循環

5.1 while循環

1. 說明：關鍵字 while ，判斷條件爲真就一直執行
2. 例子：

   1:  name = ''
   
   2:  while not name.strip():
   
   3:      name = input("Please input your name:")
   
   4:  print("Hello,", name)

5.2 for循環

1. 說明：能夠用於迭代集合中的每一個元素；
2. 例子：

   1:  #遍歷列表中的各個元素
   
    2:  >>> x = [1,2,3,4,5]
   
    3:  >>> for number in x:
   
    4:          print (number)
   
    5:  1
   
    6:  2
   
    7:  3
   
    8:  4
   
    9:  5
   
   10:  >>> 
   
   11:  
   
   12:  #使用內建函數range
   
   13:  >>> x = range(10)
   
   14:  >>> x
   
   15:  range(0, 10)
   
   16:  >>> for number in x:
   
   17:          print(number)
   
   18:  0
   
   19:  1
   
   20:  2
   
   21:  3
   
   22:  4
   
   23:  5
   
   24:  6
   
   25:  7
   
   26:  8
   
   27:  9
   
   28:  >>> 
   
   29:

5.3 循環遍歷字典元素

1. 說明：經過keys遍歷字典，或者經過values；
2. 例子：

   1:  x = {'a':'1', 'b':'2', 'c':'3'}
   
   2:  for key in x.keys():
   
   3:      print (key, x[key])
   
   4:  
   
   5:  for val in x.values():
   
   6:      print(val)
   
   7:

5.4 一些迭代工具

 

5.4.1 並行迭代

1. 說明：zip內置函數能夠將多個序列「壓縮」成一個元組的序列；
2. 例子：

   1:  >>> x = list(range(0,5))
   
    2:  >>> y = list(range(5,10))
   
    3:  >>> z = list(range(10, 15))
   
    4:  >>> z
   
    5:  [10, 11, 12, 13, 14]
   
    6:  >>> y
   
    7:  [5, 6, 7, 8, 9]
   
    8:  >>> x
   
    9:  [0, 1, 2, 3, 4]
   
   10:  
   
   11:  >>> zipped = zip(x, y, z)
   
   12:  >>> list(zipped)
   
   13:  [(0, 5, 10), (1, 6, 11), (2, 7, 12), (3, 8, 13), (4, 9, 14)]
   
   14:  >>> 
   
   15:

5.4.2 編號迭代

1. 說明：使用內建函數enumerate來進行迭代操做；
2. 例子：

   1:  >>> mylist = ['12312', '12ab', '123sa', '1231s']
   
    2:  >>> for index, string in enumerate(mylist):
   
    3:          print(index, string)
   
    4:  
   
    5:  
   
    6:  0 12312
   
    7:  1 12ab
   
    8:  2 123sa
   
    9:  3 1231s
   
   10:  >>> 
   
   11:

5.4.3 翻轉和排序迭代

1. 說明：內建函數reversed用於翻轉序列，內建函數sorted用於對序列排 序，他們都是返回操做後的序列，不對原序列進行修改；
2. 例子：

   1:  >>> data = [1,67,1,13,14,61,2]
   
   2:  >>> sorted(data)
   
   3:  [1, 1, 2, 13, 14, 61, 67]
   
   4:  >>> list(reversed(data))
   
   5:  [2, 61, 14, 13, 1, 67, 1]
   
   6:  >>> 
   
   7:

5.5 跳出循環

 

5.5.1 break

1. 說明：符合條件時直接中斷循環；
2. 例子：

   1:  >>> import math
   
   2:  >>>for x in range(99, 0, -1):
   
   3:  >>>    root = math.sqrt(x)
   
   4:  >>>    if root == int(root):
   
   5:  >>>        print ('Max number is:', x)
   
   6:  >>>        break
   
   7:  
   
   8:  Max number is 81
   
   9:

5.5.2 continue

1. 說明：結束當前循環，並跳到下一輪循環開始；
2. 例子：

   1:  #一個打印偶數的例子，不加else 語句，程序也能正確執行
   
    2:  >>> for x in range(10):
   
    3:          if x%2 == 0:
   
    4:                  print(x)
   
    5:          else:
   
    6:                  continue
   
    7:  
   
    8:  
   
    9:  0
   
   10:  2
   
   11:  4
   
   12:  6
   
   13:  8
   
   14:  >>> 
   
   15:

5.5.3 while True/break

1. 說明：while True部分實現了一個永不中止的循環，由內部的if判斷語 句控制跳出循環；
2. 例子：

   1:  while True:
   
    2:          word = input("Please enter a word:")
   
    3:          if not word:
   
    4:                  break
   
    5:          print("You input:" , word)
   
    6:  
   
    7:  Please enter a word:TEst
   
    8:  You input: TEst
   
    9:  Please enter a word:ls
   
   10:  You input: ls
   
   11:  Please enter a word:
   
   12:  >>> 
   
   13:

5.5.4 循環中的else子句

1. 說明：else子句能夠用於判斷循環操做是否始終沒有執行break操做。
2. 例子：

   1:   #設置一個奇數序列，判斷裏面是否是有偶數（一個蛋疼的程序，哈哈）
   
    2:  x = list(range(1,100,2))
   
    3:  for val in x:
   
    4:      if val%2 == 0:
   
    5:          print (x)
   
    6:          break;
   
    7:  else:
   
    8:      print("Did not break!")
   
    9:  #執行結果
   
   10:  Did not break!
   
   11: