python控制流

当我们使用任何一门语言编写程序时有时会想改变语句流的执行顺序比如根据时间打印“早上好”或者“晚上好”。在Python中有三种控制流语句——if、for和while。如果你熟悉一门编程语言如C、VB等那么这部分内容将会变的非常容易。

1.if语句
if语句用来检验一个条件, 如果条件为真,我们运行一块语句(称为 if-块 ), 否则 我们处理另外一块语句(称为 else-块 )。 else从句是可选的。
使用if语句

if expression:#冒号不能少
    statement
    statement#注意相同的语句块缩进要相同
elif expression:
    statement
    statement
elif expression:
    statement
    statement
    ...
else:
    statement
    statement
    
other statement
例 使用if语句
#!/usr/bin/python
# Filename: if.py
number = 23
guess = int(raw_input('Enter an integer : '))
if guess == number:
   print 'Congratulations, you guessed it.' # New block starts here
   print "(but you do not win any prizes!)" # New block ends here
elif guess < number:
   print 'No, it is a little higher than that' # Another block
else:
   print 'No, it is a little lower than that'
print 'Done'
# This last statement is always executed, after the if statement is executed

输出
$ python if.py
Enter an integer : 50
No, it is a little lower than that
Done
$ python if.py
Enter an integer : 22
No, it is a little higher than that
Done
$ python if.py
Enter an integer : 23
Congratulations, you guessed it.
(but you do not win any prizes!)
Done

2.while语句
只要在一个条件为真的情况下,while语句允许你重复执行一块语句。while语句是所谓循环语句的一个例子。while语句有一个可选的else从句。
使用while语句
while expression:
    statement
    statement
    ...
    statement
else:
    statement

other statement   

例 使用while语句
#!/usr/bin/python
# Filename: while.py
number = 23
running = True
while running:
   guess = int(raw_input('Enter an integer : '))
   if guess == number:
      print 'Congratulations, you guessed it.'
      running = False # this causes the while loop to stop
   elif guess < number:
      print 'No, it is a little higher than that'
   else:
      print 'No, it is a little lower than that'
else:
   print 'The while loop is over.'

print 'Done'

输出
$ python while.py
Enter an integer : 50
No, it is a little lower than that.
Enter an integer : 22
No, it is a little higher than that.
Enter an integer : 23
Congratulations, you guessed it.
The while loop is over.
Done

3.for循环
for..in是另外一个循环语句,它在一序列的对象上递归即逐一使用队列中的每个项目。python的for循环和VB的for循环类似。
使用for语句
for var in ...
    statement
    statement
else:
    statement

例 使用for语句
#!/usr/bin/python
# Filename: for.py
for i in range(1, 5):
   print i
else:
   print 'The for loop is over'
输出
$ python for.py
1
2
3
4
The for loop is over
注range函数其实有3个参数第一个参数是起点包含第二个参数是终点不含第三个参数是步长默认为1。

4.break语句
break语句是用来终止循环语句的,即哪怕循环条件不是False或序列还没有被完全递归,也停止执行循环语句。
注如果你从for或while循环中终止,任何对应的循环else块将不执行。
使用break语句
例6.4 使用break语句
#!/usr/bin/python
# Filename: break.py
while True:
  s = raw_input('Enter something : ')
  if s == 'quit':
     break
  print 'Length of the string is', len(s)
print 'Done'
(源文件:code/break.py)
输出
$ python break.py
Enter something : Programming is fun
Length of the string is 18
Enter something : When the work is done
Length of the string is 21
Enter something : if you wanna make your work also fun:
Length of the string is 37
Enter something : use Python!
Length of the string is 12
Enter something : quit
Done

5.continue语句
continue语句被用来告诉Python跳过当前循环块中的剩余语句,然后继续进行下一轮循环。
使用continue语句
例6.5 使用continue语句
#!/usr/bin/python
# Filename: continue.py
while True:
  s = raw_input('Enter something : ')
  if s == 'quit':
      break
  if len(s) < 3:
      continue
  print 'Input is of sufficient length'
  # Do other kinds of processing here...