• Built-in methods provided inside python

  max , min , sorted , map , filter

  dict1 = {
      'tank': 1000,
      'egon': 500,
      'sean': 200,
      'jason': 500
  }
  print(max(dict1, key=lambda x:dict1[x]))  #max,最大。排序根据ASCII自码表排序的
  
  print(min(dict1,key=lambda x:dict1[x]))# 最小的
  
  new_list = sorted(dict1,key=lambda x: dict1[x],reverse=True)
  print(new_list)