Python语言之控制流（if...elif...else,while,for,break,continue）

1.if...elif...else...

 1 number = 23
 2 guess = int(raw_input(‘Enter an integer : ‘))
 3
 4 if guess == number:
 5     print( ‘Congratulations, you guessed it.‘ ) # New block starts here
 6     print( "(but you do not win any prizes!)" ) # New block ends here
 7 elif guess < number:
 8     print( ‘No, it is a little higher than that‘ )# Another block
 9 else:
10     print( ‘No, it is a little lower than that‘ )
11
12 print( ‘Done‘ )
13 # This last statement is always executed, after the if statement is executed 

2.while

 1 number = 23
 2 running = True
 3
 4 while running:
 5     guess = int(raw_input(‘Enter an integer : ‘))
 6
 7     if guess == number:
 8         print ‘Congratulations, you guessed it.‘
 9         running = False # this causes the while loop to stop
10     elif guess < number:
11         print ‘No, it is a little higher than that‘
12     else:
13         print ‘No, it is a little lower than that‘
14 else:
15     print ‘The while loop is over.‘
16     # Do anything else you want to do here
17
18 print ‘Done‘

while循环条件变为False时，else块被执行。

你可能想问，这样的else块始终会被执行到，还要else干嘛。别着急，在4.break中有解答。

3.for

1 for i in range(1, 5):
2     print i
3 else:
4     print ‘The for loop is over‘

range(n, m [,stepLen]) = [n,n+stepLen,n+stepLen*2,...,n+(ceil((m-n)/stepLen)-1)*stepLen]

即从n开始，以stepLen为步长，一直到小于m的序列（不包括m）。默认情况下stepLen=1。

4.break

终止当前循环语句。

注意如果从for或while循环终止，任何对应的else将不会被执行。

5.continue

跳过当前循环块中的剩余语句，然后继续进行下一轮的循环。