IterYield.md

迭代器和生成器

• 迭代器

  • 迭代器有两个基本的方法：iter() 和 next(),且字符串，列表或元组对象都可用于创建迭代器，迭代器对象可以使用常规 for 语句进行遍历，也可以使用 next() 函数来遍历。

      str = 'lcp0578'
    
      iterObj = iter(str)
    
      for x in iterObj:
          print(x, end=' ')
    
      while True:
          try:
              print(next(iterObj)) #迭代器只能向后，所以实际在while语句并不会再输出
          except StopIteration:
              break
    

• 列表生成式

  • 语法

      [expr for iter_var in iterable] 
      [expr for iter_var in iterable if cond_expr]
    

  • 示例

      list1 = [x * x for x in range(1, 11)]
      list2 = [x * x for x in range(1, 11) if x % 2 == 0]
    
      print(list1) #[1, 4, 9, 16, 25, 36, 49, 64, 81, 100]
      print(list2) #[4, 16, 36, 64, 100]
      
      print('\n'.join([' '.join('%dx%d=%2d' % (x, y, x * y) for x in range(1, y + 1)) for y in range(1, 10)]))
      #output
      1x1= 1
      1x2= 2 2x2= 4
      1x3= 3 2x3= 6 3x3= 9
      1x4= 4 2x4= 8 3x4=12 4x4=16
      1x5= 5 2x5=10 3x5=15 4x5=20 5x5=25
      1x6= 6 2x6=12 3x6=18 4x6=24 5x6=30 6x6=36
      1x7= 7 2x7=14 3x7=21 4x7=28 5x7=35 6x7=42 7x7=49
      1x8= 8 2x8=16 3x8=24 4x8=32 5x8=40 6x8=48 7x8=56 8x8=64
      1x9= 9 2x9=18 3x9=27 4x9=36 5x9=45 6x9=54 7x9=63 8x9=72 9x9=81
    

• 生成器

    # -*- coding: UTF-8 -*-
    list1 = [x * x for x in range(10)]
    gen = (x * x for x in range(10))
  
    print(list1)
    print(gen)
  
    for num in gen:
        print(num)
  
    def my_function():
        for i in range(10):
            yield i
  
    print(my_function())
    #output
    [0, 1, 4, 9, 16, 25, 36, 49, 64, 81]
    <generator object <genexpr> at 0x10babae40>
    0
    1
    4
    9
    16
    25
    36
    49
    64
    81
    <generator object my_function at 0x10babadd0>
    
    # -*- coding: UTF-8 -*-
    def triangles(end):  # 杨辉三角形
        tr = [1]
        start = 1
        while True:
            yield tr
            tr.append(0)
            tr = [tr[i - 1] + tr[i] for i in range(len(tr))]
            start = start + 1
            if end < start:
                break
  
    n = 0
    for t in triangles(10):
        print(t)