详解Python中for循环的使用方法
for循环在Python中有遍历所有序列的项目，如列表或一个字符串。
语法：
for循环语法如下：
for iterating_var in sequence: statements(s)
如果一个序列包含一个表达式列表，计算第一个。然后，在序列中的第一项被分配给迭代变量iterating_var。接着，语句块被执行。列表中的每个项目分配给iterating_var，并且statement块被执行，直到整个序列完成(到尾部)。
流程图：
例子：
#!/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!"
当执行上面的代码，产生以下结果：
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!
通过遍历序列索引：
通过每个项目进行迭代的一种替代方法是：通过索引偏移量序列本身。下面是一个简单的例子：
#!/usr/bin/python fruits = ['banana', 'apple', 'mango'] for index in range(len(fruits)): print 'Current fruit :', fruits[index] print "Good bye!"
当执行上面的代码，产生以下结果：
Current fruit : banana Current fruit : apple Current fruit : mango Good bye!
在这里，我们采用内置函数len()，它计算元组元素的总数量以及range()内置函数给我们的实际遍历顺序。
循环使用else语句
Python支持与循环语句相关联的else语句。
- 如果else语句与for循环使用，执行else语句时，循环已经迭代完成列表。
- 如果在else语句使用while循环，当条件为假时，else语句被执行。
下面的例子演示了一个else语句，语句搜索素数从10到20的组合。
#!/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'
当执行上面的代码，产生以下结果：
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
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