Python基础第八天:各种推导式玩法

时间:2021-01-11 18:38:24

各种推导式玩法

  推导式套路

  之前我们已经学习过最简单的列表推导和生成器表达式。但是除此之外,其实还有字典推导式、集合推导式等等

  下面就是一个列表推导式为例的推导式详细格式,同样适用于其他推导式

  variable = [out_exp_res for out_exp in input_list if out_exp == 2]   out_exp_res: #列表生成式,可以是有返回值的函数
      for out_exp input_list: #迭代inp_list 将 out_exp_res 表达式中
      if out_exp == 2         #根据条件过滤那些值可以

  列表推导式

  例一:30以内所有内被3整除的数

  multioless = [i for i in range(30) if i%3 is 0]   print(multioless)
  [0, 3, 6, 9, 12, 15, 18, 21, 24, 27]

  例二:30以内所有能被3整除的数的平方

  def squard(x):   return x*x   multiples = [squard(i) for i in range(30) if i%3 is 0]   print(multiples)
  [0, 9, 36, 81, 144, 225, 324, 441, 576, 729]

  例三:找到嵌套列表中名字含有两个‘e’的所有名字

  names = [['Tom', 'Billy', 'Jefferson', 'Andrew', 'Wesley', 'Steven', 'Joe'], ['Alice', 'Jill', 'Ana', 'Wendy', 'Jennifer', 'Sherry', 'Eva']]   multiples =[named for name in names for named in name  if named.count('e') is 2]   print(multiples)
  ['Jefferson', 'Wesley', 'Steven', 'Jennifer']

  字典推导式

  例一:将一个字典的key和value对调

  macase = {'a':10,'b':34}   macas_frequency = {macase[k] : k for k in macase}   print(macas_frequency)
  {10: 'a', 34: 'b'}

  例二:合并大小写对应的value值,将k统一成小写

  mcase = {'a': 10, 'b': 34, 'A': 7, 'Z': 3}   mcase_frequency = {k.lower(): mcase.get(k.lower(), 0) + mcase.get(k.upper(), 0) for k in mcase.keys()}   print(mcase_frequency)
  {'a': 17, 'b': 34, 'z': 3}

  集合推导式

  例:计算列表中每个值的平方,自带去重功能

  squared = {x**2 for x in [1, -1, 2]}   print(squared)   # Output: set([1, 4])
  {1, 4}

  练习题:

  例1:  过滤掉长度小于3的字符串列表,并将剩下的转换成大写字母

  str_list = ['sdAfD','asOPa','sd','a','tg','AsasS','Av','qwerAtyuifAghjk']   str_list = [str.upper() for str in str_list if len(str) >= 3]   print(str_list)
  ['SDAFD', 'ASOPA', 'ASASS', 'QWERATYUIFAGHJK']

  例2:  求(x,y)其中x是0-5之间的偶数,y是0-5之间的奇数组成的元祖列表

  tupl_list = [(x, y) for x in range(0,6,2) for y in range(1,6,2)]   print(tupl_list)
  [(0, 1), (0, 3), (2, 1), (2, 3), (4, 1), (4, 3)]

  例3:  求M中3,6,9组成的列表M = [[1,2,3],[4,5,6],[7,8,9]]

  M = [[1,2,3],[4,5,6],[7,8,9]]   m = [ml[2] for ml in M ]   print(m)
  [3, 6, 9]