Python中的循环语句

1 While循环：

语法：

while expression:






   statement(s)







单语句的while循环：



while expression : statement














2 For循环：



语法：



for iterating_var in sequence:






   statements(s)







If a sequence contains an expression list, it is evaluated first. Then, 



the first item in the sequence is assigned to the iterating variable iterating_var


. Next, the statements block is executed. Each item in the list is assigned to iterating_var


, and the statements(s) block is executed until the entire sequence is exhausted.







example:







#!/usr/bin/python







for letter in 'Python':     # First Example



   print ('Current Letter :', letter)







fruits = ['banana', 'apple',  'mango']



for fruit in fruits:        # Second Example



   print ('Current fruit :', fruit)







print  ("Good bye!")







output:D


:/>python test.py



Current Letter : P



Current Letter : y



Current Letter : t



Current Letter : h



Current Letter : o



Current Letter : n



Current fruit : banana



Current fruit : apple



Current fruit : mango



Good bye!

Iterating by Sequence Index:

An alternative way of iterating through each item is by index offset into the sequence itself:

Example:

#!/usr/bin/python

fruits = ['banana', 'apple', 'mango']

for index in range(len(fruits)):

          print 'Current fruit :', fruits[index]

print "Good bye!"

3 退出循环：






Break 退出整个循环；



continue退出本次循环，继续下次循环。







4The else 
 Statement Used with Loops

Python supports to have an else statement associated with a loop statements.

• If the else statement is used with a for loop, the else statement is executed when the loop has exhausted iterating the list.

• If the else statement is used with a while loop, the else statement is executed when the condition becomes false.

Example:

The following example illustrates the combination of an else statement with a for statement that searches for prime numbers from 10 through 20.

#!/usr/bin/python for num in range(10,20): #to iterate between 10 to 20 for i in range(2,num): #to iterate on the factors of the number if num%i == 0: #to determine the first factor j=num/i #to calculate the second factor print '%d equals %d * %d' % (num,i,j) break #to move to the next number, the #first FOR else: # else part of the loop print num, 'is a prime number'

This will produce following result:

10 equals 2 * 5 11 is a prime number 12 equals 2 * 6 13 is a prime number 14 equals 2 * 7 15 equals 3 * 5 16 equals 2 * 8 17 is a prime number 18 equals 2 * 9 19 is a prime number

Similar way you can use else statement with while loop.

5 The pass Statement:

The pass statement in Python is used when a statement is required syntactically but you do not want any command or code to execute.

The pass statement is a null operation; nothing happens when it executes. The pass is also useful in places where your code will eventually go, but has not been written yet (e.g., in stubs for example):

Example:

#!/usr/bin/python for letter in 'Python' : if letter == 'h': pass print 'This is pass block' print 'Current Letter :', letter print "Good bye!"

This will produce following result:

Current Letter : P Current Letter : y Current Letter : t This is pass block Current Letter : h Current Letter : o Current Letter : n Good bye!

The preceding code does not execute any statement or code if the value of letter is 'h'. The pass statement is helpful when you have created a code block but it is no longer required.

You can then remove the statements inside the block but let the block remain with a pass statement so that it doesn't interfere with other parts of the code.

python支持连续比较 如a<b<c 他等同于a<b b<c