3-4如何进行反向迭代和如何实现反向迭代

实现反向迭代

>>> l = [1,2,3,4,5]
>>> l
[1, 2, 3, 4, 5]
>>> reversed(l)    #实际上调用了 l.__reversed()方法
<listreverseiterator object at 0x02A12D30>
>>> iter(l)     #实际上调用了 l.__iter__()方法
<listiterator object at 0x02B47A70>
reversed(l)实现了反向迭代器


>>> for x in reversed(l):
    print (x),
5 4 3 2 1
>>> 

通过类实现迭代和反射迭代

class FloatRange:
    def __init__(self,start,end,step=0.1):
        self.start = start
        self.end = end
        self.step = step

    def __iter__(self):     #定义正向迭代
        x = self.start
        while x <= self.end: 
            yield x
            x += self.step    #返回后，下一次迭代时，将当前值 加上步进

    def __reversed__(self):   #定义反向迭代
        x = self.end
        while x >= self.start:
            yield x
            x -= self.step    #返回后，下一次迭代时，将当前值减去步进

测试：

for x in FloatRange(1.0,5.0,0.5):
    print (x),
print 
print('~'*20)

for x in reversed(FloatRange(1.0,5.0,0.5)):
    print (x),

输出结果：

1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0

~~~~~~~~~~~~~~~~~~~~

5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0

 

>>> l = [1,2,3,4,5]
>>> newl = reversed(l)
>>> newl
<listreverseiterator object at 0x02A89FD0>
>>> l[::-1]
[5, 4, 3, 2, 1]
>>> l
[1, 2, 3, 4, 5]
>>> 
>>> for x in newl:
    print x,

输出结果：

5 4 3 2 1